Gerard J. Tortora Bryan Derrick son

the essentials of anatomy and physiology

eighth edition

eighth edition

Introduction to the Human Body the essentials of anatomy and physiology

GERARD J.TORTORA Bergen Community College

BRYAN DERRICKSON Valencia Community College

W I L E Y JOHN WILEY & SONS, INC.

Executive Editor Project Editor Executive Marketing Manager Development Editor Production Manager Senior Production Editor Cover Designer Text Designer Senior Illustration Editor Photo Manager Senior Media Editor Media Project Manager Program Assistant Outside Production Manager Cover image

Bonnie Roesch Lorraina Raccuia Clay Stone Karen Trost Lucille Buonocore

Elizabeth Swain Madelyn Lesure Karin Gerdes Kincheloe Anna Melhorn Hilary Newman Linda Muriello Bonnie Roth

Lauren Morris SuzanneIngrao

© 2005 Lois Greenfield

Illustration and photo credits follow the Glossary.

This book was typeset by Aptara®, Inc., and printed and bound by Quebecor Dubuque. The cover was also printed by Quebecor Dubuque.

The paper in this book was manufactured by a mill whose forest management programs include sustained yield harvesting of its timberlands. Sustained yield harvesting principles ensure that the number of trees cut each year does not exceed the amount of new growth.

Copyright © 2010, 2007, 2004, 2001, 1997, 1994, 1991, 1987 by Biological Sciences Text-books, Inc. and Bryan Derrickson

This title published by John Wi ley & Sons, Inc.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copy-right Act, without either the prior written permission of the Publisher or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (508) 750-8400, fax (508) 750-4470. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wi ley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012, (212)805-6008, E-mail: PERMREQ@WILEY.COM.

Library of Congress Cataloging-in-Publication Data

Tortora, Gerard J. Introduction to the human body: the essentials of anatomy and physiology / Gerard

J. Tortora, Bryan Derrickson. — 8th ed. p. cm.

Includes index. ISBN 978-0-470-23016-9 (cloth) 1. Human physiology. 2. Human anatomy. I. Derrickson, Bryan. II. Title. QP36.T67 2010 612—dc22

2008036347

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

ABOUT THE AUTHORS

Jerry Tortora is Professor of Biology and former Biology Coordinator at Bergen Community Col-lege in Paramus, New Jersey, where he teaches human anatomy and physiology as well as microbiology. He re-ceived his bachelor's degree in

biology from Fairleigh Dickinson University and his master's degree in science education from Montclair State College. He is a member of many professional organizations, includ-ing the Human Anatomy and Physiology Society (HAPS), the American Society of Microbiology (ASM), American As-sociation for the Advancement of Science (AAAS), National Education Association (NEA), and the Metropolitan Associa-tion of College and University Biologists (MACUB).

Above all, Jerry is devoted to his students and their aspi-rations. In recognition of this commitment, Jerry was the re-cipient of MACUB's 1992 President's Memorial Award. In 1996, he received a National Institute for Staff and Organiza-tional Development (NISOD) excellent award from the Uni-versity of Texas and was selected to represent Bergen Com-munity College in a campaign to increase awareness of the contributions of community colleges to higher education.

Jerry is the author of several best-selling science text-books and laboratory manuals, a calling that often requires an additional 40 hours per week beyond his teaching responsibil-ities. Nevertheless, he still makes time for four or five weekly aerobic workouts that include biking and running. He also enjoys attending college basketball and professional hockey games and performances at the Metropolitan Opera House.

v* Bryan Derrickson is Professor of Biology at Va-lencia Community College in Orlando, Florida, where he teaches human anatomy and physiology as well as general biology and human sexuality.

He received his bachelor's degree in biology from More-house College and his Ph.D. in Cell Biology from Duke University. Bryan's study at Duke was in the Physiology Divi-sion within the Department of Cell Biology, so while his de-gree is in Cell Biology his training focused on physiology. At Valencia, he frequently serves on faculty hiring committees. He has served as a member of the Faculty Senate, which is the governing body of the college, and as a member of the Faculty Academy Committee (now called the Teaching and

Learning Academy), which sets the standards for the acquisi-tion of tenure by faculty members. Nationally, he is a mem-ber of the Human Anatomy and Physiology Society (HAPS) and the National Association of Biology Teachers (NABT).

Bryan has always wanted to teach. Inspired by several biology professors while in college, he decided to pursue physiology with an eye to teaching at the college level. He is completely dedicated to the success of his students. He par-ticularly enjoys the challenges of his diverse student popula-tion, in terms of their age, ethnicity, and academic ability, and finds being able to reach all of them, despite their differences, a rewarding experience. His students continually recognize Bryan's efforts and care by nominating him for a campus award known as the "Valencia Professor W h o Makes Valencia A Better Place To Start." Bryan has received this award three times.

iii

PREFACE

Introduction to the Human Body: The Essentials of Anatomy and Physiology, Eighth Edition, is designed for courses in hu-man anatomy and physiology or in human biology. It assumes no previous study of the human body. The successful approach of the previous editions—to provide students with a basic understanding of the structure and functions of the human body with an emphasis on homeostasis—has been retained. In the development of the eighth edition, we focused on improving the ac-knowledged strengths of the text.

Most importantly, our students continue to remind us of their needs for—and of the power of—simplicity, directness, and clarity. To meet these needs, each chapter has been writ-ten and revised to include:

• clear, compelling, and up-to-date discussions of anatomy and physiology

• expertly executed and generously sized art

• classroom-tested pedagogy

• outstanding student study support

As we revised the content for this edition we kept our fo-cus on these important criteria for success in the anatomy and physiology classroom and have refined or added new ele-ments to enhance the teaching and learning process.

N E W T O T H I S E D I T I O N

TEXT UPDATES Every chapter in this edition of Principles of Anatomy and Physiology incorporates a host of improvements to both the text and the art developed by us and suggested by reviewers, educators, or students. Some noteworthy text changes in-clude the following:

• Chapter 3 A revision of the section on transport across the plasma membrane, which now begins with a discus-sion of passive processes (simple diffusion, facilitated dif-fusion, and osmosis) followed by a discussion of active processes (primary active transport, secondary active transport, and transport in vesicles, which includes endo-cytosis, exocytosis, and transcytosis).

• Chapter 5 A new section on tattooing and body pierc-ing and a revised section on sun damage.

• Chapter 6 A revised section on bone formation.

• Chapter 7 Revised discussions of f ibrous joints, types of synovial joints, and a new section on knee replace-ments.

• Chapter 8 A revised discussion of connective tissue components.

• Chapter 9 A ful ly revised chapter that provides a clearer understanding of nervous tissue structure and function.

• Chapter 10 Revisions that clarify how the brain and spinal cord process sensory and motor information.

• Chapter 17 Significantly revised sections on cel l-medi-ated immunity and antibody-mediated immunity along with updated illustrations.

• Chapter 21 Revised sections on tubular reabsorption and tubular secretion.

All clinical applications have been reviewed for currency and have been redesigned into Clinical Connection Boxes, to be more easily recognizable within the chapter content. Many of the entries in the Common Disorders sections at chapters' end now have new illustrations. All Medical Ter-minology and Conditions sections, also at the end of chap-ters, have been updated.

ART AND DESIGN The simple design of the eighth edition allows the il lustra-tions to be the focal point on each page. Each page or 2-page spread is carefully laid out to place related text, figures, and tables near one another, minimizing the need for page turn-ing while reading a topic. You'll notice the re-design for the updated Clinical Connections within each chapter.

An outstanding illustration program has always been a signature feature of this text. Beautiful artwork, carefully chosen photographs and photomicrographs, and unique ped-agogical enhancements all combine to make the visual appeal and usefulness of the illustration program in Introduction to the Human Body distinctive.

Continuing in this tradition, you will find exciting new three-dimensional illustrations gracing the pages of near ly every chapter in the text. Significantly, all of the illustrations in Chapter 7 on the skeleton are new, as well as all of the i l-lustrations in Chapter 8 on muscles. These new illustrations are among the best that we have ever seen in an anatomy and physiology textbook and truly support the visual learner in meeting the challenge of learning so many anatomical structures.

v

2 5 4 Chapter 9 Nervous Tissue

14. What would happen at the postsynaptic neuron if the total in-hibitory effects of the neurotransmitters were greater than the total excitatory effects?

a. A nerve impulse would be generated.

b. It would be easier to generate a nerve impulse when the next stimulus was received.

c. The nerve impulse would be rerouted to another neuron.

d. No nerve impulse would be generated.

e. The neurotransmitter would be broken down more quickly.

15. Match the following neurotransmitters with their descriptions.

a. inhibitory amino acid in the CNS A. serotonin

b. a gaseous neurotransmitter that is not packaged into synaptic vesicles

c. excitatory amino acid in the CNS

d. body's natural painkillers

e. helps regulate mood and sleep

f. neurotransmitter that activates skeletal muscle fibers

B. acetylcholine

C. endorphins

D. GABA

E. nitric oxide

F. glutamate

16. Match the following.

a. the portion of a neuron containing the nucleus

b. rounded structure at the distal end of an axon terminal

A. synaptic end bulb

B. motor neuron

C. sensory neuron

D. dendrite

c. highly branched, input part of E. a neuron

d. sac in which neurotransmitter is stored

e. neuron located entirely within the CNS

f. long, cylindrical process that conducts impulses toward another neuron

g. produces myelin sheath in PNS

h. unmyelinated gap in the myelin sheath

i. substance that increases the speed of nerve impulse conduction

j. neuron that conveys information from a receptor to the CNS

k. neuron that conveys information from the CNS to an effector

l. bundle of many axons in the PNS m. bundle of many axons in the CNS n. group of cell bodies in the PNS o. group of cell bodies in the CNS p. substance used for communication

at chemical synapses

F.

G.

H.

I.

J. K.

L.

M.

N.

O.

P.

interneuron

nucleus

myelin sheath

Schwann cell

cell body

node of Ranvier

ganglion

nerve

neurotransmitter

tract

synaptic vesicle

C R I T I C A L T H I N K I N G A P P L I C A T I O N S

1. The buzzing of the alarm clock awoke Rodrigo. He stretched, yawned, and started to salivate as he smelled the brewing cof-fee. List the divisions of the nervous system that are involved in each of these activities.

2. Prior to surgery, Marga was given a curare-like drug that tem-porarily "paralyzed" her muscles so that she could be more eas-ily intubated and would not move during surgery. What is the neurotransmitter involved and how do you think the drug pre-vents skeletal muscle contraction?

3. Sarah really looks forward to the great feeling she has after going for a nice long run on the weekends. By the end of her run, she doesn't even feel the pain in her sore feet. Sarah read in a magazine that some kind of natural brain chemical was responsible for the "runner's high" that she feels. Are there such chemicals in Sarah's brain?

4. The pediatrician was trying to educate the anxious new parents of a six-month-old baby. "No, don't worry about him not walking yet. The myelination of the baby's nervous system is not finished yet." Explain what the pediatrician means by this reassurance.

A N S W E R S T O F I G U R E Q U E S T I O N S

9.1 The total number of cranial and spinal nerves in your body is (12 X 2) + (31 X 2) = 86.

9.2 The axon conducts nerve impulses and transmits the mes-sage to another neuron or effector cell by releasing a neuro-transmitter at its axon terminals.

9.3 Most neurons in the CNS are multipolar neurons.

9.4 Sensory (afferent) neurons carry input to the CNS. Motor (efferent) neurons carry output from the CNS.

9.5 A typical value for the resting membrane potential in a neu-ron is — 70 mV.

9.6 Voltage-gated Na+ channels are open during the depolariz-ing phase, and voltage-gated K+ channels are open during the repolarizing phase of an action potential.

9.7 In some electrical synapses (gap junctions), ions may flow equally well in either direction, so either neuron may be the presynaptic one. At a chemical synapse, one neuron releases neurotransmit-ter and the other neuron has receptors that bind this chemical. Thus, the signal can proceed in only one direction.

9.8 The depolarizing phase of the action potential opens the voltage-gated Ca2+ channels in synaptic end bulbs.

axon

Some things improve with age, but

hearing is not one of them. Damage to the hair cells

that convert sound waves into nerve impulses

accumulates over a lifetime, and by the time hearing

loss is discovered, irreversible damage has already

occurred. Exposure to excessive noise is the most

common cause of hair cell damage. Damage increases

with both the intensity and duration of exposure. The

hair cells appear to be less traumatized by short periods

of loud noise, such as a fire alarm going off, than by

chronic exposure to moderately loud noise, such as the

noise of vacuum cleaners, power

tools, engines, and loud music.

Focus on Wellness: Pain Management— Sensation Modulation

SOMATIC SENSES AND SPECIAL SENSES

Consider what would

happen if you could not

feel the pain of a hot

pot handle or an

inflamed appendix, or if you could not see an oncoming

car, hear a baby's cry, smell smoke, taste your favorite

dessert, or maintain your balance on a flight of stairs.

In short, if you could not "sense" your environment and

make the necessary homeostatic adjustments, you could

not survive very well on your own.

2 9 8 Chapter 12 Somatic Senses and Special Senses

O V E R V I E W O F S E N S A T I O N S O B J E C T I V E • Define a sensation and describe the con-ditions needed for a sensation to occur.

Most of us are aware of sensory input to the central nervous sys-tem (CNS) from structures associated with smell, taste, vision, hearing, and balance. These five senses are known as the special senses. T h e other senses are termed general senses and in-clude both somatic senses and visceral senses. Somatic senses (somat- = of the body) include tactile sensations (touch, pressure, and vibration); thermal sensations (warm and cold); pain sensa-tions; and proprioceptive sensations (joint and muscle position sense and movements of the limbs and head). Visceral senses provide information about conditions within internal organs.

Def in i t ion of S e n s a t i o n Sensation is the conscious or subconscious awareness of changes in the external or internal environment. For a sensa-tion to occur, four conditions must be satisfied:

1. A stimulus, or change in the environment, capable of acti-vating certain sensory neurons, must occur. A stimulus that activates a sensory receptor may be in the form of light, heat, pressure, mechanical energy, or chemical energy.

2. A sensory receptor must convert the stimulus to an electri-cal signal, which ult imately produces one or more nerve impulses if it is large enough.

3. T h e nerve impulses must be conducted along a neural pathway from the sensory receptor to the brain.

4. A region of the brain must receive and integrate the nerve impulses into a sensation.

C h a r a c t e r i s t i c s of S e n s a t i o n s As you have learned in Chapter 10, perception is the con-scious awareness and interpretation of sensations and is pri-mari ly a function of the cerebral cortex. You seem to see with your eyes, hear with your ears, and feel pain in an injured part of your body. Th i s is because sensory nerve impulses from each part of the body arrive in a specific region of the cerebral cortex, which interprets the sensation as coming from the stimulated sensory receptors. A given sensory neu-ron carries information for one type of sensation only. Neu-rons relaying impulses for touch, for example, do not also conduct impulses for pain. T h e specialization of sensory neu-rons enables nerve impulses from the eyes to be perceived as sight and those from the ears to be perceived as sounds.

A characteristic of most sensory receptors is adaptation, a decrease in the strength of a sensation during a prolonged stim-ulus. Adaptation is caused in part by a decrease in the respon-siveness of sensory receptors. As a result of adaptation, the per-ception of a sensation may fade or disappear even though the stimulus persists. For example, when you first step into a hot shower, the water may feel very hot, but soon the sensation de-creases to one of comfortable warmth even though the stimulus (the high temperature of the water) does not change. Receptors vary in how quickly they adapt. Receptors associated with pres-sure, touch, and smell adapt rapidly. Slowly adapting receptors monitor stimuli associated with pain, body position, and the chemical composition of the blood.

T y p e s of S e n s o r y R e c e p t o r s Both structural and functional characteristics of sensory re-ceptors can be used to group them into different classes (Table 12.1). Structurally, the simplest are free nerve endings,

Table 12.1 Classi f icat ion of Sensory Receptors

Basis of C lass i f ica t ion D e s c r i p t i o n

S t ruc tu re

Free nerve e n d i n g s Bare dendr i tes are assoc ia ted wi th pain, thermal , t ickle, itch, and s o m e touch sensat ions. Encapsu la ted nerve end ings Dendr i tes enc losed in a connect ive t issue capsu le for pressure, v ibrat ion, and s o m e touch sensat ions. Separa te cel ls Receptor cel l s ynapses wi th first-order neuron; located in the ret ina o f the eye (photoreceptors) , inner ear (hair

cells), and taste buds o f the tongue (gustatory receptor cel ls).

F u n c t i o n

M e c h a n o r e c e p t o r s Detect mechan ica l pressure; prov ide sensa t ions o f touch, pressure, v ibrat ion, propr iocept ion, and hear ing and equi l ibr ium; also moni tor s t re tch ing o f b lood vesse ls and internal o rgans.

T h e r m o r e c e p t o r s Detect changes in tempera ture . Nociceptors Respond to painful st imul i resul t ing f rom physical or chemica l d a m a g e to t issue.

Photoreceptors Detect l ight that s t r ikes the ret ina o f the eye. C h e m o r e c e p t o r s Detect chemica l s in mou th (taste), nose (smel l) , and body fluids. O s m o r e c e p t o r s Sense the osmot ic pressure of body fluids.

which are bare dendrites that lack any structural specializa-tions at their ends that can be seen under a light microscope (Figure 12.1). Receptors for pain, temperature, tickle, itch, and some touch sensations are free nerve endings. Receptors for other somatic and visceral sensations, such as some touch, pressure, and vibration, have encapsulated nerve endings. Their dendrites are enclosed in a connective tissue capsule with a distinctive microscopic structure. Still other sensory recep-tors consist of specialized, separate cells that synapse with sen-sory neurons, for example, hair cells in the inner ear.

Another way to group sensory receptors is functionally— according to the type of stimulus they detect. Most stimuli are in the form of mechanical energy, such as sound waves or pressure changes; electromagnetic energy, such as light or heat; or chemical energy, such as in a molecule of glucose.

• Mechanoreceptors are sensitive to mechanical stimuli such as the deformation, stretching, or bending of cells. Mechanoreceptors provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium. They also monitor the stretching of blood vessels and internal organs.

• Thermoreceptors detect changes in temperature.

Somatic Senses 2 9 9

• Nociceptors respond to painful stimuli resulting from physical or chemical damage to tissue.

• Photoreceptors detect light that strikes the retina of the eye.

• Chemoreceptors detect chemicals in the mouth (taste), nose (smell), and body fluids.

• Osmoreceptors detect the osmotic pressure of body fluids.

• C H E C K P O I N T

1. Which senses are "special senses"?

2. How is a sensation different from a perception?

S O M A T I C S E N S E S O B J E C T I V E S • Describe the location and function of the receptors for tactile, thermal, and pain sensations.

• Identify the receptors for proprioception and describe their functions.

Somatic sensations arise from stimulation of sensory recep-tors in the skin, mucous membranes, muscles, tendons, and joints. The sensory receptors for somatic sensations are

• I T If fHT

F i g u r e 1 2 . 1 S t r u c t u r e a n d l o c a t i o n o f s e n s o r y r e c e p t o r s i n t h e s k i n a n d s u b c u t a n e o u s l a y e r .

T h e somat ic sensa t ions of touch , pressure , v ibrat ion, w a r m t h , cold, and pain ar ise f rom s e n s o r y receptors in the skin, s u b c u t a n e o u s layer, and m u c o u s m e m b r a n e s .

Nociceptor (pain receptor)

Ep idermis

Dermis

Subcu taneous layer —

Merke l (tacti le) d isc

Me issner corpusc le

Ruf f in i corpusc le

Hair root p lexus

Pac in ian corpusc le

f W h i c h receptors a re especia l ly abundant in the fingertips, pa lms, and so les?

3 0 0 Chapter 12 Somatic Senses and Special Senses

distributed unevenly. Some parts of the body surface are densely populated with receptors, and other parts contain only a few. T h e areas with the largest numbers of sensory re-ceptors are the tip of the tongue, the lips, and the fingertips.

Tact i le S e n s a t i o n s The tactile sensations (TAK-tll; tact- = touch) include touch, pressure, vibration, itch, and tickle. Although we perceive dif-ferences among these sensations, they arise by activation of some of the same types of receptors. Several types of encapsu-lated mechanoreceptors detect sensations of touch, pressure, and vibration. Other tactile sensations, such as itch and tickle sensations, are detected by free nerve endings. Tactile receptors in the skin or subcutaneous layer include Meissner corpuscles, hair root plexuses, Merkel discs, Ruffini corpuscles, pacinian corpuscles, and free nerve endings (Figure 12.1).

Touch

Sensations of touch general ly result from stimulation of tac-tile receptors in the skin or subcutaneous layer. There are two types of rapidly adapting touch receptors. Meissner cor-puscles (MIS-ner ) or corpuscles of touch are touch receptors that are located in the dermal papillae of hairless skin. Each corpuscle is an egg-shaped mass of dendrites enclosed by a capsule of connective tissue. T h e y are abundant in the finger-tips, hands, eyelids, tip of the tongue, lips, nipples, soles, clitoris, and tip of the penis. Hair root plexuses are found in hairy skin; they consist of free nerve endings wrapped around hair follicles. Hair root plexuses detect movements on the skin surface that disturb hairs. For example, an insect landing on a hair causes movement of the hair shaft that stimulates the free nerve endings.

There are also two types of slowly adapting touch recep-tors. Merkel discs, also known as tactile discs or type I cutaneous mechanoreceptors, are saucer-shaped, flattened free nerve end-ings that make contact with Merke l cells of the stratum basale (see Figure 5.2d). These touch receptors are plentiful in the fingertips, hands, lips, and external genitalia. Ruffini corpuscles or type II cutaneous mechanoreceptors are elongated, encapsulated receptors located deep in the dermis, and in l ig-aments and tendons. Present in the hands and abundant on the soles, they are most sensitive to stretching that occurs as digits or limbs are moved.

Pressure

Pressure, a sustained sensation that is felt over a larger area and occurs in deeper tissues than touch, occurs with deforma-tion of deeper tissues. Receptors that contribute to sensations of pressure include Meissner corpuscles, Merke l discs, and pacinian corpuscles. A pacinian (pa-SIN-e-an), or lamellated, corpuscle is a large oval structure composed of a multi layered connective tissue capsule that encloses a dendrite. Like Meissner corpuscles, pacinian corpuscles adapt rapidly. They

are widely distributed in the body: in the dermis and subcuta-neous layer; in tissues that underl ie mucous and serous membranes; around joints, tendons, and muscles; in the periosteum; and in the mammary glands, external genital ia, and certain viscera, such as the pancreas and urinary bladder.

Vibration

Sensations of vibration result from rapidly repetitive sensory signals from tactile receptors. The receptors for vibration sensations are Meissner corpuscles and pacinian corpuscles. Meissner corpuscles can detect lower-frequency vibrations, and pacinian corpuscles detect higher-frequency vibrations.

Itch and Tickle

The itch sensation results from stimulation of free nerve end-ings by certain chemicals, such as bradykinin, often because of a local inf lammatory response. Free nerve endings are thought to mediate the tickle sensation. Th is intriguing sen-sation typically arises only when someone else touches you, not when you touch yourself. T h e solution to this puzzle seems to lie in the impulses that conduct to and from the cerebellum when you are moving your fingers and touching yourself that don't occur when someone else is tickling you.

T h e r m a l S e n s a t i o n s Thermoreceptors are free nerve endings. Two distinct ther-mal sensations—coldness and warmth—are mediated by different receptors. Temperatures between 10° and 40°C (50-105°F) activate cold receptors, which are located in the epi-dermis. Warm receptors are located in the dermis and are acti-vated by temperatures between 32° and 48°C (90-118°F). Cold and warm receptors both adapt rapidly at the onset of a stimulus but continue to generate nerve impulses more slowly throughout a prolonged stimulus. Temperatures below 10°C and above 48°C stimulate mainly nociceptors, rather than thermoreceptors, producing painful sensations.

P a i n S e n s a t i o n s The sensory receptors for pain, called nociceptors (no' -se-SEP-tors; noci- = harmful), are free nerve endings (Figure 12.1). Nociceptors are found in practically every tissue of the

• CLINICAL CONNECTION P h a n t o m L imb S e n s a t i o n

Pat ients w h o have h a d a l imb a m p u t a t e d may still expe r i ence sensa t i ons s u c h a s i tching, pressure , t ingl ing, o r pa in a s if the l imb w e r e still there. T h i s p h e n o m e n o n is ca l led p h a n t o m l imb sensa t ion . O n e exp lana t ion for phan -t o m l imb sensa t i ons is tha t the cerebra l cor tex in te rpre ts impu l ses ar is ing in the p rox ima l po r t ions o f s e n s o r y n e u r o n s tha t prev ious ly car r ied impu l ses f r o m the l imb a s c o m i n g f r o m the nonex is ten t ( p h a n t o m ) l imb. A n o t h e r ex-p lanat ion for p h a n t o m l imb sensa t i ons is tha t n e u r o n s in the bra in tha t previ-ous ly rece ived s e n s o r y impu l ses f r o m the m iss ing l imb are still act ive, g iv ing r ise to fa lse s e n s o r y percept ions . •

Somatic Senses 3 0 1

body except the brain, and they respond to several types of stimuli. Excessive stimulation of sensory receptors, excessive stretching of a structure, prolonged muscular contractions, in-adequate blood flow to an organ, or the presence of certain chemical substances can all produce the sensation of pain. Pain may persist even after a pain-producing stimulus is removed because pain-causing chemicals l inger and because nociceptors exhibit very little adaptation. The lack of adaptation of noci-ceptors serves a protective function: If there were adaptation to painful stimuli, irreparable tissue damage could result.

There are two types of pain: fast and slow. The percep-tion of fast pain occurs very rapidly, usually within 0.1 sec-ond after a stimulus is applied. Th is type of pain is also known as acute, sharp, or pricking pain. T h e pain felt from a needle puncture or knife cut to the skin are examples of fast pain. Fast pain is not felt in deeper tissues of the body. The perception of slow pain begins a second or more after a stim-ulus is applied. It then gradually increases in intensity over a period of several seconds or minutes. Th is type of pain, which may be excruciating, is also referred to as chronic, burning, aching, or throbbing pain. Slow pain can occur both in the skin and in deeper tissues or internal organs. An exam-ple is the pain associated with a toothache.

Fast pain is very precisely localized to the stimulated area. For example, if someone pricks you with a pin, you

know exactly which part of your body was stimulated. So-matic slow pain is well localized but more diffuse (involves large areas); it usually appears to come from a larger area of the skin. In many instances of visceral pain, the pain is felt in or just deep to the skin that overlies the stimulated organ, or in a surface area far from the stimulated organ. Th is phe-nomenon is called referred pain (Figure 12.2). In general, the visceral organ involved and the area in which the pain is referred are served by the same segment of the spinal cord. For example, sensory neurons from the heart, the skin over the heart, and the skin along the medial aspect of the left arm enter spinal cord segments T 1 to T5 . Thus, the pain of a heart attack typically is felt in the skin over the heart and along the left arm.

CLINICAL CONNECTION A n a l g e s i a

S o m e pa in s e n s a t i o n s o c c u r o u t o f p r o p o r t i o n to m i n o r d a m a g e o r pe r -s i s t c h r o n i c a l l y fo r no o b v i o u s r e a s o n . In s u c h c a s e s , a n a l g e s i a ( an - = w i t h o u t ; -algesia = pa i n ) o r p a i n re l ie f is n e e d e d . A n a l g e s i c d r u g s s u c h a s asp i r i n a n d i b u p r o f e n ( for e x a m p l e , Adv i l®) b l o c k f o r m a t i o n o f s o m e c h e m i c a l s t h a t s t i m u l a t e n o c i c e p t o r s . L o c a l a n e s t h e t i c s , s u c h a s N o v o -caine®, p r o v i d e s h o r t - t e r m pa in re l ie f by b l o c k i n g c o n d u c t i o n o f n e r v e im-pu l ses . M o r p h i n e a n d o t h e r o p i a t e d r u g s a l te r t h e qua l i t y o f pa in p e r c e p -t i on in t he bra in ; pa in is sti l l s e n s e d , bu t it is no l o n g e r p e r c e i v e d a s so u n p l e a s a n t . •

F i g u r e 1 2 . 2 D i s t r i b u t i o n o f r e f e r r e d p a i n . T h e c o l o r e d p a r t s o f t h e d i a g r a m s i n d i c a t e s k i n a r e a s t o w h i c h v i s c e r a l p a i n is r e f e r r e d .

Nociceptors are present in a lmost every t i ssue of the body.

Smal l intest ine Cva ry K idney Append i x Ureter

Liver and ga l lb ladder

Ga l lb ladder

Hear t Lung and d iaph ragm

Hear t

Pancreas S t o m a c h

Ovary C o l o n Ur inary b ladder

Liver and ga l lb ladder

S t o m a c h

Liver and ga l lb ladder

K idney

(a) Anter ior v i ew (b) Poster ior v i ew

W h i c h v iscera l o rgan has t h e broadest a rea for referred pa in?

F O C U S O N W E L L N E S S

Pain M a n a g e m e n t — Sensat ion Modula t ion

P a i n is a useful sensation when it alerts up to an injury that needs attention. We pull our finger away from a hot stove, we take off shoes that are too tight, and we rest an ankle that has been sprained. We do what we can to help the injury heal and meanwhile take over-the-counter or prescription painkillers until the pain goes away.

Pain that persists for longer than two or three months despite appropri-ate treatment is known as chronic pain. The most common forms of chronic pain are low back pain and headache. Cancer, arthritis, fibromyalgia, and many other disorders are associated with chronic pain. People experiencing chronic pain often experience chronic frustration as they are sent from one specialist to another in search of a diag-nosis.

The goal of pain management pro-grams, developed to help people with chronic pain, is to decrease pain as much as possible, and then help pa-tients learn to cope with whatever pain remains. Because no single treatment works for everyone, pain management programs typically offer a wide variety

of treatments, from surgery and nerve blocks to acupuncture and exercise therapy. Following are some of the therapies that complement medical and surgical treatment for the management of chronic pain.

Counseling

Pain used to be regarded as a purely physical response to physical injury. Psychological factors are now under-stood to serve as important mediators in the perception of pain. Feelings such as fear and anxiety strengthen the pain perceptions. Pain may be used to avoid certain situations, or to gain attention. Depression and associated symptoms such as sleep disturbances can con-tribute to chronic pain. Psychological counseling techniques can help people with chronic pain confront issues that may be worsening their pain.

Relaxation and Mediation

Relaxation and mediation techniques may reduce pain by decreasing anxiety and giving people a sense of personal control. Some of these techniques in-clude deep breathing, visualization of positive images, and muscular relax-ation. Others encourage people to be-come more aware of thoughts and situ-

ations that increase or decrease pain or provide a mental distraction from the sensations of pain.

Exercise

People with chronic pain tend to avoid movement because it hurts. Inactivity causes muscles and joint structures to atrophy, which may eventually cause the pain to worsen. Regular exercise and improved fitness help to relieve pain. Why? Exercise stimulates the produc-tion of endorphins, chemicals produced by the body to relieve pain. It also im-proves self-confidence, can serve as a distraction from pain, and improves sleep quality, which is often a problem for people with chronic pain.

In what part of the nervous system do relaxation techniques have their effect?

P r o p r i o c e p t i v e S e n s a t i o n s Proprioceptive sensations (pro-pre-o-SEP-tive; proprio- = one's own) allow us to know where our head and limbs are located and how they are moving even if we are not looking at them, so that we can walk, type, or dress without using our eyes. Kinesthesia (k in ' -es-THE-ze-a ; kin- = motion; -esthesia = perception) is the perception of body movements. Proprioceptive sensations arise in receptors termed proprio-ceptors. Proprioceptors are located in skeletal muscles (mus-cle spindles), in tendons (tendon organs), in and around

synovial joints (joint kinesthetic receptors), and in the inner ear (hair cells). Those proprioceptors embedded in muscles, tendons, and synovial joints inform us of the degree to which muscles are contracted, the amount of tension on tendons, and the positions of joints. Hair cells of the inner ear monitor the orientation of the head relative to the ground and head position during movements. Propriocep-tive sensations also allow us to estimate the weight of ob-jects and determine the muscular effort necessary to per-form a task. For example, as you pick up an object you

3 0 2

Olfaction: Sense of Smell 3 0 3

quickly realize how heavy it is, and you then exert the cor-rect amount of effort needed to lift it.

Nerve impulses for conscious proprioception pass along sensory tracts in the spinal cord and brain stem and are re-layed to the primary somatosensory area (postcentral gyrus) in the parietal lobe of the cerebral cortex (see Figure 10.13). Proprioceptive impulses also pass to the cerebellum, where they contribute to the cerebellum's role in coordinating skilled movements. Because proprioceptors adapt slowly and only slightly, the brain continually receives nerve impulses related to the position of different body parts and makes ad-justments to ensure coordination.

• C H E C K P O I N T

3. W h y is it beneficial to your well-being that nociceptors and proprioceptors exhibit very little adaptation?

4. Which somatic sensory receptors detect touch sensations?

5. What is referred pain, and how is it useful in diagnosing internal disorders?

olfactory receptors, supporting cells, and basal stem cells (Figure 12.3b). Olfactory receptors are the first-order neu-rons of the olfactory pathway. Several cilia called olfactory hairs project from a knob-shaped tip on each olfactory re-ceptor. The olfactory hairs are the parts of the olfactory re-ceptor that respond to inhaled chemicals. Chemicals that have an odor and can therefore stimulate the olfactory hairs are called odorants. The axons of olfactory receptors extend from the olfactory epithelium to the olfactory bulb. Support-ing cells are columnar epithelial cells of the mucous mem-brane lining the nose. They provide physical support, nour-ishment, and electrical insulation for the olfactory receptors, and they help detoxify chemicals that come in contact with the olfactory epithelium. Basal cells are stem cells located be-tween the bases of the supporting cells and continually un-dergo cell division to produce new olfactory receptors, which live for only a month or so before being replaced. This proc-ess is remarkable because olfactory receptors are neurons, and in general, mature neurons are not replaced. Olfactory glands produce mucus that moistens the surface of the olfac-tory epithelium and serves as a solvent for inhaled odorants.

S P E C I A L S E N S E S Receptors for the special senses—smell, taste, sight, hear-ing, and equil ibrium—are housed in complex sensory or-gans such as the eyes and ears. Like the general senses, the special senses allow us to detect changes in our environ-ment. Ophthalmology (of ' - tha l -MOL-o- je ; ophthalmo- = eye; - l o g y = study o f ) is the science that deals with the eye and its disorders. The other special senses are, in large part, the concern of otorhinolaryngology ( o ' - t o - r i ' - n o -lar ' - in-GOL-o-je; oto- = ear; rhino- = nose; laryngo- = lar-ynx), the science that deals with the ears, nose, and throat and their disorders.

O L F A C T I O N : S E N S E O F S M E L L O B J E C T I V E • Describe the receptors for olfaction and the olfactory pathway to the brain.

The nose contains 10-100 million receptors for the sense of smell, or olfaction (ol-FAK-shun; olfact- = smell). Because some nerve impulses for smell and taste propagate to the lim-bic system, certain odors and tastes can evoke strong emo-tional responses or a flood of memories.

S t r u c t u r e of t h e O l f a c t o r y E p i t h e l i u m The olfactory epithelium occupies the upper portion of the nasal cavity (Figure 12.3a) and consists of three types of cells:

S t i m u l a t i o n of O l f a c t o r y R e c e p t o r s Many attempts have been made to distinguish among and classify "primary" sensations of smell. Genetic evidence now suggests the existence of hundreds of primary odors. Our ability to recognize about 10,000 different odors probably depends on patterns of activity in the brain that arise from activation of many different combinations of olfactory recep-tors. Olfactory receptors react to odorant molecules by pro-ducing an electrical signal that triggers one or more nerve impulses. Adaptation (decreasing sensitivity) to odors occurs rapidly. Olfactory receptors adapt by about 50% in the first second or so after stimulation and very slowly thereafter.

T h e O l f a c t o r y P a t h w a y On each side of the nose, about 40 bundles of the slender, unmyelinated axons of olfactory receptors extend through about 20 holes in the cribriform plate of the ethmoid bone (Figure 12.3b). These bundles of axons collectively form the right and left olfactory (I) nerves. The olfactory nerves ter-minate in the brain in paired masses of gray matter called the olfactory bulbs, which are located below the frontal lobes of the cerebrum. Within the olfactory bulbs, the axon terminals of olfactory receptors—the first-order neurons—form synapses with the dendrites and cell bodies of second-order neurons in the olfactory pathway.

The axons of the neurons extending from the olfactory bulb form the olfactory tract. Some of the axons of the ol-factory tract project to the primary olfactory area in the temporal lobe of the cerebral cortex (see Figure 10.13), where conscious awareness of smell begins. Other axons of

3 0 4 Chapter 12 Somatic Senses and Special Senses

F i g u r e 1 2 . 3 O l f a c t o r y e p i t h e l i u m a n d o l f a c t o r y r e c e p t o r s . ( a ) L o c a t i o n o f o l f a c t o r y e p i t h e l i u m in t h e n a s a l cav i t y . ( b ) A n a t o m y o f o l f a c t o r y r e c e p t o r s , w h o s e a x o n s e x t e n d t h r o u g h t h e c r i b r i f o r m p l a t e t o t h e o l f a c t o r y bu lb .

T h e o l factory ep i the l ium cons is ts of o l factory receptors, suppor t ing cel ls, and basal cel ls.

Olfactory bulb

Ol factory bulb neuron

(a) Sagit tal v i ew

Serous — secre t ion

Parts of o l factory (I) nerve

Cr ib r i fo rm plate

Bundle of axons of o l factory receptors Connec t i ve t issue Ol factory g land (produces m u c u s )

Basal cell

Ol factory receptor

Suppor t ing cel l

Dendr i te

Ol factory hair

Odoran t mo lecu le

V W h a t is t h e funct ion of basal s t e m ce l ls?

(b) En la rged aspect of o l factory receptors

the olfactory tract project to the l imbic system and hypo-thalamus; these connect ions account for emotional and memory-evoked responses to odors. Examples include sex-ual excitement upon smel l ing a certain perfume or nausea upon smel l ing a food that once made you violently i l l .

• C H E C K P O I N T

6. W h a t functions are carried out by the three types of cells in the olfactory epithelium?

7. Define the following terms: olfactory nerve, olfactory bulb, and olfactory tract.

• CLINICAL CONNECTION H y p o s m i a

H y p o s m i a ( h i - P O Z - m e - a ; hypo- = b e l o w ; -osmia = s m e l l , o d o r ) , a r e -d u c e d ab i l i t y to s m e l l , a f f e c t s h a l f o f t h o s e o v e r a g e 6 5 a n d 7 5 % o f t h o s e o v e r a g e 80 . W i t h a g i n g t h e s e n s e o f s m e l l d e t e r i o r a t e s . H y p o s -m i a a l s o c a n be c a u s e d by n e u r o l o g i c a l c h a n g e s , s u c h a s a h e a d in -ju ry , A l z h e i m e r d i s e a s e , o r P a r k i n s o n d i s e a s e ; c e r t a i n d r u g s , s u c h a s a n t i h i s t a m i n e s , a n a l g e s i c s , o r s t e r o i d s ; a n d t h e d a m a g i n g e f f e c t s o f s m o k i n g . •

G U S T A T I O N : S E N S E O F T A S T E O B J E C T I V E • Describe the receptors for gustation and the gustatory pathway to the brain.

Taste or gustation (gus-TA-shun; gust- = taste) is much simpler than olfaction because only five primary tastes can be distinguished: sour, sweet, bitter, salty, and umami (u -MAM-e) .

Gustation: Sense of Taste 3 0 5

The umami taste is described as "meaty" or "savory." All other flavors, such as chocolate, pepper, and coffee, are combinations of the five primary tastes, plus the accompa-nying olfactory and tactile (touch) sensations. Odors from food can pass upward from the mouth into the nasal cavity, where they stimulate olfactory receptors. Because olfaction is much more sensitive than taste, a given concentration of a food substance may stimulate the olfactory system thou-sands of times more strongly than it stimulates the gusta-tory system. When you have a cold or are suffering from al-lergies and cannot taste your food, it is actually olfaction that is blocked, not taste.

S t r u c t u r e of T a s t e B u d s The receptors for taste sensations are located in the taste buds (Figure 12.4). Most of the nearly 10,000 taste buds of a young adult are on the tongue, but some are also found on the roof of the mouth, pharynx (throat), and epiglottis (carti-

lage lid over the voice box). The number of taste buds de-clines with age. Taste buds are found in elevations on the tongue called papillae (pa-PIL-e; singular is papilla), which provide a rough texture to the upper surface of the tongue (Figure 12.4a,b). Vallate papillae (VAL-at = wall- l ike) form an inverted V-shaped row at the back of the tongue. Fungi-form papillae (FUN-ji-form = mushroomlike) are mushroom-shaped elevations scattered over the entire surface of the tongue. In addition, the entire surface of the tongue has fili-form papillae (FIL-i-form = threadlike), which contain touch receptors but no taste buds.

Each taste bud is an oval body consisting of three types of epithelial cells: supporting cells, gustatory receptor cells, and basal cells (Figure 12.4c). The supporting cells surround about 50 gustatory receptor cells. A single, long gustatory hair projects from each gustatory receptor cell to the exter-nal surface through the taste pore, an opening in the taste bud. Basal cells are stem cells that produce supporting cells, which then develop into gustatory receptor cells that have a

F i g u r e 1 2 . 4 T h e r e l a t i o n s h i p o f g u s t a t o r y r e c e p t o r s i n t a s t e b u d s t o t o n g u e p a p i l l a e .

Gus ta to ry ( taste) receptor cel ls a re located in tas te buds.

Epiglot t is

Palat ine tonsi l

L ingual tonsi l

Val la te papil la

Fung i fo rm papi l la

Fi l i form papi l la

(a) Dorsum of t ongue showing locat ion of papi l lae

St rat i f ied s q u a m o u s ep i the l ium

Suppor t i ng cel l

Connec t i ve t issue

(c) Structure o f a taste bud

In order, f r o m the t o n g u e to t h e brain, w h a t s t ructures f o r m t h e gus ta tory pa thway?

Taste bud

Vallate papil la

Fi l i form papi l la

Fung i form papi l la

Gus ta to ry hair

Gus ta to ry receptor cel l

Basa l cel l

Senso ry neurons

3 0 6 Chapter 12 Somatic Senses and Special Senses

n o n is t h e b a s i s for t a s t e a v e r s i o n , in w h i c h p e o p l e a n d a n i m a l s qu ick l y l e a r n to a v o i d a f o o d if it u p s e t s t he d i ges t i ve s y s t e m . B e c a u s e the d r u g s a n d rad ia t i on t r e a t m e n t s u s e d to c o m b a t c a n c e r o f t e n c a u s e n a u s e a a n d g a s t r o i n t e s t i n a l u p s e t r e g a r d l e s s o f w h a t f o o d s a re c o n s u m e d , c a n c e r p a t i e n t s m a y l ose the i r a p p e t i t e b e c a u s e t h e y d e v e l o p t as te a v e r s i o n s for m o s t f oods . •

life span of about 10 days. The gustatory receptor cells are sep-arate receptor cells. T h e y do not have an axon (like olfactory receptors) but rather synapse with dendrites of the first-order sensory neurons of the gustatory pathway.

S t i m u l a t i o n o f G u s t a t o r y R e c e p t o r s Chemica l s that st imulate gustatory receptor cells are known as tastants. Once a tastant is dissolved in saliva, i t can enter taste pores and make contact with the plasma membrane of the gustatory hairs. T h e result is an electrical signal that st imulates release of neurotransmit ter molecules f rom the gustatory receptor cell. Nerve impulses are tr ig-gered when these neurotransmit ter molecules bind to their receptors on the dendrites of the f irst-order sensory neu-ron. T h e dendrites branch profusely and contact many gus-tatory receptors in several taste buds. Individual gustatory receptor cells may respond to more than one of the five pr imary tastes. Comple te adaptation (loss of sensit ivity) to a specific taste can occur in 1 to 5 minutes of continuous st imulation.

If all tastants cause release of neurotransmit ter from many gustatory receptor cells, why do foods taste different? T h e answer to this quest ion is thought to lie in the patterns of nerve impulses in groups of f i rst-order taste neurons that synapse with the gustatory receptor cells. Different tastes arise f rom activation of different groups of taste neu-rons. In addition, a l though each individual gustatory recep-tor cell responds to more than one of the five pr imary tastes, i t may respond more strongly to some tastants than to others.

T h e G u s t a t o r y P a t h w a y Three cranial nerves contain axons of first-order gustatory neurons that innervate the taste buds. T h e facial (VII) nerve and glossopharyngeal (IX) nerve serve the tongue; the vagus (X) nerve serves the throat and epiglottis. From taste buds, impulses propagate along these cranial nerves to the medulla oblongata. From the medulla, some axons carrying taste sig-nals project to the limbic system and the hypothalamus, and others project to the thalamus. Taste signals that project from the thalamus to the primary gustatory area in the parietal lobe of the cerebral cortex (see Figure 10.13) give rise to the con-scious perception of taste.

• C H E C K P O I N T

8. How do olfactory receptors and gustatory receptor cells differ in structure and function?

9. Compare the olfactory and gustatory pathways.

V I S I O N O B J E C T I V E S • Describe the accessory structures of the eye, the layers of the eyeball, the lens, the interior of the eyeball, image formation, and binocular vision.

• Describe the receptors for vision and the visual path-way to the brain.

More than half the sensory receptors in the human body are located in the eyes, and a large part of the cerebral cortex is devoted to processing visual information. In this section of the chapter, we examine the accessory structures of the eye, the eyeball itself, the formation of visual images, the physiol-ogy of vision, and the visual pathway from the eye to the brain.

A c c e s s o r y S t r u c t u r e s o f t h e E y e The accessory structures of the eye are the eyebrows, eye-lashes, eyelids, extrinsic muscles that move the eyeballs, and lacrimal (tear-producing) apparatus. The eyebrows and eye-lashes help protect the eyeballs from foreign objects, perspi-ration, and direct rays of the sun (Figure 12.5). The upper and lower eyelids shade the eyes during sleep, protect the eyes from excessive l ight and foreign objects, and spread lu-bricating secretions over the eyeballs (by blinking). Six ex-trinsic eye muscles cooperate to move each eyeball right, left, up, down, and diagonally: the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique. Neurons in the brain stem and cerebellum coordinate and synchronize the movements of the eyes.

The lacrimal apparatus (lacrima = tear) is a group of glands, ducts, canals, and sacs that produce and drain lacrimal fluid or tears (Figure 12.5). T h e right and left lacrimal glands are each about the size and shape of an al-mond. They secrete tears through the lacrimal ducts onto the surface of the upper eyelid. Tears then pass over the sur-

• CLINICAL CONNECTION Taste Avers ion

P r o b a b l y b e c a u s e o f t as te p r o j e c t i o n s to t he h y p o t h a l a m u s a n d l imb ic s y s t e m , t h e r e is a s t r o n g l ink b e t w e e n t a s t e a n d p l e a s a n t o r u n p l e a s a n t e m o t i o n s . S w e e t f o o d s e v o k e r e a c t i o n s o f p l e a s u r e w h i l e b i t te r o n e s c a u s e e x p r e s s i o n s o f d i sgus t , e v e n in n e w b o r n bab ies . T h i s p h e n o m e -

Vision 3 0 7 1 1 1

F i g u r e 1 2 . 5 A c c e s s o r y s t ruc tures of t h e eye . A c c e s s o r y s t ructures of t h e e y e a re the eyebrows, eye lashes, eyel ids, extr insic eye muscles , and the lacr imal appara tus .

Lacr imal g land

Lacr imal duc t

V

Super ior lacr imal cana l

Lacr imal sac Inferior lacr imal canal

Naso lacr ima l duc t

Nasal cavi ty

F L O W OF T E A R S

Lacr imal g land

\ Lacr imal ducts

\ Super ior or inferior

lacr imal canal

I Lacr imal sac

I Naso lacr ima l duc t

I Nasal cavi ty

W h a t a re t h e funct ions of t ea rs?

face of the eyeball toward the nose into two lacrimal canals and a nasolacrimal duct, which allow the tears to drain into the nasal cavity.

Tears are a watery solution containing salts, some mucus, and a bacteria-killing enzyme called lysozyme. Tears clean, lubricate, and moisten the portion of the eyeball exposed to the air to prevent it from drying. Normally, tears are cleared away by evaporation or by passing into the nasal cavity as fast as they are produced. If, however, an irritating substance makes contact with the eye, the lacrimal glands are stimu-lated to oversecrete and tears accumulate. This protective mechanism dilutes and washes away the irritant. Only hu-mans express emotions, both happiness and sadness, by cry-ing. In response to parasympathetic stimulation, the lacrimal glands produce excessive tears that may spill over the edges of the eyelids and even fill the nasal cavity with fluid. This is how crying produces a runny nose.

L a y e r s of t h e Eyeba l l The adult eyeball measures about 2.5 cm (1 inch) in diameter and is divided into three layers: fibrous tunic, vascular tunic, and retina (Figure 12.6).

Fibrous Tunic

The fibrous tunic is the outer coat of the eyeball. It consists of an anterior cornea and a posterior sclera. The cornea (KOR-ne-a) is a transparent fibrous coat that covers the col-ored iris. Because it is curved, the cornea helps focus light rays onto the retina. The sclera (SKLER-a = hard), the

"white" of the eye, is a coat of dense connective tissue that covers all of the entire eyeball except the cornea. The sclera gives shape to the eyeball, makes it more rigid, and protects its inner parts. An epithelial layer called the conjunctiva (kon-junk-TI-va) covers the sclera but not the cornea and lines the inner surface of the eyelids.

Vascular Tunic

T h e vascular tunic is the middle layer of the eyeball and is composed of the choroid, ci l iary body, and iris. The choroid (KO-royd) is a thin membrane that lines most of the internal surface of the sclera. It contains many blood vessels that help nourish the retina. The choroid also con-tains melanocytes that produce the pigment melanin, which causes this layer to appear dark brown in color. Melanin in the choroid absorbs stray l ight rays, which pre-vents reflection and scattering of l ight within the eyeball. As a result, the image cast on the retina by the cornea and lens remains sharp and clear.

At the front of the eye, the choroid becomes the ciliary body (SIL-e-ar ' -e) . The ciliary body consists of the ciliary processes, folds on the inner surface of the ciliary body whose capillaries secrete a fluid called aqueous humor, and the cil-iary muscle, a smooth muscle that alters the shape of the lens for viewing objects up close or at a distance. The lens, a transparent structure that focuses light rays onto the retina, is constructed of many layers of elastic protein fibers. Zonular fibers attach the lens to the ciliary muscle and hold the lens in position.

3 0 8 Chapter 12 Somatic Senses and Special Senses

F i g u r e F i g u r e 1 2 . 6 S t ruc tu re of t h e eyeba l l .

T h e wal l of t h e eyebal l cons is ts of th ree layers: the fibrous tunic , t h e vascular tunic , and the retina.

Transve rse p lane

V

Anter ior cavi ty (conta ins a q u e o u s humor)

Sclera l venous s inus

X

Light

I Cornea

Pup i l

Lacr imal sac

Cil iary body:

Cil iary musc le

Cil iary p rocess

Zonu la r f ibers

Media l rectus musc le

V i t reous chamber (conta ins v i t reous

\\\J

M E D I A L

body)

B lood vesse ls

„ . ,,,. Opt ic d isc Fovea central is ° p t e ( I I ) n e r v e (bl ind spot)

Super ior v iew of t ransverse sect ion of r ight eyebal l

W h a t are the c o m p o n e n t s of the fibrous tun ic and vascular tun ic?

Conjunc t iva

Ret ina

Choro id M \ v \ , X \ T

/ -

Sclera

Lateral rectus musc le

L A T E R A L

if: VJ; •

• CLINICAL CONNECTION O p h t h a l m o s c o p e

U s i n g a n o p h t h a l m o s c o p e ( o f - T H A L - m o - s k o p ; ophthalmos- = e ye ; -skopeo = to e x a m i n e ) , a n o b s e r v e r c a n p e e r t h r o u g h t h e p u p i l a n d s e e a m a g n i f i e d i m a g e o f t h e r e t i n a a n d t h e b l o o d v e s s e l s t h a t c r o s s it. T h e s u r f a c e o f t h e re t i na is t h e o n l y p l a c e in t h e b o d y w h e r e b l o o d v e s s e l s c a n b e v i e w e d d i r e c t l y a n d e x a m i n e d fo r p a t h o l o g i c a l c h a n g e s , s u c h a s t h o s e t h a t o c c u r w i t h h y p e r t e n s i o n o r d i a b e t e s m e l l i t u s . •

T h e iris (= colored circle) is the colored part of the eye-ball. It includes both circular and radial smooth muscle fibers. The hole in the center of the iris, through which l ight enters the eyeball, is the pupil. The smooth muscle of the iris regulates the amount of l ight passing through the lens. W h e n the eye is stimulated by bright light, the parasympathetic di-vision of the autonomic nervous system (ANS) causes con-traction of the circular muscles of the iris, which decreases the size of the pupil (constriction). W h e n the eye must adjust to dim light, the sympathetic division of the ANS causes the radial muscles to contract, which increases the size of the pupil (dilation) (Figure 12.7).

Retina

The third and inner coat of the eyeball, the retina, lines the posterior three-quarters of the eyeball and is the beginning of

Vision 3 0 9 111

F i g u r e 1 2 . 7 R e s p o n s e s of t h e p u p i l t o l ight of v a r y i n g b r igh t -n e s s .

Contract ion of the circular musc les c a u s e s constr ic t ion of the pupi l ; cont ract ion of the radial musc les c a u s e s di lat ion of t h e pupil .

Pupi l constr ic ts as c i rcular musc les of iris cont rac t (parasympathet ic )

Pupi l Pupi l d i lates as radial musc les of iris cont rac t (sympathet ic)

Bright l ight Normal l ight

Anter ior v iews

D i m light

V W h i c h d iv is ion of t h e a u t o n o m i c nervous s y s t e m c a u s e s pupi l -lary const r ic t ion? W h i c h c a u s e s pupi l lary d i la t ion?

the visual pathway (Figure 12.8). It has two layers: the neural layer and the pigmented layer. The neural layer is a multi layered outgrowth of the brain. Three distinct layers of retinal neurons—the photoreceptor layer, the bipolar cell layer, and the ganglion cell layer—are separated by two zones, the outer and inner synaptic layers, where synaptic contacts are made. Light passes through the ganglion and bipolar cell layers and both synaptic layers before it reaches the photoreceptor layer.

The pigmented layer of the retina is a sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina. The melanin in the pigmented layer of the retina, like in the choroid, also helps to absorb stray light rays.

Photoreceptors are specialized cells that begin the process by which light rays are ultimately converted to nerve impulses. There are two types of photoreceptors: rods and cones. Rods allow us to see shades of gray in dim light, such as moonlight. Brighter lights stimulate the cones, giving rise to highly acute, color vision. Three types of cones are present in the retina: (1) blue cones, which are sensitive to blue light, (2) green cones, which are sensitive to green light; and (3) red cones, which are sensitive to red light. Color vision results

F i g u r e 1 2 . 8 M i c r o s c o p i c s t r u c t u r e o f t h e r e t i n a . T h e d o w n w a r d b l u e a r r o w a t le f t i n d i c a t e s t h e d i r e c t i o n o f t h e s i g n a l s p a s s i n g t h r o u g h t h e n e u r a l l a y e r o f t h e r e t i n a . E v e n t u a l l y , n e r v e i m p u l s e s a r i s e in g a n g l i o n c e l l s a n d p r o p a g a t e a l o n g t h e i r a x o n s , w h i c h m a k e u p t h e o p t i c ( I I ) n e r v e .

In the ret ina, v isual s igna ls pass f rom photoreceptors to bipolar cel ls to gangl ion cells.

A

Neural layer

Photoreceptors

Outer synapt ic layer

B ipo lar cel l layer

Inner synapt ic layer Gang l i on cell layer

P igmented layer

Rod

Cone

Path of D i rec t ion of l ight nerve impu lses t h rough th rough ret ina ret ina

Hor izonta l cel l B ipo lar cell Amac r i ne cell

Gang l i on cel l

Opt ic (II) nerve axons Ret inal b lood vesse l

M ic roscop ic s t ructure of the ret ina

Nerve impu lses p ropaga te a long opt ic nerve axons toward opt ic d isc

W h a t are t h e two t y p e s of photoreceptors , and h o w d o their funct ions di f fer?

3 1 0 Chapter 12 Somatic Senses and Special Senses

from the stimulation of various combinations of these three types of cones. Just as an artist can obtain almost any color by mixing them on a palette, the cones can code for different colors by differential stimulation. There are about 6 million cones and 120 million rods. Cones are most densely concen-trated in the fovea centralis, a small depression in the center of the macula lutea (MAK-u-la LOO-te-a) , or yellow spot, in the exact center of the retina. The fovea centralis is the area of highest visual acuity or resolution (sharpness of vision) because of its high concentration of cones. The main reason that you move your head and eyes while looking at something, such as the words of this sentence, is to place images of inter-est on your fovea. Rods are absent from the fovea centralis and macula lutea and increase in numbers toward the periph-ery of the retina.

From photoreceptors, information flows through the outer synaptic layer to the bipolar cells of the bipolar cell layer, and then from bipolar cells through the inner synaptic layer to the ganglion cells of the ganglion cell layer. Between 6 and 600 rods synapse with a single bipolar cell in the outer synaptic layer; a cone usually synapses with just one bipolar cell. The convergence of many rods onto a single bipolar cell increases the light sensitivity of rod vision but slightly blurs the image that is perceived. Cone vision, although less sensi-tive, has higher acuity because of the one-to-one synapses between cones and their bipolar cells. The axons of the gan-glion cells extend posteriorly to a small area of the retina called the optic disc (blind spot), where they all exit as the optic (II) nerve (see Figure 12.6). Because the optic disc con-tains no rods or cones, we cannot see an image that strikes the blind spot. Normally, you are not aware of having a blind spot, but you can easily demonstrate its presence. Cover your left eye and gaze directly at the cross below. Then increase or decrease the distance between the book and your eye. At some point, the square will disappear as its image falls on the blind spot.

+ • In ter ior of t h e Eyeba l l The lens divides the interior of the eyeball into two cavities, the anterior cavity and the vitreous chamber. The anterior cavity lies anterior to the lens and is filled with aqueous hu-mor (AK-we-us HU-mer; aqua = water), a watery fluid simi-lar to cerebrospinal fluid. Blood capillaries of the ciliary processes of the ciliary body secrete aqueous humor into the anterior cavity. It then drains into the scleral venous sinus (canal of Schlemm), an opening where the sclera and cornea meet (see Figure 12.6), and reenters the blood. The aqueous humor helps maintain the shape of the eye and nourishes the lens and cornea, neither of which has blood vessels. Nor-mally, aqueous humor is completely replaced about every 90 minutes.

Behind the lens is the second, and larger, cavity of the eyeball, the vitreous chamber. It contains a clear, jellylike substance called the vitreous body, which forms during em-bryonic life and is not replaced thereafter. This substance helps prevent the eyeball from collapsing and holds the retina flush against the choroid.

The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humor with a smaller con-tribution from the vitreous body. Intraocular pressure main-tains the shape of the eyeball and keeps the retina smoothly pressed against the choroid so the retina is well nourished and forms clear images. Normal intraocular pressure (about 16 mm Hg) is maintained by a balance between production and drainage of the aqueous humor.

Table 12.2 summarizes the structures of the eyeball.

I m a g e F o r m a t i o n a n d B i n o c u l a r V i s i o n In some ways the eye is like a camera: Its optical elements fo-cus an image of some object on a light-sensitive "film"—the retina—while ensuring the correct amount of light makes the proper "exposure." To understand how the eye forms clear images of objects on the retina, we must examine three processes: (1) the refraction or bending of light by the lens and cornea, (2) the change in shape of the lens, and (3) con-striction or narrowing of the pupil.

Refraction of Light Rays

When light rays traveling through a transparent substance (such as air) pass into a second transparent substance with a different density (such as water), they bend at the junction between the two substances. This bending is called refraction (Figure 12.9a). About 75% of the total refraction of l ight occurs at the cornea. Then, the lens of the eye further re-fracts the l ight rays so that they come into exact focus on the retina.

Images focused on the retina are inverted (upside down) (Figure 12.9b, c). They also undergo right-to-left reversal; that is, light from the right side of an object strikes the left side of the retina, and vice versa. The reason the world does not look inverted and reversed is that the brain "learns" early in life to coordinate visual images with the orientations of objects. The brain stores the inverted and reversed images we acquire when we first reach for and touch objects and inter-prets those visual images as being correctly oriented in space.

When an object is more than 6 meters (20 ft) away from the viewer, the light rays reflected from the object are nearly parallel to one another, and the curvatures of the cornea and lens exactly focus the image on the retina (Figure 12.9b). However, light rays from objects closer than 6 meters are di-vergent rather than parallel (Figure 12.9c). The rays must be refracted more if they are to be focused on the retina. This additional refraction is accomplished by changes in the shape of the lens.

Vision 3 1 1 111

S t r u c t u r e

T a b l e 1 2 . 2 S u m m a r y o f t h e S t r u c t u r e s o f t h e E y e b a l l a n d T h e i r F u n c t i o n s

F u n c t i o n

F i b r o u s t u n i c Cornea

Sc lera V a s c u l a r t u n i c

Iris

Choro id

R e t i n a

Cil iary body

Ret ina

L e n s

Lens

A n t e r i o r c a v i t y Anter ior cav i ty

V i t r e o u s c h a m b e r

Vi t reous c h a m b e r

Cornea: Adm i t s and refracts (bends) light. Sclera: Prov ides shape and protects inner parts.

Iris: Regu la tes the amoun t of l ight that en ters eyebal l . Ciliary body: Secre tes a q u e o u s humor and a l ters the shape o f the lens for near or far v is ion (accommodat ion ) . Choroid: Prov ides b lood supp ly and abso rbs scat tered light.

Rece ives light and conver ts it into nerve impulses. Prov ides output to brain via axons of gang l ion cells, wh i ch form the opt ic (II) nerve.

Ref rac ts l ight.

Con ta ins a q u e o u s humor that he lps mainta in the shape of the eyebal l and supp l ies oxygen a n d nut r ients to the lens and cornea.

Con ta ins the v i t reous body, wh i ch he lps mainta in the shape o f eyebal l and keeps the ret ina a t tached to the choro id.

3 1 2 Chapter 12 Somatic Senses and Special Senses

F i g u r e 1 2 . 9 R e f r a c t i o n o f l i g h t r a y s a n d a c c o m m o d a t i o n .

Ref rac t ion is the bend ing of l ight rays.

Light ray before ref ract ion

(a) Refract ion of l ight rays

Near ly paral lel rays f rom dis tant object

(b) V iew ing d is tant ob jec t

D ivergent rays f rom c lose ob jec t

(c) A c c o m m o d a t i o n

W h a t c h a n g e s occur dur ing a c c o m m o d a t i o n for near v is ion?

Accommodation

A surface that curves outward, like the surface of a ball, is said to be convex. T h e convex surface of a lens refracts in-coming l ight rays toward each other, so that they eventu-al ly intersect. T h e lens of the eye is convex on both its an-ter ior and posterior surfaces, and its abil ity to refract l ight increases as its curvature becomes greater. W h e n the eye is focusing on a close object, the lens becomes more convex and refracts the l ight rays more. T h i s increase in the curva-ture of the lens for near vision is cal led accommodation (Figure 12.9c).

W h e n you are v iewing distant objects, the ci l iary mus-cle of the ci l iary body is relaxed and the lens is fair ly f lat because it is stretched in all directions by taut zonular fibers. W h e n you view a close object, the ci l iary muscle contracts, which pulls the ci l iary process and choroid for-ward toward the lens. T h i s action releases tension on the lens, a l lowing it to become rounder (more convex), which increases its focusing power and causes greater conver-gence of the l ight rays.

The normal eye, known as an emmetropic eye (em'-e-TROP- ik ) , can sufficiently refract l ight rays from an object 6 m (20 ft) away so that a clear image is focused on the retina (Figure 12.10a). Many people, however, lack this ability be-cause of refraction abnormalities. Among these abnormalities are myopia (ml-O-pe-a) , or nearsightedness, which occurs when the eyeball is too long relative to the focusing power of the cornea and lens. Myopic individuals can see nearby ob-jects clearly, but not distant objects. In hyperopia (hl-per-O-pe-a) or farsightedness, also known as hypermetropia (h l ' -per-me-TRO-pe-a) , the eyeball length is short relative to the focusing power of the cornea and lens. Hyperopic individuals can see distant objects clearly, but not nearby objects. Figure 12.10b-e illustrates these conditions and shows how they are corrected. Another refract ion abnormal i ty is astigmatism (a-STIG-ma-t izm), in which either the cornea or the lens has an irregular curvature.

CLINICAL CONNECTION P r e s b y o p i a

W i t h a g i n g , t he l e n s l o s e s s o m e of i ts e las t i c i t y so i ts ab i l i ty to a c c o m m o -d a t e d e c r e a s e s . A t a b o u t a g e 40 , p e o p l e w h o h a v e no t p r e v i o u s l y w o r n g l a s s e s b e g i n to requ i re t h e m fo r c l o s e _vis ion, s u c h a s r ead ing . T h i s c o n d i t i o n is ca l l ed p r e s b y o p i a ( p r e z ' - b e - O - p e - a ; presby- = o ld ; -opia = p e r t a i n i n g to t he eye o r v i s ion ) . •

Constriction of the Pupil

Constriction of the pupil is a narrowing of the diameter of the hole through which l ight enters the eye due to contrac-tion of the circular muscles of the iris. Th is autonomic reflex occurs simultaneously with accommodation and prevents l ight rays from entering the eye through the periphery of the lens. L ight rays entering at the periphery of the lens would not be brought to focus on the retina and would result in blurred vision. The pupil, as noted earlier, also constricts in bright l ight to l imit the amount of l ight that strikes the retina.

Convergence

In humans, both eyes focus on only one set of objects, a char-acteristic called binocular vision. Th i s feature of our visual system allows the perception of depth and an appreciation of the three-dimensional nature of objects. W h e n you stare straight ahead at a distant object, the incoming l ight rays are aimed directly at the pupils of both eyes and are refracted to

Vision 3 1 3 111

F i g u r e 1 2 . 1 0 Normal a n d a b n o r m a l refract ion in t h e eyebal l . ( a ) In t h e n o r m a l ( e m m e t r o p i c ) eye , l i gh t r a y s f r o m a n o b j e c t a r e b e n t s u f f i c i e n t l y b y t h e c o r n e a a n d l e n s t o f o c u s o n t h e f o v e a c e n t r a l i s . ( b ) In t h e n e a r s i g h t e d ( m y o p i c ) eye , t h e i m a g e is f o c u s e d in f r o n t o f t h e r e t i n a . ( c ) C o r r e c t i o n is b y u s e o f a c o n c a v e l e n s t h a t d i v e r g e s e n t e r i n g l i gh t r a y s s o t h a t t h e y h a v e t o t r a v e l f u r t h e r t h r o u g h t h e e y e b a l l . ( d ) In t h e f a r s i g h t e d ( h y p e r o p i c ) e y e , t h e i m a g e is f o c u s e d b e h i n d t h e re t i na . ( e ) C o r r e c t i o n is b y a c o n v e x l e n s t h a t c a u s e s e n t e r i n g l i gh t r a y s t o c o n v e r g e .

In uncor rec ted myopia , d istant ob jects can' t be seen clearly; in uncor rec ted hyperopia , nearby ob jects can ' t b e s e e n clearly.

Lens

Cornea

(a) Normal (emmetrop ic ) eye

Normal p lane of focus Concave lens

(b) Nears igh ted (myopic) eye, (c) Nears igh ted (myopic) eye, uncor rec ted cor rec ted

C o n v e x lens

(d) Fars ighted (hyperopic) (e) Fars ighted (hyperopic) eye, uncor rec ted eye, cor rec ted

~ W h a t is presbyopia?

comparable spots on the two retinas. As you move closer to the object, your eyes must rotate toward the nose if the l ight rays from the object are to strike comparable points on both retinas. Convergence is the name for this automatic move-ment of the two eyeballs toward the midline, which is caused by the coordinated action of the extrinsic eye muscles. The nearer the object, the greater the convergence needed to maintain binocular vision.

S t i m u l a t i o n of P h o t o r e c e p t o r s After an image is formed on the retina by refraction, accom-modation, constriction of the pupil, and convergence, l ight rays must be converted into neural signals. T h e initial step in

this process is the absorption of l ight rays by the rods and cones of the retina. To understand how absorption occurs, it is necessary to understand the role of photopigments.

A photopigment (visual pigment) is a substance that can absorb l ight and undergo a change in structure. The pho-topigment in rods is called rhodopsin (rhodo- = rose; -opsin = related to vision) and is composed of a protein called opsin and a derivative of vitamin A called retinal. Any amount of l ight in a darkened room causes some rhodopsin molecules to split into opsin and retinal and initiate a series of chemical changes in the rods. W h e n the l ight level is dim, opsin and retinal recombine into rhodopsin as fast as rhodopsin is split apart. Rods usually are nonfunctional in daylight, however, because rhodopsin is split apart faster than it can be re-formed. After going from bright sunlight into a dark room, it takes about 40 minutes before the rods function maximally.

Cones function in bright l ight and provide color vision. As in rods, absorption of l ight rays causes breakdown of pho-topigment molecules. The photopigments in cones also con-tain retinal, but there are three different opsin proteins—one in each of the three types of cones. T h e cone photopigments reform much more quickly than the rod photopigment.

CLINICAL CONNECTION Color B l indness and Night B l indness

T h e c o m p l e t e l o s s o f c o n e v i s i o n c a u s e s a p e r s o n to b e c o m e l e g a l l y b l i nd . I n c o n t r a s t , a p e r s o n w h o l o s e s r od v i s i o n m a i n l y h a s d i f f i cu l t y s e e i n g in d i m l i gh t a n d t h u s s h o u l d n o t d r i v e a t n igh t . P r o l o n g e d v i t a -m i n A d e f i c i e n c y a n d t h e r e s u l t i n g b e l o w - n o r m a l a m o u n t o f r h o d o p s i n m a y c a u s e n i g h t b l i n d n e s s , a n i nab i l i t y t o s e e w e l l a t l o w l i gh t leve ls . A n i n d i v i d u a l w i t h a n a b s e n c e o r d e f i c i e n c y o f o n e o f t h e t h r e e t y p e s o f c o n e s f r o m t h e re t i na c a n n o t d i s t i n g u i s h s o m e c o l o r s f r o m o t h e r s a n d is s a i d to be c o l o r b l i n d . In t h e m o s t c o m m o n t y p e , red-green color blindness, e i t h e r r e d c o n e s o r g r e e n c o n e s a r e m i s s i n g . T h u s , t h e p e r s o n c a n n o t d i s t i n g u i s h b e t w e e n r e d a n d g r e e n . T h e i n h e r i t a n c e o f c o l o r b l i n d n e s s is i l l u s t r a t e d in F i g u r e 2 4 . 1 2 . •

T h e V i s u a l P a t h w a y After stimulation by light, the rods and cones tr igger electri-cal signals in bipolar cells. Bipolar cells transmit both excita-tory and inhibitory signals to ganglion cells. T h e ganglion cells become depolarized and generate nerve impulses. T h e axons of the ganglion cells exit the eyeball as the optic (II) nerve (Figure 12.11) and extend posteriorly to the optic chiasm (KI-azm = a crossover, as in the letter X). In the optic chi-asm, about half of the axons from each eye cross to the oppo-site side of the brain. After passing the optic chiasm, the axons, now part of the optic tract, terminate in the thalamus. Here they synapse with neurons whose axons project to the primary visual areas in the occipital lobes of the cerebral cor-tex (see Figure 10.13). Because of crossing at the optic chi-asm, the right side of the brain receives signals from both eyes for interpretation of visual sensations from the left side of an object, and the left side of the brain receives signals

3 1 4 Chapter 12 Somatic Senses and Special Senses

F i g u r e 1 2 . 1 1 V i s u a l p a t h w a y .

At the opt ic ch iasm, half of the ret inal gang l ion cell a x o n s f rom each eye cross to t h e oppos i te s ide of t h e brain.

in occipital lobes of cerebral cor tex

~ W h a t is the correct order of s t ructures that car ry ne rve impu lses ' j f r o m t h e ret ina to t h e occipi ta l lobe?

from both eyes for interpretation of visual sensations from the right side of an object.

• C H E C K P O I N T

10. How does the shape of the lens change during accomo-dation?

11. How do photopigments respond to light?

12. By what pathway do nerve impulses triggered by an ob-ject in the left half of the visual field of the left eye reach the primary visual area of the cerebral cortex?

H E A R I N G A N D E Q U I L I B R I U M O B J E C T I V E S • Describe the structures of the external, middle, and internal ear.

• Describe the receptors for hearing and equilibrium and their pathways to the brain.

The ear is a marvelously sensitive structure. Its sensory re-ceptors can convert sound vibrations into electrical signals

1000 times faster than photoreceptors can respond to light. Beside receptors for sound waves, the ear also contains recep-tors for equilibrium (balance).

S t r u c t u r e of t h e E a r The ear is divided into three main regions: (1) the external ear, which collects sound waves and channels them inward; (2) the middle ear, which conveys sound vibrations to the oval window; and (3) the internal ear, which houses the re-ceptors for hearing and equilibrium.

External (Outer) Ear

The external (outer) ear collects sound waves and passes them inward (Figure 12.12). It consists of an auricle, exter-nal auditory canal, and eardrum. The auricle, the part of the ear that you can see, is a skin-covered flap of elastic car-tilage shaped like the flared end of a trumpet. It plays a small role in collecting sound waves and directing them to-ward the external auditory canal (audit- = hearing), a curved tube that extends from the auricle and directs sound waves toward the eardrum. The canal contains a few hairs and ceruminous glands (se-RU-mi-nus; cer- = wax), which secrete cerumen (se-RU-men) (earwax). The hairs and ceru-men help prevent foreign objects from entering the ear. The eardrum, also called the tympanic membrane (tim-PAN-ik; tympan- = adrum), is a thin, semitransparent parti-tion between the external auditory canal and the middle ear. Sound waves cause the eardrum to vibrate. Tearing of the tympanic membrane, due to trauma or infection, is called a perforated eardrum.

Middle Ear

The middle ear is a small, air-fil led cavity between the eardrum and inner ear (Figure 12.12). An opening in the anterior wall of the middle ear leads directly into the auditory tube, commonly known as the eustachian tube, which connects the middle ear with the upper part of the throat. When the auditory tube is open, air pressure can equalize on both sides of the eardrum. Otherwise, abrupt changes in air pressure on one side of the eardrum might cause it to rupture. During swallowing and yawning, the tube opens, which explains why yawning can help equalize the pressure changes that occur while f ly ing in an airplane.

Extending across the middle ear and attached to it by means of ligaments are three tiny bones called auditory ossi-cles (OS-si-kuls) that are named for their shapes- the malleus (MAL-e-us), incus (ING-kus), and stapes (STA-pez), com-monly called the hammer, anvil, and stirrup (Figure 12.12). Equally tiny skeletal muscles control the amount of move-ment of these bones to prevent damage by excessively loud

Hearing and Equil ibrium 3 1 5 m

F i g u r e 1 2 . 1 2 S t ruc ture of t h e ear.

I T h e ear has th ree pr incipal r e g i o n s : t h e external ear, t h e midd le ear, and the internal ear.

Frontal — p lane

Semic i rcu lar canal Tempora l bone

Aur ic le

^ External ear

Midd le ear

In ternal ear

Ves t ibu lococh lear (VIII) nerve:

Vest ibu lar b ranch

Coch lea r b ranch

C o c h l e a

Elast ic car t i lage C e r u m e n

External audi tory canal Tympan i c m e m b r a n e

T o nasopha rynx

Aud i to ry tube

Frontal sec t ion t h rough the r ight s ide of the skul l show ing the th ree pr incipal reg ions of the ear

W h e r e a re the receptors for hear ing and equi l ibr ium located?

noises. The stapes fits into a small opening in the thin bony partition between the middle and internal ear called the oval window, where the inner ear begins.

Internal (Inner) Ear

The internal (inner) ear is divided into the outer bony labyrinth and inner membranous labyrinth (Figure 12.13). The bony labyrinth (LAB-i-r inth) is a series of cavities in the temporal bone, including the cochlea, vestibule, and semicircular canals. The cochlea is the sense organ for hearing, and the vestibule and semicircular canals are the sense organs for equil ibrium and balance. The bony labyrinth contains a fluid called perilymph. This fluid sur-rounds the inner membranous labyrinth, a series of sacs and tubes with the same general shape as the bony labyrinth. The membranous labyrinth contains a fluid called endolymph.

The vestibule (VES-ti-bul) is the oval-shaped middle part of the bony labyrinth. The membranous labyrinth in the vestibule consists of two sacs called the utricle (U-tri-kul = little bag) and saccule (SAK-ul = little sac). Behind the vestibule are the three bony semicircular canals. The ante-rior and posterior semicircular canals are both vertical, and the lateral canal is horizontal. One end of each canal enlarges into a swelling called the ampulla (am-PUL-la = little jar). The portions of the membranous labyrinth that lie inside the bony semicricular canals are called the semicircular ducts, which connect with the utricle of the vestibule.

A transverse section through the cochlea (KOK-le-a = snail's shell), a bony spiral canal that resembles a snail's shell, shows that it is divided into three channels: cochlear duct, scala vestibuli, and scala tympani. The cochlear duct is a con-tinuation of the membranous labyrinth into the cochlea; it is filled with endolymph. The channel above the cochlear duct

3 1 6 Chapter 12 Somatic Senses and Special Senses

F i g u r e 1 2 . 1 3 D e t a i l s o f t h e r i g h t i n t e r n a l e a r . ( a ) R e l a t i o n s h i p o f t h e s c a l a t y m p a n i , c o c h l e a r d u c t , a n d s c a l a v e s t i b u l i . T h e a r r o w s i n d i c a t e t h e t r a n s m i s s i o n o f s o u n d w a v e s . ( b ) D e t a i l s o f t h e s p i r a l o r g a n ( o r g a n o f C o r t i ) .

T h e th ree channe ls in t h e c o c h l e a a re the sca la vest ibul i , sca la tympan i , and cochlear duct .

Utricle

S tapes in oval w i n d o w

Vest ibu lar m e m b r a n e

Coch lear duct

Basi lar m e m b r a n e

R o u n d w i n d o w

Scala t ympan i

M E D I A L

Coch lea

Scala t ympan i Coch lear duct Scala vest ibul i

( a ) Sec t ions th rough the coch lea

Tector ia l m e m b r a n e

Ha i rs

Ou te r hair cell

Suppor t i ng cel ls

Basi lar m e m b r a n e Cel ls l in ing sca la t ympan i

V (b) En la rgement o f spiral o rgan (organ o f Cort i )

W h a t s t ructures separa te t h e external ear f r o m the midd le e a r ? T h e midd le ear f r o m the internal e a r ?

Hai r cel l

Sensory and motor f ibers in coch lear b ranch o f ves t ibu lococh lear (VIII) nerve

is the scala vestibuli, which ends at the oval window. The channel below the cochlear duct is the scala tympani, which ends at the round window (a membrane-covered opening di-rectly below the oval window). Both the scala vestibuli and scala tympani are part of the bony labyrinth of the cochlea

and are filled with perilymph. The scala vestibuli and scala tympani are completely separated, except for an opening at the apex of the cochlea. Between the cochlear duct and the scala vestibuli is the vestibular membrane. Between the cochlear duct and scala tympani is the basilar membrane.

Hearing and Equil ibrium 3 1 7 m

Resting on the basilar membrane is the spiral organ (organ of Corti), the organ of hearing (Figure 12.13b). The spiral organ consists of supporting cells and hair cells. The hair cells, the receptors for auditory sensations, have long processes at their free ends that extend into the en-dolymph of the cochlear duct. T h e hair cells form syn-apses with sensory and motor neurons in the cochlear branch of the vestibulocochlear (VIII) nerve. The tectorial membrane, a flexible gelatinous membrane, covers the hair cells.

P h y s i o l o g y of H e a r i n g The events involved in stimulation of hair cells by sound waves are as follows (Figure 12.14):

O The auricle directs sound waves into the external audi-tory canal.

Q Sound waves striking the eardrum cause it to vibrate. The distance and speed of its movement depend on the intensity and frequency of the sound waves. More intense (louder) sounds produce larger vibrations. The eardrum

vibrates slowly in response to low-frequency (low-pitched) sounds and rapidly in response to high-frequency (high-pitched) sounds.

Q The central area of the eardrum connects to the malleus, which also starts to vibrate. The vibration is transmitted from the malleus to the incus and then to the stapes.

Q As the stapes moves back and forth, it pushes the oval window in and out.

Q The movement of the oval window sets up fluid pressure waves in the perilymph of the cochlea. As the oval window bulges inward, it pushes on the perilymph of the scala vestibuli.

O The fluid pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the mem-brane covering the round window, causing it to bulge outward into the middle ear. (See Q in the figure.)

Q As the pressure waves deform the walls of the scala vestibuli and scala tympani, they also push the vestibular membrane back and forth, creating pressure waves in the endolymph inside the cochlear duct.

F i g u r e 1 2 . 1 4 P h y s i o l o g y o f h e a r i n g s h o w n i n t h e r i g h t e a r . T h e n u m b e r s c o r r e s p o n d t o t h e e v e n t s l i s t e d in t h e tex t . T h e c o c h l e a h a s b e e n u n c o i l e d in o r d e r t o m o r e e a s i l y v i s u a l i z e t h e t r a n s m i s s i o n o f s o u n d w a v e s a n d t h e i r s u b s e -q u e n t d i s t o r t i o n o f t h e v e s t i b u l a r a n d b a s i l a r m e m b r a n e s o f t h e c o c h l e a r d u c t .

S o u n d w a v e s or ig inate f r o m v ibrat ing objects .

Mal leus Incus Stapes v ibrat ing He l ico t rema Coch lea

Sound waves

Sca la t ympan i Sca la vest ibul i

Bas i la r m e m b r a n e

Spiral o rgan (o rgan of Cor t i )

Tector ial m e m b r a n e

Vest ibular m e m b r a n e

Tympan ic m e m b r a n e

Coch lear duc t (conta ins e n d o l y m p h )

R o u n d w i n d o w

Midd le ear Aud i to ry t ube

V W h a t is the funct ion of hair ce l ls?

3 1 8 Chapter 12 Somatic Senses and Special Senses

O T h e pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane. Bending of their hairs st imulates the hair cells to release neuro-transmitter molecules at synapses with sensory neurons that are part of the vestibulocochlear (VIII) nerve (see Figure 12.13b). Then , the sensory neurons generate nerve impulses that conduct along the vestibulocochlear (VIII) nerve.

Sound waves of various frequencies cause certain re -gions of the basilar membrane to vibrate more intensely than other regions. Each segment of the basilar membrane is "tuned" for a part icular pitch. Because the membrane is narrower and stiffer at the base of the cochlea (closer to the oval window), h igh-f requency (high-pitched) sounds in-duce maximal vibrat ions in this region. Toward the apex of the cochlea, the basi lar membrane is wider and more flexi-ble; low-frequency ( low-pitched) sounds cause maximal vi-bration of the basi lar membrane there. Loudness is deter-mined by the intensity of sound waves. High- intens i ty sound waves cause larger vibrat ions of the basilar mem-brane, which leads to a h igher frequency of nerve impulses reaching the brain. Louder sounds also may stimulate a larger number of hair cells.

A u d i t o r y P a t h w a y Sensory neurons in the cochlear branch of each vestibulo-cochlear (VIII) nerve terminate in the medulla oblongata on the same side of the brain. From the medulla, axons ascend to the midbrain, then to the thalamus, and finally to the pri-mary auditory area in the temporal lobe (see Figure 10.13). Because many auditory axons cross to the opposite side, the right and left primary auditory areas receive nerve impulses from both ears.

P h y s i o l o g y of E q u i l i b r i u m You learned about the anatomy of the internal ear structures for equil ibrium in the previous section. In this section we will cover the physiology of balance, or how you are able to stay on your feet after tripping over your roommate's shoes.

There are two types of equilibrium (balance). One kind, called static equilibrium, refers to the maintenance of the position of the body (mainly the head) relative to the force of gravity. Body movements that stimulate the receptors for static equil ibrium include ti lt ing the head and linear accelera-tion or deceleration, such as when the body is being moved in an elevator or in a car that is speeding up or slowing down. The second kind, dynamic equilibrium, is the maintenance of body position (mainly the head) in response to rotational acceleration or deceleration. Collectively, the receptor organs for equilibrium, which include the saccule, utricle, and mem-branous semicircular ducts, are called the vestibular appara-tus (ves-TIB-u-lar) .

Static Equilibrium

T h e walls of both the utricle and the saccule contain a small, thickened region called a macula (MAK-u-la; macula = spot). T h e two maculae (plural), which are perpendicular to one another, are the receptors for static equi l ibrium. T h e maculae provide sensory information on the position of the head in space and help maintain appropriate posture and balance. T h e maculae also contribute to some aspects of dynamic equi l ibr ium by detecting l inear acceleration and deceleration.

T h e two maculae consist of two kinds of cells: hair cells, which are the sensory receptors, and supporting cells (Figure 12.15). T h e hairs of the hair cells protrude into a thick, jel lyl ike substance called the otolithic membrane. A layer of dense calcium carbonate crystals, cal led otoliths (oto- = ear; -liths = stones), extends over the entire surface of the otolithic membrane. If you tilt your head forward, gravity pulls the membrane (and the otoliths) so it slides over the hair cells in the direction of the tilt. Th i s st imu-lates the hair cells and tr iggers nerve impulses that conduct along the vestibular branch of the vestibulocochlear (VIII) nerve (see Figure 12.12).

Dynamic Equilibrium

The three membranous semicircular ducts lie at r ight angles to one another in three planes (see Figure 12.13 a). T h e posi-tioning permits detection of rotational acceleration or decel-eration. T h e dilated portion of each duct, the ampulla, con-tains a small elevation called the crista (KRIS-ta = crest; plural is cristae) (Figure 12.16). Each crista contains a group of hair cells and supporting cells. Covering the crista is a mass of gelat inous material called the cupula (KU-pu-la) . W h e n the head moves, the attached membranous semicircular ducts and hair cells move with it. However, the endolymph within the membranous semicircular ducts is not attached and lags behind due to its inertia. As the moving hair cells drag along the stationary endolymph, the hairs bend. Bend-ing of the hairs causes electrical signals in the hair cells. In turn, these signals tr igger nerve impulses in sensory neurons

• CLINICAL CONNECTION Otoacoust ic Emiss ions

B e s i d e s its ro le in d e t e c t i n g s o u n d s , t h e c o c h l e a h a s t h e s u r p r i s i n g ab i l i ty t o produce s o u n d s , w h i c h a r e c a l l e d o t o a c o u s t i c e m i s s i o n s . T h e s e s o u n d s a r i se f r o m v i b r a t i o n s o f t h e ha i r ce l l s t h e m s e l v e s , c a u s e d in p a r t by s i g n a l s f r o m motorneurons t ha t s y n a p s e w i t h t h e ha i r ce l ls . A sens i t i ve m i c r o p h o n e p l a c e d nex t to t h e e a r d r u m c a n p ick u p t h e s e v e r y -l o w - v o l u m e s o u n d s . D e t e c t i o n o f o t o a c o u s t i c e m i s s i o n s is a fast , i nex -p e n s i v e , a n d n o n i n v a s i v e w a y t o s c r e e n n e w b o r n s for h e a r i n g de fec ts . I n d e a f bab ies , o t o a c o u s t i c e m i s s i o n s a r e no t p r o d u c e d or a re g rea t l y r e d u c e d in s ize . •

_ . _ " T l p f , Hearing and Equil ibrium 3 1 9

F i g u r e 1 2 . 1 5 L o c a t i o n a n d s t r u c t u r e o f r e c e p t o r s in t h e m a c u l a e o f t h e r i g h t e a r . B o t h s e n s o r y n e u r o n s ( b l u e ) a n d m o t o r n e u r o n s ( r e d ) s y n a p s e w i t h t h e h a i r ce l l s .

M o v e m e n t s of the otol i thic m e m b r a n e s t imulate t h e hair cel ls.

Otol i ths Otol i th ic m e m b r a n e

Hairs

Hair cel l

• 0 3 5 3 ®

Suppor t ing cell

Loca t ion of utr ic le a n d saccu le (conta in macu lae )

Vest ibu lar b ranches of ves t ibu lococh lear (VI I I ) nerve

(a ) Overal l s t ructure of a sec t ion of the macu la

(b) Posi t ion of macu la wi th head upr ight (left) and t i l ted fo rward (right)

W h a t is t h e funct ion of the m a c u l a e ?

that are part of the vestibular branch of the vestibulocochlear (VIII) nerve.

E q u i l i b r i u m P a t h w a y s Most of the vestibular branch axons of the vestibulocochlear (VIII) nerve enter the brain stem and then extend to the medulla or the cerebellum, where they synapse with the next neurons in the equilibrium pathways. From the medulla, some axons conduct nerve impulses along the cranial nerves

that control eye movements and head and neck movements. Other axons form a spinal cord tract that conveys impulses for regulation of muscle tone in response to head move-ments. Various pathways among the medulla, cerebellum, and cerebrum enable the cerebellum to play a key role in main-taining equilibrium. The cerebellum continuously receives sensory information from the utricle and saccule. In re-sponse, the cerebellum makes adjustments to the signals go-ing from the motor cortex to specific skeletal muscles to maintain equilibrium.

3 2 0 Chapter 12 Somatic Senses and Special Senses

F i g u r e 1 2 . 1 6 L o c a t i o n a n d s t r u c t u r e o f t h e m e m b r a n o u s s e m i c i r c u l a r d u c t s o f t h e r i g h t e a r . B o t h s e n s o r y n e u r o n s ( b l u e ) a n d m o t o r n e u r o n s ( r e d ) s y n a p s e w i t h t h e h a i r ce l l s . T h e a m p u l l a r y n e r v e s a r e b r a n c h e s o f t h e v e s t i b u l a r d i v i s i o n o f t h e v e s t i b u l o c o c h l e a r ( V I I I ) n e r v e .

T h e posi t ions of t h e m e m b r a n o u s semic i rcular ducts permit detect ion of rotat ional m o v e m e n t s . fe™

Semic i rcu lar duc t

Ampu l l a

Locat ion of ampu l l ae of semic i rcu lar duc ts (conta in cr istae)

Sensory f iber Mo to r f iber

Ha i r bundle

Suppor t i ng cel l

Cr is ta

Ampu l la ry nerve

(a) Detai ls of a cr ista

Cupu la

Ampu l la

As h e a d rotates in one d i rect ion, cupu la is d ragged th rough e n d o l y m p h a n d bent in oppos i te d i rect ion

Head in still posi t ion H e a d rotat ing

(b) Posi t ion of a cupu la wi th the head in the still posi t ion (left) and w h e n the head rotates (right)

f W i t h wh ich t y p e of equi l ibr ium are the m e m b r a n o u s semic i rcular duc ts assoc ia ted?

Hearing and Equil ibrium 3 2 1 m

Table 12.3 summarizes the structures of the ear related to hearing and equilibrium.

• C H E C K P O I N T

13. W h a t is the pathway for auditory impulses from the cochlea to the cerebral cortex?

14. Compare the function of the maculae in mainta in ing static equil ibrium with the role of the cristae in maintain-ing dynamic equilibrium.

Now that our exploration of the nervous system and sen-sations is completed, you can appreciate the many ways that the nervous system contributes to homeostasis of other body systems by examining Focus on Homeostasis: The Nervous System. Next, in Chapter 13, we will see how the hormones released by the endocrine system also help maintain home-ostasis of many body processes.

Table 12.3 S u m m a r y of St ructures of the Ear Related to Hear ing and Equi l ibr ium R e g i o n s of t h e Ear a n d Key S t ruc tu res F u n c t i o n s

Externa l Ear

Aur ic le

External audi tory canal

Tympan i c m e m b r a n e

Auricle: Col lects sound waves. External auditory canal: Di rects sound w a v e s to the eardrum. Eardrum (tympanic membrane): Sound waves cause it to v ibrate, wh ich , in turn, causes the ma l leus to v ibrate.

M i d d l e Ear Audi to ry oss ic les

Aud i to ry tube

In te rna l Ear

Semic i rcu lar duc ts Coch lea

Auditory ossicles: Transmit and ampl i fy v ib ra t ions f rom t ympan ic m e m b r a n e to oval w indow. Auditory (eustachian) tube: Equa l izes air pressure on both s ides o f the t ympan ic membrane .

Cochlea: Con ta ins a ser ies of fluids, channe ls , and m e m b r a n e s that t ransmi t v ibrat ions to the spiral o rgan (organ o f Cort i ) , the o rgan of hear ing; hair cel ls in the spiral o rgan t r igger nerve im-pu lses in the coch lear branch o f the ves t ibu lococh lear (VIII) nerve. Vestibular apparatus: Inc ludes semic i rcu lar ducts, utricle, and saccule, wh ich genera te nerve impu lses that p ropagate a long the vest ibular branch o f the ves t ibu lococh lear (VIII) nerve.

Semicircular ducts: Conta in cr istae, s i tes of hair cel ls for dynamic equi l ibr ium. Utricle: Con ta ins macula , site o f hair cel ls for stat ic equi l ibr ium. Saccule: Con ta ins macula , site o f hair cel ls for stat ic equi l ibr ium.

Saccu le

FOCUS ON HOMEOSTASIS

B O D Y S Y S T E M

THE NERVOUS SYSTEM

C O N T R I B U T I O N O F T H E N E R V O U S S Y S T E M

F o r al l b o d y s y s t e m s

I n t e g u m e n t a r y s y s t e m

Ske le ta l s y s t e m

Together wi th h o r m o n e s f rom the endocr ine sys tem, nerve impulses provide c o m m u n i c a t i o n and regulat ion of most body t issues.

Sympathe t ic nerves of the a u t o n o m i c nervous s y s t e m ( A N S ) control contract ion of s m o o t h m u s -cles a t tached to hair fol l icles a n d secre t ion of perspirat ion f r o m sweat g lands.

Nociceptors (pain receptors) in b o n e t issue w a r n of bone t r a u m a or d a m a g e .

M u s c u l a r s y s t e m Somat ic motor neurons rece ive instruct ions f rom motor a reas of the brain a n d st imulate con-t ract ion of skeletal musc les to br ing about body m o v e m e n t s . T h e basal gangl ia a n d ret icular for-mat ion set the level of musc le t o n e . T h e cerebe l lum coord inates ski l led m o v e m e n t s .

E n d o c r i n e s y s t e m

The hypotha lamus regulates secre t ion of h o r m o n e s f rom the anter ior a n d poster ior p i tu i tary .The A N S regulates secre t ion of h o r m o n e s f rom the adrena l medu l la a n d pancreas .

C a r d i o v a s c u l a r s y s t e m

The card iovascular center in t h e medu l la ob longata provides nerve impulses to the A N S that govern heart rate a n d t h e forcefu lness of the hear tbeat . Nerve impulses f rom t h e A N S a lso regu-late b lood pressure and b lood flow through b lood vesse ls .

L y m p h a t i c s y s t e m a n d i m m u n i t y

R e s p i r a t o r y s y s t e m

D i g e s t i v e s y s t e m

Certa in neurot ransmit ters help regulate i m m u n e responses . Act iv i ty in the nervous s y s t e m m a y increase or d e c r e a s e i m m u n e responses .

Respi ra tory areas in the brain s tem control breath ing rate a n d depth . The A N S helps regulate the d iameter of a i rways.

The A N S a n d enter ic nervous s y s t e m (ENS) he lp regulate d i g e s t i o n . T h e parasympathe t ic divi-s ion of the A N S st imulates m a n y d igest ive processes .

U r i n a r y s y s t e m The A N S helps regulate b lood flow to k idneys, thereby inf luencing t h e rate of ur ine format ion; brain a n d spinal cord centers govern empty ing of ur inary bladder.

R e p r o d u c t i v e s y s t e m s

3 2 2

The hypotha lamus a n d l imbic s y s t e m g o v e r n a var iety of sexual behaviors; the A N S br ings about erect ion of t h e penis in males and the cl i toris in females a n d e jaculat ion of s e m e n in males . The hypotha lamus regulates re lease of anter ior pi tui tary h o r m o n e s that control the go-nads (ovaries a n d testes) . Nerve impulses el ic i ted by touch st imul i f rom suckl ing infants c a u s e re lease of oxytoc in and mi lk e ject ion in nursing mothers .

Medical Terminology and Conditions 3 2 3

C O M M O N D I S O R D E R S

Cata rac ts A common cause of blindness is a loss of transparency of the lens known as a cataract (CAT-a-rakt). The lens becomes cloudy (less transparent) due to changes in the structure of the lens proteins. Cataracts often occur with aging but may also be caused by injury, excessive exposure to ultraviolet rays, certain medications (such as long-term use of steroids), or complications of other diseases (for example, diabetes). People who smoke also have increased risk of de-veloping cataracts. Fortunately, sight can usually be restored by surgi-cal removal of the old lens and implantation of an artificial one.

G l a u c o m a In glaucoma (glaw-KO-ma), the most common cause of blindness in the United States, a buildup of aqueous humor within the ante-rior cavity causes an abnormally high intraocular pressure. Persis-tent pressure results in a progression from mild visual impairment to irreversible destruction of the retina, damage to the optic nerve, and blindness. Because glaucoma is painless, and because the other eye initially compensates to a large extent for the loss of vision, a person may experience considerable retinal damage and loss of vi-sion before the condition is diagnosed.

D e a f n e s s Deafness is significant or total hearing loss. Sensorineural deafness is caused by either impairment of hair cells in the cochlea or damage

of the cochlear branch of the vestibulocochlear nerve. This type of deafness may be caused by atherosclerosis, which reduces blood supply to the ears; repeated exposure to loud noise, which destroys hair cells of the spiral organ; or certain drugs such as aspirin and streptomycin. Conduction deafness is caused by impairment of the outer and middle ear mechanisms for transmitting sounds to the cochlea. It may be caused by otosclerosis, the deposition of new bone around the oval window; impacted cerumen; injury to the eardrum; or aging, which often results in thickening of the eardrum and stiffening of the joints of the auditory ossicles.

Meniere 's D isease Meniere's disease (men'-e-ARZ) results from an increased amount of endolymph that enlarges the membranous labyrinth. Among the symptoms are fluctuating hearing loss (caused by distortion of the basilar membrane of the cochlea) and roaring tinnitus (ringing). Vertigo (a sensation of spinning or whirling) is characteristic of Meniere's disease. Almost total destruction of hearing may occur over a period of years.

Otit is Med ia Otitis media is an acute infection of the middle ear caused primarily by bacteria and associated with infections of the nose and throat. Symp-toms include pain; malaise (discomfort or uneasiness); fever; and a red-dening and outward bulging of the eardrum, which may rupture unless prompt treatment is received (this may involve draining pus from the middle ear). Bacteria from the nasopharynx passing into the auditory tube is the primary cause of all middle ear infections. Children are more susceptible than adults to middle ear infections because their auditory tubes are almost horizontal, which decreases drainage.

M E D I C A L T E R M I N O L O G Y A N D C O N D I T I O N S

Age-related macular disease (AMD) Degeneration of the macula lutea of the retina in persons 50 years of age and older.

Anosmia (an-OZ-me-a; a- = without; -osmi = smell, odor) Total lack of the sense of smell.

Cochlear implant A device that translates sounds into electrical signals that can be interpreted by the brain. It is especially useful for peo-ple with deafness caused by damage to hair cells in the cochlea.

Conjunctivitis (pinkeye) An inflammation of the conjunctiva; when caused by bacteria such as pneumococci, staphylococci, or He-mophilus influenzae, it is very contagious and more common in children. May also be caused by irritants, such as dust, smoke, or pollutants in the air, in which case it is not contagious.

Detached retina Detachment of the neural portion of the retina from the pigment epithelium due to trauma, disease, or age-re-lated degeneration. The result is distorted vision and blindness.

LASIK (laser-assisted in-situ keratomileusis) Surgery with a laser to correct the curvature of the cornea for conditions such as nearsightedness, farsightedness, and astigmatism.

Nystagmus (nis-TAG-mus; nystagm- = nodding or drowsy) A rapid involuntary movement of the eyeballs, possibly caused by a dis-ease of the central nervous system. It is associated with condi-tions that cause vertigo.

Otalgia (o-TAL-je-a; ot- = ear; -algia = pain) Earache. Retinoblastoma (ret-i-no-blas-TO-ma; -oma = tumor) A tumor

arising from immature retinal cells; it accounts for 2% of child-hood cancers.

Scotoma (sko-TO-ma = darkness) An area of reduced or lost vision in the visual field.

Strabismus (stra-BIZ-mus) An imbalance in the extrinsic eye mus-cles that causes a misalignment of one eye so that its line of vi-sion is not parallel with that of the other eye (cross-eyes) and both eyes are not pointed at the same object at the same time; the condition produces a squint.

Tinnitus (ti-NI-tus) A ringing, roaring, or clicking in the ears. Trachoma (tra-KO-ma) A serious form of conjunctivitis and the

greatest single cause of blindness in the world. It is caused by the bacterium Chlamydia trachomatis. The disease produces an excessive growth of subconjunctival tissue and invasion of blood vessels into the cornea, which progresses until the entire cornea is opaque, causing blindness.

Vertigo (VER-ti-go = dizziness) A sensation of spinning or move-ment in which the world seems to revolve or the person seems to revolve in space.

3 2 4 Chapter 12 Somatic Senses and Special Senses

\ S T U D Y O U T L I N E

O v e r v i e w of S e n s a t i o n s

1. Sensation is the conscious or subconscious awareness of exter-nal and internal stimuli.

2. Two general classes of senses are (1) general senses, which include somatic senses and visceral senses, and (2) special senses, which include smell, taste, vision, hearing, and equilibrium (balance).

3. The conditions for a sensation to occur are reception of a stim-ulus by a sensory receptor, conversion of the stimulus into one or more nerve impulses, conduction of the impulses to the brain, and integration of the impulses by a region of the brain.

4. Sensory impulses from each part of the body arrive in specific regions of the cerebral cortex.

5. Adaptation is a decrease in sensation during a prolonged stimu-lus. Some receptors are rapidly adapting; others are slowly adapting.

6. Receptors can be classified structurally by their microscopic features as free nerve endings, encapsulated nerve endings, or separate cells. Functionally, receptors are classified by the type of stimulus they detect as mechanoreceptors, thermore-ceptors, nociceptors, photoreceptors, osmoreceptors, and chemoreceptors.

S o m a t i c S e n s e s 1. Somatic sensations include tactile sensations (touch, pressure,

vibration, itch, and tickle), thermal sensations (heat and cold), pain sensations, and proprioceptive sensations (joint and mus-cle position sense and movements of the limbs). Receptors for these sensations are located in the skin, mucous membranes, muscles, tendons, and joints.

2. Receptors for touch include Meissner corpuscles, hair root plexuses, Merkel discs, and Ruffini corpuscles. Receptors for pressure and vibration are pacinian corpuscles. Tickle and itch sensations result from stimulation of free nerve endings.

3. Thermoreceptors, free nerve endings in the epidermis and der-mis, adapt to continuous stimulation.

4. Nociceptors are free nerve endings that are located in nearly every body tissue; they provide pain sensations.

5. Proprioceptors inform us of the degree to which muscles are contracted, the amount of tension present in tendons, the posi-tions of joints, and the orientation of the head.

O l f a c t i o n : S e n s e of S m e l l

1. The olfactory epithelium in the upper portion of the nasal cav-ity contains olfactory receptors, supporting cells, and basal stem cells.

2. Individual olfactory receptors respond to hundreds of different odorant molecules by producing an electrical signal that trig-gers one or more nerve impulses. Adaptation (decreasing sensi-tivity) to odors occurs rapidly.

3. Axons of olfactory receptors form the olfactory nerves, which convey nerve impulses to the olfactory bulbs. From there, im-

pulses conduct via the olfactory tracts to the limbic system, hy-pothalamus, and cerebral cortex (temporal lobe).

G u s t a t i o n : S e n s e of T a s t e 1. The receptors for gustation, the gustatory receptor cells, are

located in taste buds.

2. To be tasted, substances must be dissolved in saliva.

3. The five primary tastes are salty, sweet, sour, bitter, and umami.

4. Gustatory receptor cells trigger impulses in cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus). Impulses for taste conduct to the medulla oblongata, limbic system, hypothala-mus, thalamus, and the primary gustatory area in the parietal lobe of the cerebral cortex.

V i s i o n

1. Accessory structures of the eyes include the eyebrows, eyelids, eyelashes, the lacrimal apparatus (which produces and drains tears), and extrinsic eye muscles (which move the eyes).

2. The eyeball has three layers: (a) fibrous tunic (sclera and cornea), (b) vascular tunic (choroid, ciliary body, and iris), and (c) retina.

3. The retina consists of a neural layer (photoreceptor layer, bipo-lar cell layer, and ganglion cell layer) and a pigmented layer (a sheet of melanin-containing epithelial cells).

4. The anterior cavity contains aqueous humor; the vitreous chamber contains the vitreous body.

5. Image formation on the retina involves refraction of light rays by the cornea and lens, which focus an inverted image on the central fovea of the retina.

6. For viewing close objects, the lens increases its curvature (ac-commodation), and the pupil constricts to prevent light rays from entering the eye through the periphery of the lens.

7. Improper refraction may result from myopia (nearsightedness), hypermetropia (farsightedness), or astigmatism (irregular cur-vature of the cornea or lens).

8. Movement of the eyeballs toward the nose to view an object is called convergence.

9. The first step in vision is the absorption of light rays by pho-topigments in rods and cones (photoreceptors). Stimulation of the rods and cones then activates bipolar cells, which in turn activate the ganglion cells.

10. Nerve impulses arise in ganglion cells and conduct along the optic nerve, through the optic chiasm and optic tract to the thalamus. From the thalamus, the optic radiations extend to the primary visual area in the occipital lobe of the cerebral cortex.

H e a r i n g a n d E q u i l i b r i u m

1. The external ear consists of the auricle, external auditory canal, and eardrum.

2. The middle ear consists of the auditory (eustachian) tube, audi-tory ossicles, oval window, and round window.

3. The internal ear consists of the bony labyrinth and membra- 6. nous labyrinth. The internal ear contains the spiral organ (or-gan of Corti), the organ of hearing.

4. Sound waves enter the external auditory canal, strike the 7. eardrum, pass through the ossicles, strike the oval window, set up pressure waves in the perilymph, strike the vestibular mem-brane and scala tympani, increase pressure in the endolymph, g vibrate the basilar membrane, and stimulate hair cells in the spiral organ. 9

5. Hair cells release neurotransmitter molecules that can initiate nerve impulses in sensory neurons.

J I Self-Quiz 3 2 5

ffiiflls; Sensory neurons in the cochlear branch of the vestibulocochlear nerve terminate in the medulla oblongata. Auditory signals then pass to the midbrain, thalamus, and temporal lobes. Static equilibrium is the orientation of the body relative to the pull of gravity. The maculae of the utricle and saccule are the sense organs of static equilibrium.

Dynamic equilibrium is the maintenance of body position in response to rotational acceleration and deceleration.

Most vestibular branch axons of the vestibulocochlear nerve enter the brain stem and terminate in the medulla and pons; other axons extend to the cerebellum.

S E L F - Q U I Z

1. You enter a sauna and it feels awfully hot, but soon the temper-ature feels comfortably warm. What have you have experi-enced?

a. damage to your thermoreceptors

b. sensory adaptation c. a change in the temperature of the sauna

d. inactivation of your thermoreceptors

e. temporary damage to sensory neurons

2. The lacrimal glands produce , which drain(s) into the

a. tears; anterior cavity b. tears; nasal cavity

c. the vitreous body; vitreous chamber

d. aqueous humor; anterior cavity e. aqueous humor; scleral venous sinus

3. The spiral organ (organ of Corti)

a. contains hair cells

b. is responsible for equilibrium c. is filled with perilymph

d. is another name for the auditory (eustachian) tube

e. transmits auditory nerve impulses to the brain

4. Equilibrium and the activities of muscles and joints are moni-tored by

a. olfactory receptors b. nociceptors

c. tactile receptors

d. proprioceptors e. thermoreceptors

5. In the retina, cone photoreceptors

a. are more numerous than rods

b. contain the photopigment rhodopsin

c. are more sensitive to low light levels than are rods d. tend to be highly concentrated in the optic disc

e. provide higher acuity of vision than do rods

6. Which of the following is NOT required for a sensation to occur?

a. the presence of a stimulus

b. a receptor specialized to detect a stimulus c. the presence of slowly adapting receptors

d. a sensory neuron to conduct impulses

e. a region of the brain for integration of the nerve impulse

7. Match each receptor with its function.

a. color vision A. pacinian corpuscle b. taste B. Meissner corpuscle c. smell C. rod photoreceptor d. dynamic equilibrium D. nociceptor e. vision in dim light E. gustatory receptor cell f. stretch in a muscle F. olfactory receptor

g. static equilibrium G. muscle spindle h. pressure H. maculae i. touch I. cristae

detection of pain J. cone photoreceptors

g. The type of pain that can be precisely localized is a. referred pain b. fast pain c. slow pain d. chronic pain e. none of the above because pain cannot be localized

9. Which of the following characteristics of taste is NOT true?

a. Olfaction can affect taste.

b. Three cranial nerves conduct the impulses for taste to the brain.

c. Taste adaptation occurs quickly. d. Humans can recognize about 10 primary tastes.

e. Taste receptors are located in taste buds on the tongue, on the roof of the mouth, in the throat, and in the epiglottis.

3 2 6 Chapter 12 Somatic Senses and Special Senses

10. An ice skater is spinning rapidly on the ice. What is occurring in her inner ear?

a. The hair cells on the macula are responding to changes in static equilibrium.

b. The hair cells in the cochlea are responding to changes in dynamic equilibrium.

c. The cristae of each semicircular duct are responding to changes in dynamic equilibrium.

d. The cochlear branch of the vestibulocochlear (VIII) nerve is transmitting nerve impulses to the brain related to equilibrium.

e. The auditory (eustachian) tube is adjusting for varying air pressures.

11. Kinesthesia is the

a. perception of body movements

b. ability to identify an object by feeling it

c. sensation of weightlessness that occurs in outer space

d. decrease in sensitivity of receptors to a prolonged stimulus

e. movement of body parts in a rhythmic manner

12. Which of the following is NOT true about nociceptors?

a. They respond to stimuli that may cause tissue damage.

b. They consist of free nerve endings.

c. They can be activated by excessive stimuli from other sensations.

d. They are found in virtually every body tissue except the brain.

e. They adapt very rapidly.

13. The sense of smell

a. requires the presence of dissolved odorants

b. is transmitted through olfactory nerves and olfactory tracts

c. evokes emotional responses because of limbic system involvement

d. is initiated by stimulating olfactory hairs

e. is described by all of the above characteristics

14. Transmission of vibrations (sound waves) from the tympanic membrane to the oval window is accomplished by

a. neurons

b. the tectorial membrane

c. the auditory ossicles

d. the endolymph

e. the auditory (eustachian) tube

15. Match the following:

a. focuses light rays onto the retina

b. regulates the amount of light entering the eye

c. contains aqueous humor

d. contains blood vessels that help nourish the retina

e. hole in the middle of the iris

f. dense connective tissue that provides shape to the eye

g. contains photoreceptors

A. sclera

B. choroid

C. pupil

D. lens

E. retina

F. iris

G. anterior cavity

16. Which of the following structures refracts light rays entering the eye?

a. cornea b. sclera c. pupil

d. retina e. conjunctiva

17. Your 45-year-old neighbor has recently begun to have difficulty reading the morning newspaper. You explain that this condition is known as and is due to .

a. myopia, inability of his eyes to properly focus light on his retinas

b. night blindness, a vitamin A deficiency

c. binocular vision, the eyes focusing on two different objects

d. astigmatism, an irregularity in the curvature of the lens

e. presbyopia, the loss of elasticity in the lens

18. Damage to cells in the fovea centralis would interfere with

а. convergence b. accommodation c. visual acuity

d. ability to see in dim light e. intraocular pressure

19. Place the following events concerning the visual pathway in the correct order:

1. Nerve impulses exit the eye via the optic nerve.

2. Optic tract axons terminate in the thalamus.

3. Light reaches the retina.

4. Rods and cones are stimulated.

5. Synapses occur in the thalamus and continue to the primary visual area in the occipital lobe.

б. Ganglion cells generate nerve impulses.

a. 4, 1, 2, 5, 6, 3 b. 5, 4, 1, 3, 2, 6 c. 3, 4, 6, 1, 5, 2 d. 3, 4, 6, 1, 2, 5 e. 3, 4, 5, 6, 1, 2

20. Place the following events of the auditory pathway in the correct order:

1. Hair cells in the spiral organ bend as they rub against the tectorial membrane.

2. Movement in the oval window begins movement in the perilymph.

3. Nerve impulses exit the ear via the vestibulocochlear (VIII) nerve.

4. The eardrum and auditory ossicles transmit vibrations from sound waves.

5. Pressure waves from the perilymph cause bulging of the round window and formation of pressure waves in the endolymph.

a. 4, 2, 5, 1, 3 b. 4, 5, 2, 3, 1 c. 5, 3, 2, 4, 1

d. 3, 4, 5, 1, 2 e. 2, 4, 1, 5, 3

Answers To Figure Questions 3 2 7 m

C R I T I C A L T H I N K I N G A P P L I C A T I O N S

1. Evelyn is preparing her 6-month-old infant for bed. She gives him a warm bath, dries him off, dresses him and gives him a quick tickle to make him smile. As she lays him in his crib she gives him a light kiss on his lips and gently strokes his arms un-til he dozes off. What are some of the baby's receptors that have been activated by his mom's actions?

2. Cliff works the night shift and sometimes falls asleep in A&P class. What is the effect on the structures in his internal ear when his head falls backward as he slumps in his seat?

3. A medical procedure used to improve visual acuity involves shaving of a thin layer off the cornea. How could this proce-dure improve vision?

4. The optometrist put drops in Latasha's eyes during her eye exam. When Latasha looked in the mirror after the exam, her pupils were very large and her eyes were sensitive to the bright light. How did the eye drops produce this effect on Latasha's eyes?

A N S W E R S T O F I G U R E Q U E S T I O N S

12.1 Meissner corpuscles are abundant in the fingertips, palms, and soles.

12.2 The kidneys have the broadest area for referred pain.

12.3 Basal stem cells undergo cell division to produce new olfac-tory receptors.

12.4 The gustatory pathway: gustatory receptor cells ^ cranial nerves VII, IX, and X ^ medulla oblongata ^ thalamus ^ primary gustatory area in the parietal lobe of the cerebral cortex.

12.5 Tears clean, lubricate, and moisten the eyeball.

12.6 The fibrous tunic consists of the cornea and sclera; the vas-cular tunic consists of the choroid, ciliary body, and iris.

12.7 The parasympathetic division of the autonomic nervous sys-tem causes pupillary constriction; the sympathetic division causes pupillary dilation.

12.8 The two types of photoreceptors are rods and cones. Rods provide black-and-white vision in dim light; cones provide high visual acuity and color vision in bright light.

12.9 During accommodation, the ciliary muscle contracts, zonu-lar fibers slacken, and the lens becomes more rounded (convex) and refracts light more.

12.10 Presbyopia is the loss of elasticity in the lens that occurs with aging.

12.11 Structures carrying visual impulses from the retina: axons of ganglion cells ^ optic (II) nerve ^ optic chiasm ^ optic tract ^ thalamus ^ primary visual area in occipital lobe of the cerebral cortex.

12.12 The receptors for hearing and equilibrium are located in the inner ear: cochlea (hearing) and semicircular ducts (equilibrium).

12.13 The eardrum (tympanic membrane) separates the outer ear from the middle ear. The oval and round windows separate the middle ear from the internal ear.

12.14 Hair cells convert a mechanical force (stimulus) into an elec-trical signal (depolarization and repolarization of the hair cell membrane).

12.15 The maculae are the receptors for static equilibrium.

12.16 The membranous semicircular ducts function in dynamic equilibrium.