the human body in health and illness

The Human Body in Health and Illness, 4th edition

Barbara Herlihy

Chapter 13:

Sensory System

Lesson 13 Objectives

• State the functions of the sensory system.

• Define the five types of sensory receptors.

• Describe the four components involved in the perception of a sensation.

• Describe the five general senses.

Copyright © 2011, 2007 by Saunders,

an imprint of Elsevier Inc. All rights

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Lesson 13 Objectives

• Describe the five special senses.

• Describe the structure of the eye.

• Explain the movement of the eyes.

• Describe how the size of the pupils changes.



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Sensory System

• Sensory system allows us to experience the world.

• External information

• Sound of a dog barking

• Internal information

• Sudden change in blood pressure



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Five Types of Sensory Receptors

• Receptor: Specialized area of a sensory neuron that detects a specific stimulus

– Chemoreceptors

– Pain receptors (nociceptors)

– Thermoreceptors

– Mechanoreceptors

– Photoreceptors



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Four Components of Sensation



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Two Characteristics of Sensation

• Projection

• Adaptation



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Projection

• Brain refers sensation back to its source

A. Ordinary injury

B. Phantom limb pain



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Adaptation

• Adaptation: With continuous stimulation, sensory receptors become less responsive.

• Receptors vary in their ability to adapt.

Smell and temperature receptors adapt well.

Pain receptors do not adapt at all.



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Five General Senses

• Pain

• Touch

• Pressure

• Temperature

• Proprioception



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Pain Receptors or Nociceptors

• Consist of free nerve endings stimulated by tissue injury, chemicals, tissue hypoxia

• Widely distributed throughout the skin, viscera, other internal tissues

• Do not adapt



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Sites of Referred Pain

• Compare heart’s location with possible sites of pain during a heart attack.

• Gallbladder attack may present with shoulder pain.



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Touch and Pressure Receptors

• Touch (tactile) receptors


– Found mostly in skin

• Pressure receptors


– Located in the skin, subcutaneous tissue, and deep tissue



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Thermoreceptors

• Receptors for cold and heat

• Located in free nerve endings and other specialized sensory cells in the skin

• Quick adaptation

• Temperature extremes experienced as pain



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Proprioception

• Proprioception: Sense of orientation or position in space

• Receptors

– Located in muscles, tendons, joints, and inner ear

– Sensory information parietal lobe, cerebellum



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Five Special Senses

• Smell

• Taste

• Sight

• Hearing

• Balance



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Olfactory Sense: Smell

• Chemoreceptors in nasal tissue

• Nerve impulses travel on CN I to temporal lobe for interpretation



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Gustatory Sense: Taste

• Taste receptors are chemoreceptors.

• Nerve impulses move along three cranial nerves to parietal and temporal lobes.



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Vision: Sense of Sight

• Visual accessory structures

• Primary visual structures are the eye and visual pathway.



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Visual Accessory Structures

• Eyebrows

• Eyelids

• Conjunctiva

• Eyelashes

• Lacrimal apparatus

• Extrinsic eye muscles



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Eye: Organ of Vision



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Eyeball: Three Layers

• Sclera

• Choroid

• Retina



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Sclera

• Tough outer layer in posterior eyeball

• Forward extension becomes cornea

• Extrinsic eye muscles attach here



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Choroid

• Middle layer in the posterior eyeball

• Forward extension becomes ciliary body and iris

• Highly vascular to nourish retina



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Retina

• Inner layer in posterior eyeball

• Site of photoreceptors

Rods

Cones

• Optic disc

Exit of CN II



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Retinal Photoreceptors

• Rods

– Located on periphery

– Responsible for black and white or night vision

• Cones

– Located on central part of posterior eye

– Concentrated in fovea centralis in center of macula lutea

– Responsible for color vision



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Cavities of Eyeball • Posterior cavity

– Between lens and retina

– Contains vitreous humor

• Anterior cavity

– Between lens and cornea

– Contains aqueous humor



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Formation and Drainage of Aqueous Humor

• Formed by ciliary body

• Circulates through pupil behind cornea

• Drains through canals of Schlemm



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Muscles of the Eye

• Extrinsic muscles: Move eyeball in its bony orbit

• Intrinsic muscles: Move structures within eyeball

– Iris

– Ciliary muscles



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Extrinsic Muscles of the Eye

• Four rectus muscles

• Two obliques

• Primary innervation

from CN III



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Three Intrinsic Eye Muscles

• Iris

Circular muscle

• Miosis

• Muscarinic receptors

Radial muscle

• Mydriasis

• Alpha 1 receptors

• Ciliary muscles



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Eye Disorders



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Refraction: Lens

• Ciliary muscles pull on suspensory ligaments.

• Suspensory ligaments pull on lens.

• Lens changes shape.



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Refraction

• Bending light rays to focus on retina

• Lens, primary refracting structure

• Focal point on retina



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Errors of Refraction

• Myopia, focal point in front of retina

• Hyperopia, focal point behind retina

• Astigmatism, result of irregularly curved cornea



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Visual Pathway

• Photoreceptors generate nerve impulse

• Nerve impulse travels along CN II to occipital lobe

• Occipital lobe “sees” Rover



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Visual Pathway: Optic Chiasm

• Lateral fibers of CN II ascend to same side of brain.

• Medial fibers of CN II cross to opposite sides, forming the optic chiasm.

• The brain sees one image.



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How Seeing Occurs

• Pathway of light

Cornea aqueous humor pupil lens vitreous humor rods and cones

• Pathway of nerve impulses

Rods and cones CN II occipital lobe



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Lesson 13-3 Objectives

• Describe the three divisions of the ear.

• Describe the functions of the parts of the ear involved in hearing.

• Explain the role of the ear in maintaining the body’s equilibrium.



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Sense of Hearing: Three Parts of Ear

• External to tympanic membrane

• Middle from tympanic membrane to oval window

• Inner behind oval window



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Three Parts of Ear: Contents

• External

– Auricle, external auditory canal

• Middle

– Malleus, incus, stapes

– Eustachian tube

• Inner

– Cochlea, vestibule, semicircular canals, origin of CN VIII



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Eustachian Tube

• Connects throat with middle ear

• Equalizes pressure across tympanic membrane

• In young child, short and horizontal

• In adults, longer and more vertical



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Receptors : Organ of Corti

• Mechanoreceptors (hairs) in cochlea of inner ear

• Organ of Corti in endolymph

• Cochlear branch of CN VIII



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How Hearing Occurs

• Pathway of vibrations

Sound waves tympanic membrane ossicles oval window organ of Corti

• Pathway of nerve impulses

Organ of Corti CN VIII (cochlear branch) temporal lobe



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Recap: How Hearing Occurs



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Receptors for Balance

• Mechanoreceptors (hairs) in vestibule and semicircular canals of inner ear

• Hairs in endolymph

• Vestibular branch of CN VIII



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the human body in health and illness

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