Chapter 12– Nervous Tissue

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Ch. 12 Nervous Tissue– Study Guide

1. Critically read Chapter 12 pp. 442-461 before 12.5 Synapses.

2. Comprehend Terminology (those in bold in the textbook) within the reading scope above

3. Study-- Figure questions, Think About It questions, and Before You Go On (section-ending) questions (within the reading scope above) . Before You Go On Questions.

4. Do end-of-chapter questions—– Testing Your Recall— 1-4, 7, 11-17– True or False– 1-4, 6, 8

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12.1 Overview of the nervous system

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§ Maintaining internal coordination by two body systems

1. Homeostasis? How?

2. Endocrine and nervous systems--A. Endocrine sys. – slower– chemical messengers (hormones) delivered

to the bloodstream– Example-- insulin

B. Nervous sys. – quicker– chemical and electrical means– Example– in cold environment,

vasoconstriction/shivering 12-4

§ Subdivisions of Nervous SystemTwo major ANATOMICAL subdivisions:

• Central nervous system (CNS)– brain and spinal cord enclosed in bony

coverings

• Peripheral nervous system (PNS)– all nervous tissue outside the CNS; made up of:– Nerves-- bundles of axons in connective

tissue; emerge from the CNS; carry signals– Ganglia-- knotlike swellings in nerves

Fig. @12.1

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Part of PNS (next slide)

• Sensory (Afferent) divisions (receptors to CNS)– carry signals to the CNS– somatic division—Ex. – visceral sensory division—Ex.

• Motor (Efferent) division (CNS to effectors)1.somatic motor division

Effectors: skeletal muscles2.visceral motor division (also called ANS)

Effectors: glands and cardiac/smooth muscles• sympathetic division/parasympathetic

divisionFig. 12.2

§ Functional divisions of PNS (SAME)

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Brain

CNS PNS

Spinalcord

Sensorydivision

Motordivision

Visceralsensorydivision

Somaticsensorydivision

Visceralmotor

division

Somaticmotor

division

Sympatheticdivision

Parasympatheticdivision

12.2 Properties of neurons

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§ Universal properties of neuronsNerves made up of nerve cells (neurons);

neurons’ properties include:

• Excitability– ability to respond to stimuli by producing

action potential

• Conductivity– produce traveling electrical signals

• Secretion– Where? Why?– What is secreted?

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§ Functional Classes of Neurons1. Sensory (afferent) neurons

– detect changes in body and external environment– Ex.

2. Interneurons (association neurons)– Confine ENTIRELY in CNS– 90% of our neurons are interneurons– process, store and retrieve information

3. Motor (efferent) neuron– send signals out to muscles and gland cells

(effectors carry out body responses)– Ex. Fig. 12.3

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Fig.12.3 Three Classes of Neurons

Example--Detecting your own pulse at wrist

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§ Neuron• Cell body = soma

– Nucleus

• Dendrites (1-many)

– Function

• Axon (single; nerve fiber)

– Function

Fig. 12.4 c-d-e 12-13

Neurofibrils

Axon(d)

Figure 12.4d

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Myelin sheath

Axolemma

Axoplasm

Neurilemma

(c)

Schwann cellnucleus

§ Variation in Neuron Structure–

No. of processes from the soma: (Fig. 16.34)

• Multipolar neuron– most common

• Bipolar neuron– one dendrite/one axon

• Unipolar neuron– Ex. sensory from skin to

spinal cord directly• Anaxonic neuron

– many dendrites/no axon– Ex. help in visual

processes12-17

12.3 Supportive cells (Neuroglia)

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§ Introduction

• How many?! Neurons are outnumbered by neuroglia (1:50) in the nervous sys.

• Functions- protect the neurons and help them function

• Example– in the fetus, guide young migrating neurons to their destinations

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§ 4 Types of Neuroglial Cells (CNS)1. Astrocytes (star-shaped)

– most abundant glial cells - form framework of CNS– contribute to blood-brain barrier and regulate

composition of brain tissue fluid2. Oligodendrocytes form myelin sheaths in

CNS; distinguish these from Schwann cells3. Ependymal cells (epithelial cells) line

ventricles of the brain and central canal of the spinal cord; produce CSF

4. Microglia formed from monocytes; engulf invading microbes– in areas of infection, trauma or stroke

Fig. 12.612-21

Neuroglial Cells of CNS

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§ 2 Types of Neuroglial Cells (PNS)

1. Schwann cells -- myelinate fibers of PNS; assist in the regeneration of damaged fibers

2. Satellite cells – surround cell bodies in ganglia; regulate the chemical environment of the neurons

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§ Myelin• Insulating layer around a nerve fiber;

analogy– the rubber insulation on a wire

• In CNS,– Each oligodendrocyte myelinate several

fibers (Fig. 12.7a)

• In PNS, – The ___________ cell wraps the nerve fiber – outermost coil is called neurilemma

containing bulging body of the Schwann cell (nucleus and most of its cytoplasm) (Fig. 12.7b) 12-24

Figure 12.7b

Myelin Sheath in CNS

Myelin Sheath in PNS

NeurilemmaMyelin sheath

Axon

Schwann cell

Myelin Sheath in PNSNode of Ranvier

(gaps)-- between Schwann cells (also in CNS)

Internodes–

from one gap to the next

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• Locations?CNS, PNS, both (circle one)

• One Schwann cell harbors ______ small fibers

• The Schwann cell– folds once around each fiber

Fig. 12.8

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§ Unmyelinated nerve fibers

Schwann cell

Basal lamina

Unmyelinatednerve fibers

§ Myelination and Speed of Nerve Signal

• Diameter of fiber and presence of myelin– large fibers have more surface area for signal

conduction

• Speeds– small, unmyelinated fibers = 0.5 - 2.0 m/sec– small, myelinated fibers = 3 - 15.0 m/sec– large, myelinated fibers = up to 120 m/sec

• Functions– slow signals supply the stomach and dilate pupil– fast signals supply skeletal muscles and transport

sensory signals for vision and balance 12-30

12.4A Electrophysiology of neurons

KEY issues–

1.How does a neuron generate an electrical signal?

2.Cellular mechanisms for producing electrical potential and currents

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§ Electrical Potentials & Currents1. Electrical potential – a difference in the

concentration of charged particles between one point and another

2. Electrical current– flow of charged particles from one point to another

3. Living cells have electrical potentials (are polarized)– resting membrane potential is -70 mV with a

negative charge on the _______ of membrane; why? (next slide)

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§ Resting Membrane Potential of all cells (RMP; -70 mV)

Factors contribute to RMP: unequal distribution of electrolytes in ECF & ICF

1. Diffusion of ions down their conc. gradient2. Selective permeability of the cell mem.3. Cations and anions attract to each other

Details--• Membrane very permeable to K+ • Membrane much less permeable to Na+

• Cytoplasmic anions can not escape— EX. Proteins, phosphates etc.

Fig. 12.1112-33

• Na+ more concentrated in the ECF

• K+ more concentrated in the ICF 12-34

Negative charge (-70 mV)

Fig. 12.11--Ion basis of the resting membrane potential

§ When a neuron is stimulated• Local potentials– changes in membrane

potential when a neuron is stimulated – Causes– How?– Results– depolarization– Ionic bases--

• Na+ rushes into/out of (circle one) the cell• Na+ diffuses for short distance inside

membrane producing a local potential

Fig. 12.12 and X

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For example– a chemical (pain signal; ligand) stimulates a neuron

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Magnitude

of stimulus

Resting mem.Potential (-70 mV)Time

Stimuli A B C D

Local potential-- Local, graded, and decremental

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§ Action potentials if stimulus is strong (Fig. 12.13)

1. Threshold reached2. Depolarization

(sodium channels open)

3. Repolarization (Sodium channels close and K+ gates fully open)

4. Hyperpolarization5. Resting membrane

potential restores

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Figure 12.13a

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§ Action potentials vs. local potentials (Table 12.5)

Local Potentials Action Potentials

Local ?

Graded + reversible

?

Decremental ?

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Ionic base

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§ The refractory period of the action potential (AP)

Period of resistance to stimulation for another AP

• Absolute refractory period– as long as Na+ gates are open– no stimulus will trigger AP

• Relative refractory period– as long as K+ gates are open– only especially strong

stimulus will trigger new AP

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12.4B--Conduction of a nerve signal in an unmyelinated fiber

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• Where are ion gates?

• First action potential occurs at?

• The next action potential occurs at?

• Chain reactions continue until _____

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12.4C--Conduction of a nerve signal in a myelinated fiber

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§ Saltatory conduction (Fig. 12.17a-b)

A. Most (99%) of the voltage-regulated ion gates are at the _______________.– Slow but nondecremental

B. At the internodes– – nerve signals travel very ______ (diffusion

of ions) and decremental.

C. Most of the axon is covered with myelin (internodes)– nerve signal is faster at 120m/sec (than

unmyelinated ones (up to 2 m/sec)12-46

•Action potentials occurs only at the _________________•It is called saltatory conduction meaning _____________.

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