Nervous System Corresponding textbook pages: 436-440, 442-454, 456-459

Nervous System

• Function:

– Maintain coordination through the use of electrical and chemical processes.

• Characteristics:

– Excitability

– Conductivity

– Secretion

• Divisions

– Central vs. Peripheral Nervous System

– Somatic vs. Autonomic Nervous System

Neurons

• “Functional” Cell of the nervous system

• Extremely excitable, and excite other cells

• Types:

– Sensory

– Motor

– Interneurons

Fig. 12.3

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1

2

3

Peripheral nervous system Central nervous system

Sensory (afferent)

neurons conduct

signals from

receptors to the CNS.

Motor (efferent)

neurons conduct

signals from the CNS

to effectors such as

muscles and glands.

Interneurons

(association

neurons) are

confined to

the CNS.

Neuron Anatomy

• Note: These structures are typical of a motor neuron, sensory neurons can be shaped slightly different.

• Soma or cell body

• Dendrites

• Axon hillock

• Axon

• Terminal Arborization

• Synaptic Knobs

• Nissl Bodies

Fig. 12.5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Dendrites

Dendrites

Dendrites

Axon

Axon

Dendrites

Axon

Unipolar neuron

Multipolar neurons

Bipolar neurons

Anaxonic neuron

Neuroglial cells

• Oligodendrocytes

• Ependymal Cells

• Microglia

• Astrocytes

• Satellite Cells

• Schwann cells

Schwann cells

• Coat entire axon

• Found in the PNS

• Myelin Sheath

– Myelin

• Neurilemma

• Nodes of Ranvier

Fig. 12.4c

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Myelin sheath

Axolemma

Axoplasm

Neurilemma

(c)

Schwann cell

nucleus

Fig. 12.9-5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Endoneurium Myelin sheath

Local trauma

Muscle fiber

Macrophages

Growth processes

Degenerating axon

Schwann cells

Growth processes

Normal nerve fiber

Injured fiber

Degeneration of severed fiber

Early regeneration

Late regeneration

1

2

3

4

5

Neuromuscular

junction

Degenerating

terminal

Degenerating

Schwann cells

Regeneration

tube Atrophy of

muscle fibers

Retraction of

growth processes

Fig. 12.6

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Ependymal cell

Cerebrospinal fluid

Neurons

Astrocyte

Perivascular feet

Microglia

Oligodendrocyte

Capillary

Myelinated axon

Myelin (cut)

Nerve Impulse

Membrane/Electrical Potential

A difference in the concentration of charged particles across the mebrane.

Potential for energy via a current.

Polarized: if a cell has potential it is called polarized.

Resting Potential

When a nerve impulse is not conducting an impulse. Potential is usually -70mV.

Stimulus

Anything that changes a resting potential.

Threshold Potential

The critical voltage point. Need to reach this point in order to send an impulse.

Nerve Impulse

Depolarization Membrane Potential shifts away, becomes less negative from

the resting potential. There a decrease in difference in concentration of charged particles.

Repolarization Membrane Potential shifts back to resting potential. There is

an increase in difference in concentration of charged particles. Counteraction to Depolarization.

Hyperpolarization Opposite of Depolarization. There is a greater increase in

difference between concentrations

Action Potential A complete cycle of Depolarization and Repolarization in

response to a very strong stimulus.

Steps in an Action Potential

• An adequate stimulus is applied to the neuron. Threshold potential (-55mV) is reached.

• Sodium channels open and Na flows into the axon causing the membrane to depolarize.

• As the depolarizing occurs, more Na channels open and more Na flows in. Membrane depolarizes even more.

• At a certain voltage (+35mV) the Na channels start closing.

Steps in an Action Potential

Once +35mV is reached, there is a reversal of polarity and repolarization begins.

Potassium channels fully open and K rushes out of the cell. This counter acts the movement of Na.

The inside of the cell again becomes less positive.

Repolarization is aided by Na/K pumps. The pump removes 3 Na from the cell and brings in 2 K.

Hyperpolarization occurs.

Finally go back to resting state.

Key Points

• One action potential triggers another, like a domino effect.

• Action potentials originate in the axon hillock and travel down the axon.

• All or nothing event. Either the change reaches threshold or it doesn’t.

• Not reversible!

• Key players: Na and K

• Key voltages: -70mV, -55mV, +35mV

Steps in an Action Potential

• Action Potential

Fig. 12.13a

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Time

–70

Depolarization Repolarization

Hyperpolarization

Threshold mV

+35

0

–55

(a)

7

2

6

3

4

5

1

Local

potential

Resting membrane

potential

Action

potential

Nerve Conduction

• Local Potential

– Short range change, reversible effects.

• Saltatory Conduction

– Conduction via jumping and skipping.

• Refractory Period

– Absolute Refractory

– Relative refractory

Fig. 12.15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Threshold

mV

Time

+35

–55

–70

0

Absolute

refractory

period

Relative

refractory

period

Resting membrane

potential

Fig. 12.17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

+ + + +

+ +

+ +

+ +

+ +

+ + + +

+ +

+ +

+ +

+ +

+ + + +

+ +

+ +

+ +

+ +

– – – –

– – – –

– – – –

– – – –

– – – –

+ +

+ +

– – – –

+ +

+ +

– – – –

– – – –

+ +

+ +

– – – –

– –

– –

– –

– –

– –

– –

(a)

(b)

Na+inflow at node

generates action potential

(slow but nondecremental)

Na+ diffuses along inside

of axolemma to next node

(fast but decremental)

Excitation of voltage-

regulated gates will

generate next action

potential here

+ +

+ +

– – – –

+ +

+ +

– – – –

+ +

+ +

– – – –

+ +

+ +

– – – –

+ +

+ +

– – – –

+ +

+ +

– – – –

Action potential

in progress

Refractory

membrane

Excitable

membrane

Synapse

• Region where a neuron carries info toward another structure like a muscle or a gland.

• 3 components:

– Axon (Pre-synaptic structure)

– Synaptic cleft

– Post-synaptic structure

• Release of neurotransmitters.

• One neuron can be both a pre and post-synaptic structure.

Fig. 12.20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Axon of presynaptic neuron

Postsynaptic neuron

Postsynaptic neuron

Mitochondria

Synaptic cleft

Synaptic knob

Microtubules

of cytoskeleton

Synaptic vesicles

containing neurotransmitter

Neurotransmitter

receptor

Neurotransmitter

release

Testing Your Recall on page 472

Questions #1-4, 7, 11-16