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Copyright 2010, John Wiley & Sons, Inc.

Chapter 9

Nervous Tissue


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End of Chapter 9

Copyright 2010 John Wiley & Sons, Inc.

All rights reserved. Reproduction or translation of this

work beyond that permitted in section 117 of the 1976

United States Copyright Act without express permission

of the copyright owner is unlawful. Request for furtherinformation should be addressed to the Permission

Department, John Wiley & Sons, Inc. The purchaser may

make back-up copies for his/her own use only and not

for distribution or resale. The Publishers assumes noresponsibility for errors, omissions, or damages caused

by the use of theses programs or from the use of the

information herein.


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Structures of the Nervous System

Brain: neurons enclosed within skull

Spinal cord:connects to brain and enclosedwithin spinal cavity

Nerves: bundles of many axons of neurons

Cranial nerves (12 pairs) emerge from brain Spinal nerves (31 pairs) emerge from spinal cord

Ganglia: groups of neuron cell bodies locatedoutside of brain and spinal cord

Enteric plexuses:networks in digestive tract Sensory receptors:monitor changes in internal

or external environments


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Structures of the Nervous System


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Functions of the Nervous System

Sensory receptors and sensory nerves Carry information into brain and spinal cord

Integration: information processing

Perception = awareness of sensory input

Analyzing and storing information to help lead to

appropriate responses

Motor activity: efferent nerves

Signals to muscles and glands (effectors)


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Organization of the Nervous System

Central Nervous System (CNS) Brain and spinal cord

Peripheral Nervous System (PNS)

All nervous system structures outside of the CNS


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Histology of the Nervous System

Neurons Can respond to stimuli and convert stimuli to

electrical signals (nerve impulses) that travel along

neurons

Neuroglia cells: support, nourish and protectneurons

Neuroglia critical for homeostasis of interstitial fluid

around neurons


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Neuronal Structure

Cell body: nucleus, cytoplasm with typicalorganelles

Dendrites: highly branched structures that

carry impulses to the cell body

Axon: conducts away from cell body toward

another neuron, muscle or gland

Emerges at cone-shaped axon hillock

Axon terminals: contain synaptic vesiclesthat can release neurotransmitters


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Neuronal

Structure


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Structural Classes of Neurons

Multipolar

Have several or many dendrites and one axon

Most common type in brain and spinal cord

Bipolar

Have one dendrite and one axon

Example: in retina of eye and inner ear

Unipolar

Have fused dendrite and axon Sensory neurons of spinal nerves


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Functional Classes of Neurons

Sensory (afferent)

Convey impulses into CAN (brain or spinal cord)

Motor (efferent)

Convey impulses from brain or spinal cord out

through the PNS to effectors (muscles or glands)

Interneurons (association neurons)

Most are within the CNS

Transmit impulses between neurons, such asbetween sensory and motor neurons


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Neuroglia

Cells smaller but much more numerous thanneurons

Can multiply and divide and fill in brain areas

Gliomas: brain tumors derived from neuroglia

Functions

Do not conduct nerve impulses

Do support, nourish and protect neurons


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Neuroglia

Astrocytes: help form blood brain barrier

Oligodendrocytes: produce myelin in CNS

Microglia: protect CNS cells from disease

Ependymal cells: form CSF in ventricles

Schwann: produce myelin around PNS

neurons; help to regenerate PNS axons

Satellite cells: support neurons in PNS ganglia


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Myelination

Axons covered with a myelin sheath Many layers of lipid and protein: insulates neurons

Increases speed of nerve conduction

Appears white (in white matter)

Nodes of Ranvier: gaps in the myelin

Nodes are important for rapid signal conduction

Some diseases destroy myelin:

Multiple sclerosis

Tay-Sachs


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Collections of Nervous Tissue

Clusters of neuron cell bodies

Ganglion: cluster of cell bodies in PNS

Nucleus: cluster of cell bodies in CNS

Bundles of axons

Nerve: bundle of axons in PNS

Tract: bundle to axons in CNS


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Gray and White Matter

White matter: primarily myelinated axons

Gray matter: cell bodies, dendrites,

unmyelinated axons, axon terminals, neuroglia

Locations of gray and white matter

Spinal cord: white matter (tracts) surround centrally

located gray matter H of butterfly

Brain: gray matter in thin cortex surrounds white

matter (tracts)


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Neuron Regeneration

Regeneration of PNS neuronsAxons and dendrite in the PNS can be repaired if

cell body is intact and Schwann cells functional.

These form a regeneration tube and grow axons

or dendrites if scar tissue does not fill the tube Regeneration of CNS neurons

Very limited even if cell body is intact

Inhibited by neuroglia and by lack of fetal growth-stimulators


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Central nervous system (CNS) structures: Brain

Spinal cord

Peripheral nervous system (PNS) structures: Cranial nerves and branches

Spinal nerves and branches

Ganglia

Sensory receptors


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Peripheral nervous system (PNS) divisions Somatic (SNS)

Sensory neurons from head, body wall, limbs, special

sense organs

Motor neurons to skeletal muscle: voluntary

Autonomic (ANS) nervous systems Sensory neurons from viscera

Motor neurons to viscera (cardiac muscle, smooth

muscle, glands): involuntary

Sympathetic: fight-or-flight

Parasympathetic: rest-and-digest

Enteric nervous system (ENS): brain of the gut


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Peripheral nervous system (PNS), Enteric nervous system (ENS): brain of the gut

Sensory neurons monitor chemical changes and

stretching of GI wall

Motor neurons regulate contractions, secretions and

endocrine secretions (involuntary)


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Structure and Function of the Nervous SystemInteractions Animation

Introduction to Structure and Function of the

Nervous System

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Action Potentials

Action potentials = nerve impulses

Require

A membrane potential: a charge difference across

cell membrane (polarization)

Ion channels: allow ions to move by diffusion fromhigh to low concentration

Leakage channels: allow ions to leak through membrane;

there are more for K+than for Na+

Gated channels Open and close on command

Respond to changes in membrane so can generate and

conduct action potentials


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Resting Membrane Potential Typically70 mV

Inside of membrane more negative than outside

Caused by presence of ions: Inside (more negative) because cytosol has:

Many negative ions (too large to leak out): amino acids

(in cellular proteins) and phosphates (as in ATP)

K+ that easily leaks out through many K+ channels

Outside (more positive) because interstitial fluidhas:

Few negative ions Na+that does not leak out of cell: few Na+ channels

Membrane pumps that quickly pump out Na+ thatdoes leak (diffuse) into cell


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Resting Membrane Potential


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Action Potential

Series of events that activate cell membrane

in neuron or muscle fiber

An initial event (stimulus) is required

Triggers resting membrane to become more

permeable to Na+

Causes enough Na+to enter cell so that cell

membrane reaches threshold (~55 mv)

If so, the following events occur: action potentialwhich spreads along neuron or muscle fiber


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Action Potential Depolarizing phase

Na+channels openas more Na+enters cell,

membrane potential rises and becomes positive

(700+ 30 mv)

Repolarizing phase K+channels openas more K+leave cell,

membrane potential is returned to resting value

(+ 30070 mv)

May overshoot: hyperpolarizing phase

Typically depolarization and repolarization take

place in about 1 millisecond (1/1000 sec)


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Action Potential


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Action Potential

Recovery

Levels of ions back to normal by action of Na+/K+

pump

Refractory period (brief): even with adequate

stimulus, cell cannot be activated All-or-none principle

If a stimulus is strong enough to cause

depolarization to threshold level, the impulse will

travel the entire length of the neuron at a constant

and maximum strength.


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Membrane PotentialsInteractions Animations

Membrane Potentials

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Conduction of Nerve Impulses

Nerve impulse conduction (propagation) Each section triggers the next locally as even more

Na+channels are opened (like row of dominos)

Types of conduction

Continuous conduction In unmyelinated fibers; slower form of conduction

Saltatory conduction In myelinated fibers; faster as impulses leap between

nodes of Ranvier

Factors that increase rate of conduction Myelin, large diameter and warm nerve fibers


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Conduction

of Nerve

Impulses


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Conduction

of Nerve

Impulses


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Synaptic Transmission

Similar sequence of events occurs at Synapse (neuron-neuron)

Neuromuscular junction (neuron-muscle fiber:chapter 8)

Neuroglandular junction (neuron-gland)

Triggered by action potential (nerve impulse)

Components of synapse:

Sending neuron: presynaptic neuron (releases

neurotransmitter) Space between neurons: synaptic cleft

Receiving neuron: postsynaptic neuron


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Action potential arrives at presynaptic

neurons end bulb

Opens voltage gated Ca2+channelsCa2+

flows into presynaptic cytosol

Increased Ca2+concentrationexocytosis

of synaptic vesicles

Neurotransmitter (NT) released into cleft

NT diffuses across cleft and binds toreceptors in postsynaptic cell membrane


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NT serves as chemical trigger (stimulus) of

ion channels

Postsynaptic cell membrane may bedepolarized or hyperpolarized

Depends on type of NT and type of postsynapticcell

1000+ neurons converge on synapse; the sum ofall of their NTs determines effect

If threshold reached, then postsynaptic cellaction potential results


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One-way transmission only because

Only presynaptic cells release NT

Only postsynaptic cells have receptors for NT

binding

Finally, NT must be removed from the cleft.Three possible mechanisms

Diffusion out of cleft

Destruction by enzymes (such as ACh-ase) in cleft

Transport back (recycling) into presynaptic cell


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Copyright 2010, John Wiley & Sons, Inc.Copyright 2009 John Wiley & Sons, Inc. 38

Signal Transmission at the Chemical

Synapse


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Neurotransmitters

Acetylcholine (ACh): common in PNS Stimulatory (on skeletal muscles)

Inhibitory (on cardiac muscle)

Amino acids

Glutamate, aspartate, gamma aminobutyric acid(GABA), glycine

Modified amino acids

Norepinephrine (NE), dopamine (DA), serotonin

Neuropeptides such as endorphins

Nitric oxide (NO)


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End of Chapter 9

Copyright 2010 John Wiley & Sons, Inc.

All rights reserved. Reproduction or translation of this

work beyond that permitted in section 117 of the 1976

United States Copyright Act without express permission

of the copyright owner is unlawful. Request for furtherinformation should be addressed to the Permission

Department, John Wiley & Sons, Inc. The purchaser may

make back-up copies for his/her own use only and not

for distribution or resale. The Publishers assumes noresponsibility for errors, omissions, or damages caused

by the use of theses programs or from the use of the

information herein.

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