Transcript HA5_MM_ch12_3 - El Camino College

PowerPoint® Lecture Slides
prepared by Leslie Hendon,
University of Alabama,
Birmingham
12
HUMAN
ANATOMY
fifth edition
MARIEB | MALLATT | WILHELM
PART 3
Fundamentals
of the
Nervous
System and
Nervous
Tissue
Copyright © 2008 Pearson Education, Inc.,
publishing as Benjamin Cummings
Myelin Sheaths



Segmented structures composed of the lipoprotein
myelin
Surround thicker axons
Form an insulating layer
 Prevent leakage of electrical current

Increase the speed of impulse conduction
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Myelin Sheaths in the PNS



Formed by Schwann cells (neurolemmacytes)
Develop during fetal period and in the first year of
postnatal life
Schwann cells wrap in concentric layers around the
axon
 Cover the axon in a tightly packed coil of
membranes

Neurilemma
 Material external to myelin layers
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Myelin Sheaths in the PNS
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Figure 12.14a, b
Myelin Sheaths in the PNS
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Figure 12.14c, d
Myelin Sheaths in the PNS
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Figure 12.15a
Myelin Sheaths in the PNS

Nodes of Ranvier – gaps along axon
 Thick axons are myelinated
 Thin axons are unmyelinated
 Conduct impulses more slowly
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Myelin Sheaths in the CNS

Oligodendrocytes form the myelin sheaths in the
CNS
 Have multiple processes
 Coil around several different axons
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Figure 12.15b
Gray and White Matter in the CNS

Gray matter
 Is gray-colored and surrounds hollow central
cavities of the CNS
 Forms H-shaped region in the spinal cord
 Dorsal half contains cell bodies of interneurons
 Ventral half contains cell bodies of motor neurons
 Primarily composed of neuronal cell bodies,
dendrites, unmyelinated axons
 Surrounds white matter of CNS in cerebral cortex
and cerebellum
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Gray and White Matter in the CNS

White matter
 Lies external to the gray matter of the CNS
 Composed of myelinated axons
 Consists of axons passing between specific regions
of the CNS
 Tracts are bundles of axons traveling to similar
destinations
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Nerves

Nerves – cablelike organs in the PNS
 Consists of numerous axons wrapped in connective
tissue
 Axon is surrounded by Schwann cells

You see many nerves in lab
 Nerves of Brachial Plexus
 Radial, axillary, median, musculocutaneous, ulnar
 Nerves of lumbosacral plexus
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Nerves


Endoneurium – layer of delicate connective
tissue surrounding the axon
Perineurium – connective tissue wrapping
surrounding a nerve fascicle
 Nerve fascicles – groups of axons bound into
bundles

Epineurium – whole nerve is surrounded by
tough fibrous sheath
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Structure of a Nerve
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Figure 12.16a
Integration Between the PNS and CNS


The CNS and PNS are functionally interrelated
Nerves of the PNS
 Information pathways to and from body periphery
 Afferent PNS fibers respond to sensory stimuli
 Efferent PNS fibers transmit motor stimuli from
CNS to muscles and glands
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Integration Between the PNS and CNS

Nerves of the CNS
 Composed on interneurons that
 Process and receive sensory information
 Direct information to specific CNS regions
 Initiate appropriate motor responses
 Transport information from one area of the CNS to
another
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Reflex Arcs

Reflex arcs – simple chains of neurons
 Explain reflex behaviors
 Determine structural plan of the nervous system
 Responsible for reflexes
 Rapid, autonomic motor responses
 Can be visceral or somatic
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Five Essential Components to the Reflex Arc



Receptor – site where stimulus acts
Sensory neuron – transmits afferent impulses to
the CNS
Integration center – consists of one or more
synapses in the CNS
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Five Essential Components to the Reflex Arc


Motor neuron – conducts efferent impulses from
integration center to an effector
Effector – muscle or gland cell
 Responds to efferent impulses
 Contracting or secreting
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Five Essential Components to the Reflex Arc
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Figure 12.17
Types of Reflexes

Monosynaptic reflex
 Simplest of all reflexes
 Just one synapse
 The fastest of all reflexes
 Knee-jerk reflex
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Types of Reflexes

Polysynaptic reflex
 More common type of reflex
 Most have a single interneuron between the sensory
and motor neuron
 Withdrawal reflexes
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Types of Reflexes
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Figure 12.18a, b
Simplified Design of the Nervous System

Three-neuron reflex arcs
 Basis of the structural plan of the nervous system
 Similar reflexes are associated with the brain
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Simplified Design of the Nervous System

Sensory neurons – located dorsally
 Cell bodies outside the CNS in sensory ganglia
 Central processes enter dorsal aspect of the spinal
cord

Motor neurons – located ventrally
 Axons exit the ventral aspect of the spinal cord
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Simplified Design of the Nervous System

Interneurons – located centrally
 Synapse with sensory neurons
 Interneurons are neurons confined to CNS
 Long chains of interneurons between sensory and
motor neurons
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Simplified Design of the Nervous System
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 12.19
Neuronal Circuits



Diverging circuit – one presynaptic neuron
synapses with several other neurons (divergence)
Converging circuit – many neurons synapse on a
single postsynaptic neuron (convergence)
Reverberating circuit – circuit that receives
feedback via a collateral axon from a neuron in the
circuit
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Neuronal Circuits
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Figure 12.20
Input Processing – not in notes

Serial processing
 Neurons pass a signal to a specific destination
along a single pathway from one to another


Parallel processing
Input is delivered along many pathways; a single
sensory stimulus results in multiple perceptions
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Neural Processing
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Figure 12.21
Disorders of the Nervous System

Multiple sclerosis
 Common cause of neural disability
 An autoimmune disease
 Immune system attacks the myelin around axons in
the CNS
 Varies widely in intensity among those affected
 More women than men are affected
 When men are affected disease develops quicker and
is more devastating
 Cause is incompletely understood
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Nervous Tissue Throughout Life

Nervous system develops from the dorsal
ectoderm
 Invaginates to form the neural tube and neural crest
 Neural tube walls begin as neuroepithelial cells
 These cells divide and become neuroblasts
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Nervous Tissue Throughout Life
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Figure 12.22
Neuronal Regeneration


Neural injuries may cause permanent dysfunction
If axons alone are destroyed, cells bodies often
survive and the axons may regenerate
 PNS – macrophages invade and destroy axon distal
to the injury
 Axon filaments grow peripherally from injured site
 Partial recovery is sometimes possible
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Neuronal Regeneration
 CNS – neuroglia never form bands to guide regrowing axons and may hinder axon growth with
growth-inhibiting chemicals
 No effective regeneration after injury to the spinal
cord and brain
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Regeneration of the Peripheral Nerve Fiber
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Figure 12.23