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Transcript
7
The Nervous System
PART A
PowerPoint® Lecture Slide Presentation by Jerry L. Cook, Sam Houston University
ESSENTIALS
OF HUMAN
ANATOMY
& PHYSIOLOGY
EIGHTH EDITION
ELAINE N. MARIEB
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functions of the Nervous System
 Sensory input – gathering information
 To monitor changes occurring inside and
outside the body
 Changes = stimuli = sensory input
 Integration
 To process and interpret sensory input and
decide if action is needed
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functions of the Nervous System
 Motor output
 A response to integrated stimuli
 The response activates muscles or glands
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structural Classification of the Nervous
System
 Central nervous system (CNS)
 Brain
 Spinal cord
 Act as integrating & command centers (interpret sensory
input & issue commands)
 Peripheral nervous system (PNS)
 Nerve outside the brain and spinal cord
 Spinal nerves carry impulses to & from spinal cord
 Cranial nerves carry impulses to & from brain
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of the Peripheral
Nervous System
 Sensory (afferent) division
 Nerve fibers that carry information to the
central nervous system
Figure 7.1
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of the Peripheral
Nervous System
 Sensory fibers that carry impulses from skin,
skeletal muscle & joints = somatic sensory
fibers.
 Those that carry impulses from the visceral
organs = visceral sensory fibers
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of the Peripheral
Nervous System
 Motor (efferent) division
 Nerve fibers that carry impulses away
from the central nervous system to effector
organs, the muscles & glands
Figure 7.1
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of the Peripheral
Nervous System
 Motor (efferent) division
 Two subdivisions
 Somatic nervous system = voluntary
 Autonomic nervous system = involuntary
Figure 7.1
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of the Peripheral
Nervous System
 Autonomic nervous system = involuntary
 2 parts
 Sympathetic – mobilizes the body during
extreme situations – called the “fight or
flight” division
 Parasympathetic – allows us to “unwind”
& conserve energy – called the
craniosacral division
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Organization of the Nervous System
Figure 7.2
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue: Support Cells
(Neuroglia = literally “nerve glue”)
 Astrocytes
 Abundant, star-shaped cells
 Brace neurons;
anchor to capillaries
 Form barrier
between capillaries
and neurons
 Control the chemical
environment of
the brain
Figure 7.3a
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue: Support Cells
 Microglia
 Spider-like phagocytes
 Dispose of debris
(dead brain cells, bacteria)
 Ependymal cells
 Line cavities of the
brain and spinal cord
 Beating cilia
circulate cerebrospinal fluid
Figure 7.3b–c
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue: Support Cells
 Oligodendrocytes
 Produce myelin sheath around nerve fibers
in the central nervous system
 Unable to transmit nerve impulses
 Never lose their
ability to divide
 Most brain tumors
are gliomas tumors formed by
neuroglia
Figure 7.3d
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue: Support Cells
 Satellite cells
 Protect neuron cell bodies
 Schwann cells
 Form myelin sheath in the peripheral
nervous system
Figure 7.3e
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue: Neurons
 Neurons = nerve cells
 Cells specialized to transmit messages
 Major regions of neurons
 Cell body – nucleus and metabolic center of
the cell
 Processes – fibers that extend from the cell
body
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Anatomy
 Cell body
 Nissl substance
– specialized
rough
endoplasmic
reticulum
 Neurofibrils –
intermediate
cytoskeleton
that maintains
cell shape
Figure 7.4a
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Anatomy
 Cell body
 Nucleus
 Large nucleolus
 Lacks centrioles –
confirms amitotic
nature of most
neurons
Figure 7.4a–b
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Anatomy
 Extensions outside
the cell body – from
microscopic to 3-4
feet in length
 Dendrites –
conduct impulses
toward the cell
body – may
number in the
100’s
 Axons – conduct
impulses away
from the cell
body – only one
Figure 7.4a
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Axons and Nerve Impulses
 Axons end in axon terminals
 Axon terminals contain vesicles with
neurotransmitters – impulses stimulate their
release
 Axon terminals are separated from the next
neuron by a gap
 Synaptic cleft – gap between adjacent
neurons
 Synapse – junction between nerves
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nerve Fiber Coverings
 Schwann cells –
produce myelin
sheaths in jelly-roll
like fashion on axons
outside the CNS
 Neurilemma – part of
Schwann cell external
to myelin sheath
 Nodes of Ranvier –
gaps in myelin sheath
along the axon
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 7.5
Nerve Fiber Coverings
 Oligodendrocytes form CNS myelin sheaths
 CNS sheaths lack a neurilemma
 Because the neurilemma remains intact (for
the most part) when a peripheral nerve fiber is
damaged, it plays an important role in fiber
regeneration, an ability that is largely lacking
in the CNS
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Cell Body Location
 Most are found in the central nervous system
 Gray matter – cell bodies and unmylenated
fibers
 Nuclei – clusters of cell bodies within the
white matter of the central nervous system
 Ganglia – collections of cell bodies outside
the central nervous system
 Bundles of nerve fibers in the CNS are called
tracts; whereas in the PNS they are called
nerves
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of Neurons
 Sensory (afferent) neurons
 Cell bodies always found in the ganglion outside
the CNS
 Carry impulses from the sensory receptors to
CNS
 Cutaneous sense organs (skin)
 Pain receptors – bare dendrite endings –
most numerous
 Proprioceptors – detect stretch or tension
(muscles & tendons) – help maintain balance
& normal posture
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of Neurons
 Motor (efferent) neurons
 Carry impulses from the CNS to viscera,
&/or muscles & glands
 Cell bodies of motor neurons are always
located in the CNS
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Classification of Neurons
 Interneurons (association neurons)
 Found in neural pathways in the central
nervous system
 Connect sensory and motor neurons
 Cell bodies are always located in the CNS
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Classification
Figure 7.6
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structural Classification of Neurons
 Multipolar neurons – many extensions
from the cell body
 All motor & interneurons (association
neurons) are multipolar
 Most common
Figure 7.8a
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structural Classification of Neurons
 Bipolar neurons – one axon and one dendrite
 Rare in adults - found only in some special
sense organs (eye, nose)
Figure 7.8b
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structural Classification of Neurons
 Unipolar neurons – have a short single
process leaving the cell body
Figure 7.8c
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Structural Classification of Neurons
 Short, divides almost immediately into
proximal (central) & distal (peripheral)
processes
 Dendrite = small process at end of peripheral
process; remainder = axon
 Axon conducts impulses both away from &
toward cell body
 Sensory neurons found in PNS ganglia are
unipolar
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Functional Properties of Neurons
 Irritability – ability to respond to stimuli
 Conductivity – ability to transmit an impulse
 The plasma membrane at rest is polarized
 Fewer positive ions are inside the cell than
outside the cell
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Starting a Nerve Impulse
 Depolarization – a
stimulus depolarizes the
neuron’s membrane
 A deploarized membrane
allows sodium (Na+) to
flow inside the
membrane
 The exchange of ions
initiates an action
potential in the neuron
Figure 7.9a–c
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The Action Potential
 If the action potential (nerve impulse) starts,
it is propagated over the entire axon
 Potassium ions rush out of the neuron after
sodium ions rush in, which repolarizes the
membrane
 The sodium-potassium pump restores the
original configuration
 This action requires ATP
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Nerve Impulse Propagation
 The impulse
continues to move
toward the cell body
 Impulses travel faster
when fibers have a
myelin sheath
Figure 7.9d–f
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Continuation of the Nerve Impulse
between Neurons
 Impulses are able to cross the synapse to
another nerve
 Neurotransmitter is released from a
nerve’s axon terminal
 The dendrite of the next neuron has
receptors that are stimulated by the
neurotransmitter
 An action potential is started in the
dendrite
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
How Neurons Communicate at Synapses
Figure 7.10
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings