Chapter 9
The Nervous System
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Objectives
• List the organs and divisions of the nervous
system and describe the generalized
functions of the system as a whole
• Identify the major types of cells in the nervous
system and discuss the function of each
• Identify the anatomical and functional
components of a three-neuron reflex arc.
Compare and contrast the propagation of a
nerve impulse along a nerve fiber and across
a synaptic cleft.
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Objectives
• Identify the major anatomical components of
the brain and spinal cord and briefly comment
on the function of each
• Identify and discuss the coverings and fluid
spaces of the brain and spinal cord
• Compare and contrast spinal and cranial
nerves
• Discuss the structure and function of the two
divisions of the autonomic nervous system
• Describe major nervous system disorders
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Organs and Divisions
of the Nervous System
• Central nervous system (CNS)—brain
and spinal cord
• Peripheral nervous system (PNS)—all
nerves
• Autonomic nervous system (ANS)
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Cells of the Nervous System
• Neurons
– Consist of three parts
• Cell body of neuron—main part
• Dendrites—branching projections that conduct
impulses to cell body of neuron
• Axon—elongated projection that conducts
impulses away from cell body of neuron
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Cells of the Nervous System
• Neurons
– Neurons classified according to function or
direction of impulse
• Sensory neurons: conduct impulses to the spinal cord
and brain; also called afferent neurons
• Motor neurons: conduct impulses away from brain and
spinal cord to muscles and glands; also called efferent
neurons
• Interneurons: conduct impulses from sensory neurons to
motor neurons; also called central or connecting neurons
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Cells of the Nervous System
• Glia (neuroglia)
– Support cells, bringing the cells of nervous
tissue together structurally and functionally
– Three main types of connective tissue cells
of the CNS
• Astrocytes—star-shaped cells that anchor small blood
vessels to neurons
• Microglia—small cells that move in inflamed brain tissue
carrying on phagocytosis
• Oligodendrocytes—form myelin sheaths on axons in the
CNS (Schwann cells form myelin sheaths in PNS only) 9
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Cells of the Nervous System
• Disorders of nervous tissue
– Multiple sclerosis—characterized by myelin
loss in central nerve fibers and resulting
conduction impairments
– Tumors
• General name for nervous system tumors is
neuroma
• Most neuromas are gliomas, glial tumors
• Multiple neurofibromatosis—characterized by
numerous benign tumors
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Nerves
• Nerve—bundle of peripheral axons
– Tract—bundle of central axons
– White matter—brain or cord tissue
composed primarily of myelinated axons
(tracts)
– Gray matter—brain or cord tissue
composed primarily of cell bodies and
unmyelinated fibers
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Nerves
• Nerve coverings—fibrous connective
tissue
– Endoneurium—surrounds individual fibers
within a nerve
– Perineurium—surrounds a group (fascicle)
of nerve fibers
– Epineurium—surrounds the entire nerve
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Reflex Arcs
• Nerve impulses are conducted from receptors to
effectors over neuron pathways or reflex arcs;
conduction by a reflex arc results in a reflex (i.e.,
contraction by a muscle or secretion by a gland)
• The simplest reflex arcs are two-neuron arcs—
consisting of sensory neurons synapsing in the spinal
cord with motor neurons; three-neuron arcs consist of
sensory neurons synapsing in the spinal cord with
interneurons that synapse with motor neurons
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Nerve Impulses
• Definition—self-propagating wave of
electrical disturbance that travels along
the surface of a neuron membrane;
sometimes called action potentials
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Nerve Impulses
• Mechanism
– At rest, the neuron’s membrane is slightly
positive on the outside—polarized—from a
slight excess of Na+ on the outside
– A stimulus triggers the opening of Na+
channels in the plasma membrane of the
neuron
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Nerve Impulses
• Mechanism
– Inward movement of Na+ depolarizes the
membrane by making the inside more positive
than the outside at the stimulated point; this
depolarization is a nerve impulse (action potential)
– The stimulated section of membrane immediately
repolarizes, but by that time the depolarization has
already triggered the next section of membrane to
depolarize, thus propagating a wave of electrical
disturbances (depolarizations) all the way down
the membrane
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The Synapse
• Definition—the place where impulses are
transmitted from one neuron to another (the
postsynaptic neuron)
• Synapse made of three structures—synaptic
knob, synaptic cleft, and plasma membrane
• Neurotransmitters bind to specific receptor
molecules in the membrane of a postsynaptic
neuron, opening ion channels and thereby
stimulating impulse conduction by the
membrane
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The Synapse
• Names of neurotransmitters—acetylcholine,
catecholamines (norepinephrine, dopamine,
and serotonin), endorphins, enkephalins,
nitric oxide (NO), and other compounds
• Parkinson disease (PD)—characterized by
abnormally low levels of dopamine in motor
control areas of the brain; patients usually
exhibit involuntary trembling and muscle
rigidity
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Central Nervous System
• Divisions of the brain
– Brainstem
• Consists of three parts, named in ascending
order: medulla oblongata, pons, and midbrain
• Structure—white matter with bits of gray matter
scattered through it
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Central Nervous System
• Divisions of the brain
– Brainstem
• Functions
– All three parts of brainstem are two-way conduction
paths
– Sensory tracts in the brainstem conduct impulses to
the higher parts of the brain
– Motor tracts conduct from the higher parts of the
brain to the spinal cord
– Many important reflex centers lie in the brainstem
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Central Nervous System
– Diencephalon
• Structure and function of the hypothalamus
– Consists mainly of the posterior pituitary gland,
pituitary stalk, and gray matter
– Acts as the major center for controlling the ANS;
therefore, helps control the functioning of most
internal organs
– Controls hormone secretion by anterior and posterior
pituitary glands; therefore, it indirectly helps control
hormone secretion by most other endocrine glands
– Contains centers for controlling appetite,
wakefulness, pleasure, etc.
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Central Nervous System
• Structure and function of the thalamus
– Dumbbell-shaped mass of gray matter extending into
each cerebral hemisphere
– Relays sensory impulses to cerebral cortex sensory
areas
– In some way produces the emotions of pleasantness
or unpleasantness associated with sensations
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Central Nervous System
– Cerebellum
• Second largest part of the human brain
• Helps control muscle contractions to produce
coordinated movements so that we can
maintain balance, move smoothly, and sustain
normal postures
• Recent evidence shows the coordinating
effects of the cerebellum may be more
extensive, also assisting the cerebrum and
other regions of the brain
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Central Nervous System
– Cerebrum
• Largest part of the human brain
• Outer layer of gray matter is the cerebral cortex;
made up of lobes; composed mainly of dendrites and
cell bodies of neurons
• Interior of the cerebrum composed mainly of white
matter (i.e., nerve fibers arranged in bundles called
tracts)
• Functions of the cerebrum—mental processes of all
types, including sensations, consciousness, memory,
and voluntary control of movements
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Central Nervous System
• Brain disorders
– Destruction of brain tissue
• Cerebrovascular accident (CVA)—hemorrhage
from or cessation of blood flow through cerebral
blood vessels; a “stroke”
• Cerebral palsy (CP)—condition in which
damage to motor control areas of the brain
before, during, or shortly after birth causes
paralysis (usually spastic) of one or more limbs
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Central Nervous System
• Brain disorders
– Dementia—progressive loss of memory, shortened
attention span, personality changes, reduced
intellectual capacity, motor control deficit
• Alzheimer disease (AD)—brain disorder of the middle
and late adult years characterized by dementia
• Huntington disease (HD)—inherited disorder
characterized by chorea (purposeless movement)
progressing to severe dementia
• HIV (also causes AIDS) can infect neurons and thus
cause dementia
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Central Nervous System
• Brain disorders
– Seizure disorders
• Seizure—sudden burst of abnormal neuron
activity that results in temporary changes in
brain function
• Epilepsy—many forms, all characterized by
recurring seizures
• Electroencephalogram—graphic representation
of voltage changes in the brain used to
evaluate brain activity
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Central Nervous System
• Spinal cord
– Columns of white matter, composed of bundles of myelinated
nerve fibers, form the outer portion of the H-shaped core of the
spinal cord; bundles of axons called tracts
– Interior composed of gray matter made up mainly of neuron
dendrites and cell bodies (Figure 9-18)
– Spinal cord tracts provide two-way conduction paths—
ascending and descending
– Spinal cord functions as the primary center for all spinal cord
reflexes; sensory tracts conduct impulses to the brain, and
motor tracts conduct impulses from the brain
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Central Nervous System
• Coverings and fluid spaces of the brain and
spinal cord
– Coverings
• Cranial bones and vertebrae
• Cerebral and spinal meninges—the dura mater,
arachnoid mater, and the pia mater
– Fluid spaces
• Subarachnoid spaces of meninges
• Central canal inside cord
• Ventricles in brain
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Peripheral Nervous System
• Cranial nerves
– 12 pairs—attached to undersurface of the brain
– Connect brain with the neck and structures in the
thorax and abdomen
• Spinal nerves
– 31 pairs—contain dendrites of sensory neurons
and axons of motor neurons
– Conduct impulses necessary for sensations and
voluntary movements
– Skin surface area supplied by a single nerve is
called a dermatome
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Peripheral Nervous System
• Peripheral nerve disorders
– Neuritis—general term referring to nerve
inflammation
• Sciatica is inflammation of the sciatic nerve that
innervates the legs
• Neuralgia, or muscle pain, often accompanies
neuritis
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Peripheral Nervous System
• Peripheral nerve disorders
– Trigeminal neuralgia—recurring episodes
of stabbing pain along one or more
branches of the trigeminal (fifth cranial)
nerve in the head
– Bell palsy—paralysis of facial features
resulting from damage to the facial
(seventh cranial) nerve
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Peripheral Nervous System
• Peripheral nerve disorders
– Herpes zoster or shingles
• Viral infection caused by chickenpox virus that
has invaded the dorsal root ganglion and
remained dormant until an episode of shingles
• Usually affects a single dermatome, producing
characteristic painful plaques or vesicles
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Autonomic Nervous System
• Autonomic nervous system—motor neurons that
conduct impulses from the central nervous system to
cardiac muscle, smooth muscle, and glandular
epithelial tissue; regulates body’s automatic or
involuntary functions
• Autonomic neurons—preganglionic autonomic
neurons conduct from spinal cord or brainstem to an
autonomic ganglion; postganglionic neurons conduct
from autonomic ganglia to cardiac muscle, smooth
muscle, and glandular epithelial tissue
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Autonomic Nervous System
• Autonomic or visceral effectors—tissues
to which autonomic neurons conduct
impulses (i.e., cardiac and smooth
muscle and glandular epithelial tissue)
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Autonomic Nervous System
• Composed of two divisions: the sympathetic
system and the parasympathetic system
• Autonomic conduction paths
– Consist of two-neuron relays (i.e., preganglionic
neurons from the CNS to autonomic ganglia,
synapses, postganglionic neurons from ganglia to
visceral effectors)
– In contrast, somatic motor neurons conduct all the
way from the CNS to somatic effectors with no
intervening synapses
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Autonomic Nervous System
• Sympathetic nervous system
– Dendrites and cell bodies of sympathetic
preganglionic neurons located in gray matter of
thoracic and upper lumbar segments of spinal
cord
– Axons leave spinal cord in the anterior roots of
spinal nerves, extend to sympathetic or collateral
ganglia, and synapse with several postganglionic
neurons whose axons extend to spinal or
autonomic nerves to terminate in visceral effectors
– A chain of sympathetic ganglia is in front of and at
each side of the spinal column
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Autonomic Nervous System
– Functions of the sympathetic nervous
system
• Serves as the emergency or stress system,
controlling visceral effectors during strenuous
exercise and when strong emotions (anger,
fear, hate, or anxiety) are elicited
• Group of changes induced by sympathetic
control is called the fight-or-flight response
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Autonomic Nervous System
• Parasympathetic nervous system
– Structure
• Parasympathetic preganglionic neurons have dendrites
and cell bodies in the gray matter of the brainstem and
the sacral segments of spinal cord
• Parasympathetic preganglionic neurons terminate in
parasympathetic ganglia located in the head and the
thoracic and abdominal cavities close to visceral
effectors
• Each parasympathetic preganglionic neuron synapses
with postganglionic neurons to only one effector
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Autonomic Nervous System
• Parasympathetic nervous system
– Function—dominates control of many
visceral effectors under normal, everyday
conditions
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Autonomic Nervous System
• Autonomic neurotransmitters
– Cholinergic fibers—preganglionic axons of
parasympathetic and sympathetic systems
and parasympathetic postganglionic axons
release acetylcholine
– Adrenergic fibers—axons of sympathetic
postganglionic neurons release
norepinephrine (noradrenaline)
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Autonomic Nervous System
• Autonomic nervous system as a whole
– Regulates the body’s automatic functions
in ways that maintain or quickly restore
homeostasis
– Many visceral effectors are doubly
innervated (i.e., they receive fibers from
parasympathetic and sympathetic divisions
and are influenced in opposite ways by the
two divisions)
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Autonomic Nervous System
• Disorders of the autonomic nervous
system
– Stress-induced disease
• Prolonged or excessive response to stress can
disrupt normal functioning throughout the body
• Examples of stress-induced conditions include
heart disease, digestive problems, and reduced
resistance to disease
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Autonomic Nervous System
• Disorders of the autonomic nervous
system
– Neuroblastoma—highly malignant tumor of
the sympathetic nervous system, primarily
affecting young children
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