Unit 2
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
• Motor neurons: conduct impulses away from brain and
spinal cord to muscles and glands
• Interneurons: conduct impulses from sensory neurons to
motor neurons
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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
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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—
– 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
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Central Nervous System
• Divisions of the brain
– Brainstem
• Functions
–All three parts of brainstem are twoway conduction paths
–Many important reflex centers lie in
the brainstem
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Central Nervous System
Structure and function of the hypothalamus
– Consists mainly of the posterior pituitary
gland
– 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
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Central Nervous System
– Cerebrum
• Largest part of the human brain
• Composed mainly of dendrites and cell bodies of
neurons
• 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
– 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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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
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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
• regulates body’s automatic or
involuntary functions
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Autonomic Nervous System
• Composed of two divisions: the sympathetic
system and the parasympathetic system
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Autonomic Nervous System
– Functions of the sympathetic nervous
system
• Serves as the emergency or stress system,
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
– Function—dominates control under normal,
everyday conditions
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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
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