Download Chapter 8 The Nervous System

Chapter 8
The Nervous System
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ORGANS AND DIVISIONS OF THE
NERVOUS SYSTEM (Figure 8-1)
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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
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Neurons
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Consist of three main parts—dendrites; cell body of neuron;
axon (Figure 8-2)
• 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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Neurons classified according to function
• Sensory (afferent) neurons—conduct impulses to the spinal cord
and brain
• Motor (efferent) neurons—conduct impulses away from brain and
spinal cord to muscles and glands
• Interneurons—conduct impulses from sensory neurons to motor
neurons or among a network of interneurons; also known as
central or connecting neurons
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CELLS OF THE NERVOUS
SYSTEM (cont.)
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Glia (neuroglia)
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Support cells, bringing the cells of nervous tissue
together structurally and functionally
 Three main types of glial cells of the CNS (Figure 8-3)
• 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
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Schwann cells form myelin sheaths on axons of the
PNS (Figure 8-2)
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NERVES AND TRACTS
(Figure 8-4)
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Nerve—bundle of peripheral axons
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Tract—bundle of central axons
 White matter—tissue composed primarily of
myelinated axons (nerves or tracts)
 Gray matter—tissue composed primarily of cell
bodies and unmyelinated fibers
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Nerve coverings—fibrous connective tissue
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Endoneurium—surrounds individual fibers within a
nerve
 Perineurium—surrounds a group (fascicle) of nerve
fibers
 Epineurium—surrounds the entire nervea
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REFLEX ARCS
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Nerve impulses are conducted from receptors
to effectors over neuron pathways or reflex arcs;
conduction by a reflex arc results in a reflex
(that is, 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 (Figure 8-5)
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NERVE IMPULSES
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Definition—self-propagating wave of electrical disturbance that
travels along the surface of a neuron membrane (also called
action potential)
Mechanism
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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
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
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Definition—the place where impulses are transmitted
from one neuron to another (the postsynaptic neuron)
(Figure 8-7)
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
Names of neurotransmitters—acetylcholine,
catecholamines (norepinephrine, dopamine, and
serotonin), endorphins, enkephalins, nitric oxide
(NO), and other compounds
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CENTRAL NERVOUS SYSTEM
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Divisions of the brain (Figure 8-9 and Table 8-1)
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Brainstem
• Consists of three parts of brain; named in ascending order:
the medulla oblongata, pons, and midbrain
• Structure—white matter with bits of gray matter scattered
through it
• Functions
 All three parts of brainstem conduct impulses to the
higher parts of the brain
– 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
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CENTRAL NERVOUS SYSTEM (cont.)
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Divisions of the brain (cont.)
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Cerebellum
• Structure
Second largest part of the human brain
 Gray matter outer layer is thin but highly folded, forming a
large surface area for processing information
 Arbor vitae—internal, treelike network of white matter tracts
• Function
 Helps control muscle contractions to produce coordinated
movements for maintaining balance, moving smoothly, and
sustaining normal postures
 Variety of additional coordinating effects, assisting the
cerebrum and other regions of the brain

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CENTRAL NERVOUS SYSTEM (cont.)
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Divisions of the brain (cont.)
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Diencephalon
• Hypothalamus
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Consists mainly of the posterior pituitary gland,
pituitary stalk, and gray matter
Acts as the major center for controlling the ANS;
therefore, it 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 body temperature,
appetite, wakefulness, and pleasure
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CENTRAL NERVOUS SYSTEM (cont.)
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Divisions of the brain (cont.)
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Diencephalon (cont.)
• Thalamus
Dumbbell-shaped mass of gray matter extending toward
each cerebral hemisphere
 Relays sensory impulses to cerebral cortex sensory areas
 In some way produces the emotions of pleasantness or
unpleasantness associated with sensations
• Pineal gland (pineal body)
 Small body resembling a pine nut behind the thalamus
 Adjusts output of “time-keeping hormone” melatonin in
response to changing levels of external light (sunlight and
moonlight)

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CENTRAL NERVOUS SYSTEM (cont.)
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Divisions of the brain (cont.)
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Cerebrum
• Largest part of the human brain
• Outer layers of gray matter are 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
 Tracts—nerve fibers arranged in bundles
 Basal nuclei—islands of gray matter regulate
automatic movements and posture
• 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 (cont.)
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Spinal cord (Figure 8-11)
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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
 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 (cont.)
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Coverings and fluid spaces of the brain and spinal
cord
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Coverings (Figure 8-13)
• Cranial bones and vertebrae
• Cerebral and spinal meninges—the dura mater, the
pia mater, and the arachnoid mater
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Fluid spaces
• Subarachnoid spaces of meninges
• Central canal inside cord
• Ventricles in brain
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PERIPHERAL NERVOUS SYSTEM
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Cranial nerves (Figure 8-16 and Table 8-2)
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Twelve pairs—attached to undersurface of the brain
 Connect brain with the neck and structures in the thorax
and abdomen
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Spinal nerves
Thirty-one pairs—contain dendrites of sensory neurons
and axons of motor neurons
 Conduct impulses necessary for sensations and voluntary
movements
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Dermatome—skin surface area supplied by a single
cranial or spinal nerve
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AUTONOMIC NERVOUS SYSTEM
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Functional anatomy
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Autonomic nervous system—motor neurons that conduct
impulses from the central nervous system to cardiac muscle,
smooth muscle, and glandular epithelial tissue; regulates the
body’s automatic or involuntary functions (Figure 8-18)
 Autonomic neurons—preganglionic autonomic neurons
conduct impulses from spinal cord or brainstem to an
autonomic ganglion; postganglionic neurons conduct from
autonomic ganglia to cardiac muscle, smooth muscle, and
glandular epithelial tissue
 Autonomic or visceral effectors—tissues to which autonomic
neurons conduct impulses (i.e., cardiac and smooth muscle
and glandular epithelial tissue)
 Composed of two divisions—the sympathetic system and the
parasympathetic system
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AUTONOMIC NERVOUS SYSTEM (cont.)
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Autonomic conduction paths
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Consist of two-neuron relays (that is, preganglionic
neurons from the central nervous system 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 (cont.)
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Sympathetic nervous system
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Dendrites and cell bodies of sympathetic preganglionic neurons are
located in the gray matter of the thoracic and upper lumbar segments
of the spinal cord
Axons leave the 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
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 triggered
• Group of changes induced by sympathetic control is called the
fight-or-flight response
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AUTONOMIC NERVOUS SYSTEM (cont.)
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Parasympathetic nervous system
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Structure
• Parasympathetic preganglionic neurons have dendrites
and cell bodies in the gray matter of the brainstem and
the sacral segments of the 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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Function—dominates control of many visceral
effectors under normal, everyday conditions,
counterbalances sympathetic function
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AUTONOMIC NERVOUS SYSTEM (cont.)
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Autonomic neurotransmitters
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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 as a whole
Regulates the body’s automatic functions in ways that
maintain or quickly restore homeostasis
 Many visceral effectors are doubly innervated (that is, they
receive fibers from parasympathetic and sympathetic
divisions and are influenced in opposite ways by the two
divisions)
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