Chapter 14
Peripheral Nervous System
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Spinal Nerves

Overview


Thirty-one pairs of spinal nerves are connected to the spinal cord
(Figure 14-1)
No special names; are numbered by level of vertebral column at
which they emerge from the spinal cavity
•
•
•
•
•


Eight cervical nerve pairs (C1 through C8)
Twelve thoracic nerve pairs (T1 through T12)
Five lumbar nerve pairs (L1 through L5)
Five sacral nerve pairs (S1 through S5)
One coccygeal nerve pair
Lumbar, sacral, and coccygeal nerve roots descend from point of
origin to lower end of spinal cord (level of first lumbar vertebra)
before reaching the intervertebral foramina of the respective
vertebrae, through which the nerves emerge
Cauda equina—describes the appearance of the lower end of the
spinal cord and its spinal nerves as a horse’s tail
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Spinal Nerves

Structure of spinal nerves

Each spinal nerve attaches to spinal cord by a
ventral (anterior) root and a dorsal (posterior) root

Dorsal root ganglion—swelling in the dorsal root of
each spinal nerve

All spinal nerves are mixed nerves
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Spinal Nerves

Structure of spinal nerves (cont.)
 Ramus
• One of several large branches formed after each spinal nerve
emerges from the spinal cavity (Figure 14-2)
• Dorsal ramus—supplies somatic motor and sensory fibers to
smaller nerves that innervate the muscles and skin of the
posterior surface of the head, neck, and trunk
• Ventral ramus


Structure is more complex than that of dorsal ramus
Autonomic motor fibers split from the ventral ramus and head
toward a ganglion of the sympathetic chain
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Spinal Nerves

Nerve plexuses

Plexuses—complex networks formed by the ventral
rami of most spinal nerves (not T2 through T12)
subdividing and then joining together to form individual
nerves

Each individual nerve that emerges contains all the
fibers that innervate a particular region of the body

In plexuses, spinal nerve fibers are rearranged
according to their ultimate destination, reducing the
number of nerves needed to supply each body part
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Spinal Nerves
 There
are four major pairs of plexuses:
• Cervical plexus (Figure 14-3)

Located deep within the neck

Made up of ventral rami of C1 through C4 and a branch of the
ventral ramus of C5

Individual nerves emerging from cervical plexus innervate the
muscles and skin of the neck, upper shoulders, and part of the head

Phrenic nerve exits the cervical plexus and innervates the
diaphragm
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Spinal Nerves
• Brachial plexus (Figure 14-4)

Located deep within the shoulder

Made up of ventral rami of C5 through T1

Individual nerves emerging from brachial plexus innervate
the lower part of the shoulder and the entire arm
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Spinal Nerves

Four major pairs of plexuses (cont.):
• Lumbar plexus (Figure 14-5)

Located in the lumbar region of the back in the psoas muscle

Formed by intermingling fibers of L1 through L4

Femoral nerve exits the lumbar plexus, divides into many
branches, and supplies the thigh and leg
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• Sacral plexus and coccygeal plexus (Figure 14-5)

Located in the pelvic cavity in the anterior surface of the
piriformis muscle

Formed by intermingling of fibers from L4 through S4

Tibial, common peroneal, and sciatic nerves exit the sacral
plexus and supply nearly all the skin of leg, posterior thigh
muscles, and leg and foot muscles
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Spinal Nerves

Dermatomes and myotomes (Figure 14-6)

Dermatome—region of skin surface area supplied
by afferent (sensory) fibers of a given spinal nerve
(Figure 14-7)

Myotome—skeletal muscle(s) supplied by efferent
(motor) fibers of a given spinal nerve (Figure 14-8)
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Cranial Nerves
(Tables 14-2 and 14-3)

Overview

Twelve pairs of cranial nerves connect to the brain, mostly
the brainstem (Figure 14-9)

Identified by name (determined by either distribution or
function) and/or number (order in which they emerge,
anterior to posterior)

Made up of bundles of axons
• Mixed cranial nerve—axons of sensory and motor neurons
• Sensory cranial nerve—axons of sensory neurons only
• Motor cranial nerve—mainly axons of motor neurons and a
small number of sensory fibers (proprioceptors)
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Cranial Nerves


Olfactory nerve (I)

Composed of axons of neurons whose dendrites and cell
bodies lie in nasal mucosa and terminate in olfactory bulbs

Carries information about sense of smell
Optic nerve (II)

Composed of axons from the innermost layer of sensory
neurons of the retina

Carries visual information from the eyes to the brain
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Cranial Nerves


Oculomotor nerve (III)

Fibers originate from cells in the oculomotor nucleus and
extend to some of the external eye muscles

Efferent autonomic fibers are also present, which extend to
the intrinsic muscles of the eye to regulate amount of light
entering eye and aid in focusing on near objects

Sensory fibers from proprioceptors in the eye muscles are
also present
Trochlear nerve (IV)

Motor fibers originate in cells of the midbrain and extend to
the superior oblique muscles of the eye

Also contains afferent fibers from proprioceptors in the
superior oblique muscles of the eye
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Cranial Nerves

Trigeminal nerve (V) (Figure 14-10)
 Has
three branches: ophthalmic nerve, maxillary
nerve,
and mandibular nerve
 Sensory neurons carry afferent impulses from skin
and mucosa of head and teeth to cell bodies in the
trigeminal ganglion
 Motor fibers originate in trifacial motor nucleus and
extend
to the muscles of mastication through the
mandibular nerve
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Cranial Nerves

Abducens nerve (VI)
 Motor
nerve with fibers originating from a
nucleus in the pons on the floor of the fourth
ventricle and extending to the lateral rectus
muscles of the eye
 Contains afferent fibers from proprioceptors in
the lateral rectus muscles
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Cranial Nerves

Facial nerve (VII)

Motor fibers originate from a nucleus in the lower part of
the pons and extend to superficial muscles of the face
and scalp (Figure 14-11)

Autonomic fibers extend to submaxillary and sublingual
salivary glands

Also contains sensory fibers from taste buds of anterior
two thirds
of the tongue
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Cranial Nerves

Vestibulocochlear nerve (VIII)

Two distinct divisions that are both sensory:
vestibular nerve
and cochlear nerve:
• Vestibular nerve fibers originate in the semicircular
canals in inner ear and transmit impulses that result in
sensations of equilibrium
• Cochlear nerve fibers originate in the organ of Corti in the
cochlea of the inner ear and transmit impulses that result
in sensations of hearing
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Cranial Nerves

Glossopharyngeal nerve (IX)

Composed of sensory, motor, and autonomic
nerve fibers

Supplies fibers to tongue, pharynx, and carotid
sinus
(Figure 14-12)
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Cranial Nerves

Vagus nerve (X)

Composed of sensory and motor fibers with many
widely distributed branches

Sensory fibers supply pharynx, larynx, trachea,
heart, carotid body, lungs, bronchi, esophagus,
stomach, small intestine, and gallbladder (Figure
14-13)

Somatic motor fibers innervate the pharynx and
larynx and are mostly autonomic fibers
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Cranial Nerves


Accessory nerve (XI)

Motor nerve that is an “accessory” to the vagus nerve

Innervates thoracic and abdominal viscera, pharynx, larynx,
trapezius, and sternocleidomastoid (Figure 14-14)
Hypoglossal nerve (XII)

Composed of motor and sensory fibers

Motor fibers innervate the muscles of the tongue

Contains sensory fibers from proprioceptors in muscles
of the tongue
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Divisions of the
Peripheral Nervous System


There are two functional divisions of the
peripheral nervous system:

Afferent (sensory) division

Efferent (motor) division
Efferent division is divided further into the somatic
motor nervous system and the efferent portions of
the autonomic nervous system
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Somatic Motor Nervous System

Basic principles of somatic motor pathways

Somatic nervous system—includes all voluntary motor
pathways outside the central nervous system

Somatic effectors—skeletal muscles
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Somatic Motor Nervous System

Somatic reflexes
 Nature
of a reflex
• Reflex—action that results from a nerve impulse passing
over a reflex arc; predictable response to a stimulus


Cranial reflex—center of reflex arc is in the brain
Spinal reflex—center of reflex arc is in the spinal cord
• Reflex consists of either muscle contraction or glandular
secretion


Somatic reflex—contraction of skeletal muscles
Autonomic (visceral) reflex—either contraction of smooth
or cardiac muscle or secretion by glands
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Somatic Motor Nervous System

Somatic reflexes (cont.)

Some somatic reflexes of clinical importance—reflexes deviate
from normal in certain diseases, and reflex testing is a valuable
diagnostic aid
• Knee jerk (also known as patellar reflex)—extension of the lower leg in
response to tapping the patellar tendon; tendon and muscles are stretched,
stimulating muscle spindles and initiating conduction over a two-neuron reflex
arc (Figure 14-15); may be classified in several different ways
• Ankle jerk (also known as Achilles reflex)—extension of the foot in response to
tapping the Achilles tendon; tendon reflex and deep reflex mediated by twoneuron spinal arcs; centers lie in first and second sacral segments of the cord
• Babinski reflex—extension of great toe, with or without fanning of other toes, in
response to stimulation of outer margin of sole; present in normal infants until
approximately 11⁄2 years of age and then becomes suppressed when
corticospinal fibers become fully myelinated; in humans over 11⁄2 years of age,
a positive Babinski reflex is one of the pyramidal signs indicating destruction of
corticospinal (pyramidal tract) fibers
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Somatic Motor Nervous System

Somatic reflexes (cont.)
• Plantar reflex—plantar flexion of all toes and a slight turning in and flexion
of anterior part of foot in response to stimulation of outer edge of sole
• Corneal reflex—winking in response to touching the cornea; mediated by
reflex arcs with sensory fibers in ophthalmic branch of fifth cranial nerve,
centers in pons, and motor fibers in seventh cranial nerve
• Abdominal reflex—drawing in of abdominal wall in response to stroking
side of abdomen; superficial reflex; mediated by arcs with sensory and
motor fibers in T9 through T12 and centers in these segments of the cord;
decreased or absent reflex may involve lesions of pyramidal tract upper
motor neurons

Spinal cord reflex—center of reflex arc located in spinal cord gray matter

Segmental reflex—mediating impulses enter and leave at same cord segment

Ipsilateral reflex—mediating impulses come from and go to the same side of the body

Stretch or myotatic reflex—result of type of stimulation used to evoke reflex

Extensor reflex—produced by extensors of the lower leg

Tendon reflex—tapping the tendon is stimulus that elicits reflex

Deep reflex—result of deep location of receptors stimulated to produce reflex
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The Autonomic Nervous System

Overview

Contains afferent (sensory) and efferent (motor) components
(the efferent components are emphasized here)

Carries fibers to and from the autonomic effectors

Major function—to regulate heartbeat, smooth muscle contraction,
and glandular secretions to maintain homeostasis

Two efferent divisions—sympathetic division and parasympathetic
division

Sympathetic division consists of neural pathways that are separate
from parasympathetic pathways

Many autonomic effectors are dually innervated, which allows
remarkably precise control of effector
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The Autonomic Nervous System

Structure of the autonomic nervous system

Basic plan of efferent autonomic pathways (Figure 14-16)
• Each pathway is made up of autonomic nerves, ganglia, and plexuses,
which are made of efferent autonomic neurons
• All autonomic neurons function in reflex arcs
• Efferent autonomic regulation ultimately depends on feedback from
sensory receptors
• Relay of two efferent autonomic neurons conducts information from
central nervous system to autonomic effectors:

Preganglionic neuron—conducts impulses from central nervous system to
an autonomic ganglion

Postganglionic neuron—efferent neuron with which a preganglionic neuron
synapses within autonomic ganglion
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The Autonomic Nervous System

Structure of the sympathetic pathways
• Sympathetic chain ganglia

Most ganglia of the sympathetic division lie along either side of the anterior
surface of the vertebral column and are joined with the other ganglia
located on the same side

Each chain runs from second cervical vertebra to level of coccyx

Usually there are 22 sympathetic chain ganglia on each side of vertebral
column: 3 cervical, 11 thoracic, 4 lumbar, and 4 sacral
• Thoracolumbar division

Sympathetic preganglionic neurons with dendrite and cell bodies in lateral
gray horns of thoracic and lumbar segments of spinal cord

Axons leave the cord by way of ventral roots of thoracic and first four
lumbar spinal nerves and split away from other spinal nerve fibers by the
white ramus to a sympathetic chain ganglion
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The Autonomic Nervous System

Structure of the sympathetic pathways (cont.)
• Preganglionic fiber may take one of three paths once inside the
sympathetic chain ganglion:

Synapse with sympathetic postganglionic neuron

Send ascending or descending branches through the sympathetic trunk to
synapse with postganglionic neurons in other chain ganglia

Pass through one or more chain ganglia without synapsing
• Sympathetic postganglionic neurons

Dendrites and cell bodies are mostly in sympathetic chain ganglia or
collateral ganglia

Gray ramus—short branch by which some postganglionic axons return to a
spinal nerve
• In the sympathetic division, preganglionic neurons are relatively short,
and postganglionic neurons are relatively long
• Axon of one sympathetic preganglionic neuron synapses with many
postganglionic neurons, terminating in widely spread organs
(Figure 14-17)
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The Autonomic Nervous System

Structure of the parasympathetic pathways
• Parasympathetic preganglionic neurons—cell bodies are located in nuclei in the
brainstem or lateral gray columns of the sacral cord; extend a considerable
distance before synapsing with postganglionic neurons
• Parasympathetic postganglionic neurons—dendrites and cell bodies are located
in parasympathetic ganglia, which are embedded in or near autonomic effectors
• Parasympathetic postganglionic neurons synapse with postganglionic neurons
that each lead to a single effector (Figure 14-17)

Autonomic neurotransmitters (Figures 14-18, 14-20)
• Axon terminal of autonomic neurons release either of two neurotransmitters:
norepinephrine or acetylcholine
• Adrenergic fibers—release norepinephrine; axons of postganglionic sympathetic
neurons
• Cholinergic fibers—release acetylcholine; axons of preganglionic sympathetic
neurons and of preganglionic and postganglionic parasympathetic neurons
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The Autonomic Nervous System
 Autonomic
neurotransmitters (cont.)
• Norepinephrine affects visceral effectors by first binding to one
of two types of adrenergic receptors in plasma membranes:
alpha receptors or beta receptors

Binding of norepinephrine to alpha receptors in smooth muscle of
blood vessels is stimulating, causing the vessels to constrict

Binding of norepinephrine to beta receptors in smooth muscle of
blood vessels is inhibitory, causing blood vessels to dilate; in
cardiac muscle, has stimulating effect
• Epinephrine also stimulates adrenergic receptors, enhancing
and prolonging effects of sympathetic stimulation
• Effect of a neurotransmitter on any postsynaptic cell is
determined by characteristics of the receptors, not by the
neurotransmitter
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The Autonomic Nervous System

Autonomic neurotransmitters (cont.)
• Termination of actions of norepinephrine and epinephrine

Monoamine oxidase (MAO)—enzyme that breaks up
neurotransmitter molecules taken back up by the synaptic knobs

Catechol-O-methyl transferase (COMT)—enzyme that breaks down
the remaining neurotransmitter
• Acetylcholine binds to two types of cholinergic receptors: nicotinic
receptors and muscarinic receptors
• Termination of action of acetylcholine is by the enzyme
acetylcholinesterase
• Autonomic neurotransmitters and receptors may influence
different types of presynaptic and postsynaptic receptors at
synapses with dually innervated effectors—this summation of
effects increases precision of control
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The Autonomic Nervous System

Functions of the autonomic nervous system

Overview of autonomic function
• The autonomic nervous system functions to regulate
visceral effectors in ways that tend to maintain or quickly
restore homeostasis
• Sympathetic and parasympathetic divisions are tonically
active, often exerting antagonistic influences on visceral
effectors
• Doubly innervated effectors continually receive both
sympathetic and parasympathetic impulses, and the
summation of the two determine the controlling effect
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The Autonomic Nervous System

Functions of the autonomic nervous system
(cont.)

Functions of the sympathetic division
• Under resting conditions, the sympathetic division can act to maintain
the normal functioning of doubly innervated autonomic effectors
• Sympathetic impulses function to maintain normal tone of the smooth
muscle in blood vessel walls
• Major function of sympathetic division is that it serves as an
“emergency” system—the “fight-or-flight” reaction (review Table 14-8)

Functions of the parasympathetic division (review Table 14-7)
• Dominant controller of most autonomic effectors most of the time
• Acetylcholine—slows heartbeat and acts to promote digestion and
elimination
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The Big Picture: The Peripheral Nervous
System and the Whole Body

The peripheral nervous system is made of all the afferent
nervous pathways coming into the central nervous system
and all the efferent pathways going out of the central
nervous system

Peripheral pathways are pathways that lead from the
integrator central nervous system to the effectors

Peripheral motor pathways serve as an informationcarrying network that allows the central nervous system to
communicate regulatory information to all of the nervous
effectors in the body

Every major organ is influenced, directly or indirectly, by
peripheral nervous system output
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