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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; numbered by level of
vertebral column at which they emerge from
the spinal cavity
Eight cervical nerve pairs (C1 through C8)
 12 thoracic nerve pairs (T1 through T12)
 Five lumbar nerve pairs (L1 through L5)
 Five sacral nerve pairs (S1 through S5)
 One coccygeal nerve pair

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SPINAL NERVES (cont.)
 Lumbar,
sacral, and coccygeal nerve roots
descend from point of origin to the lower end
of the 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 (cont.)

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 (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

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SPINAL NERVES (cont.)

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
 Some autonomic fibers synapse with neurons that
continue on to autonomic effectors through splanchnic
nerves; others synapse with neurons whose fibers rejoin
the ventral ramus
 Sympathetic rami: splitting and rejoining of autonomic
fibers
 Motor and sensory fibers innervate muscles and glands
in the extremities and lateral and ventral portions of neck
and trunk

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SPINAL NERVES: PLEXUSES (cont.)

Nerve plexuses
 Plexus:
complex network formed by the ventral rami
of most spinal nerves (not T2 through T12),
subdividing and then joining together to form
individual nerves
 Fibers from several different rami join together to form
individual nerves
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SPINAL NERVES: PLEXUSES (cont.)
 Four major pairs of plexuses
 Cervical plexus (Figure 14-3)
 Located deep within the neck
 Composed 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
 Brachial plexus (Figure 14-4)
 Located deep within the shoulder
 Composed 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: 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
 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 the leg,
posterior thigh muscles, and leg and foot muscles
 Coccygeal plexus arises from S5 and S4 and supplies
the skin over the coccyx bone
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SPINAL NERVES (cont.)

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 or muscles
supplied by efferent (motor) fibers of a given
spinal nerve (Figure 14-8)
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CRANIAL NERVES

Overview (Tables 14-2 and 14-3)
 12
pairs of cranial nerves connect to the brain,
mostly on the brainstem (Figure 14-9)
 Identified by name (determined by either distribution
or function) or number (order in which they emerge,
anterior to posterior) or both
 Composed 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 (cont.)

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 (cont.)
 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,
extending to the intrinsic muscles of the eye
to regulate amount of light entering eye and
help focus on near objects
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CRANIAL NERVES (cont.)

Trochlear nerve (IV)
 Motor
fibers originate in cells of the midbrain
and extend to the superior oblique muscles of
the eye
 Means “pulley” since the muscle passes
through a pulley like ligament
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CRANIAL NERVES (cont.)

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 (cont.)

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
 Abducts the eye
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CRANIAL NERVES (cont.)

Facial nerve (VII)
 Motor
fibers originate from a nucleus in lower
part of 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 (cont.)

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 organ of
Corti in the cochlea of the inner ear and
transmit impulses resulting in sensations of
hearing
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CRANIAL NERVES (cont.)

Glossopharyngeal nerve (IX)
 Composed
of sensory, motor, and autonomic nerve
fibers
 Supplies fibers to tongue, pharynx, and carotid sinus
(Figure 14-12)

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 (cont.)

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)
 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

Two functional divisions of the peripheral
nervous system
 Afferent
 Efferent

(sensory) division
(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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DIVISIONS OF THE PERIPHERAL
NERVOUS SYSTEM (cont.)

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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DIVISIONS OF THE PERIPHERAL NERVOUS
SYSTEM (cont.)

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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DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM
(cont.)
 Somatic
reflexes of clinical importance: reflexes
deviate from normal in certain diseases, and reflex
testing is a valuable diagnostic aid (Figure 14-16)
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)
 Ankle jerk (also known as Achilles reflex): extension of the
foot in response to tapping the Achilles tendon

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DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM: SOMATIC
REFLEXES (cont.)
Plantar reflex: curling of all toes and a slight turning in
and flexion of the anterior part of the foot in response to
stimulation of the outer edge of the sole; compare to
Babinski sign
 Babinski sign: extension of great toe, with or without
fanning of other toes, in response to stimulation of outer
margin of sole of foot; present in normal infants until
approximately 1½ years of age and then becomes
suppressed when corticospinal fibers become fully
myelinated; in human beings older than 1½ years, a
positive Babinski reflex is one of the pyramidal signs
indicating destruction of corticospinal (pyramidal tract)
fibers

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DIVISIONS OF THE PERIPHERAL NERVOUS
SYSTEM: SOMATIC REFLEXES (cont.)

Corneal reflex: winking in response to the cornea
being touched; mediated by reflex arcs with
sensory fibers in the ophthalmic branch of the fifth
cranial nerve, centers in the pons, and motor fibers
in the seventh cranial nerve
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DIVISIONS OF THE PERIPHERAL NERVOUS SYSTEM:
SOMATIC REFLEXES (cont.)

Abdominal reflex: drawing in of the abdominal wall in
response to stroking the side of the abdomen; 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 tendon is stimulus that elicits reflex
Deep reflex: result of deep location of receptors stimulated to produce
reflex
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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 separate
from parasympathetic pathways
 Many autonomic effectors are dually innervated, which
allows remarkably precise control of effector
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AUTONOMIC NERVOUS SYSTEM (cont.)

Structure of the autonomic nervous system
 Basic plan of efferent autonomic pathways (Figure 14-17)
 Each pathway is composed 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 the central
nervous system to an autonomic ganglion
 Postganglionic neuron: efferent neuron with which a
preganglionic neuron synapses within autonomic ganglion
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AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
 Structure of the sympathetic
 Sympathetic chain ganglia




pathways
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 the second cervical vertebra to the level of the
coccyx
22 sympathetic chain ganglia are usually on each side of vertebral
column: three cervical, 11 thoracic, four lumbar, and four sacral
Thoracolumbar division


Sympathetic preganglionic neurons with dendrite and cell bodies in
lateral gray horns of the thoracic and lumbar segments of the spinal
cord
Axons leave the cord by way of the ventral roots of the 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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AUTONOMIC NERVOUS SYSTEM: STRUCTURE (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 14-18)

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AUTONOMIC NERVOUS SYSTEM: STRUCTURE (cont.)
 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 preganglionic neurons synapse with
postganglionic neurons that each lead to a single
effector (Figure 14-18)

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AUTONOMIC NERVOUS SYSTEM:
STRUCTURE (cont.)
 Autonomic neurotransmitters (Figures 14-18 and 14-20)
 Axon terminal of autonomic neurons releases 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
 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
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AUTONOMIC NERVOUS SYSTEM:
STRUCTURE (cont.)
 Autonomic neurotransmitters (cont.)
 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
 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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AUTONOMIC NERVOUS SYSTEM (cont.)

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;
summation of the two determines the controlling
effect

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AUTONOMIC NERVOUS SYSTEM:
FUNCTIONS (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 as an “emergency”
system—the “fight or flight” reaction (Table 14-7)

 Functions
of the parasympathetic division (Table 14-6)
Dominant controller of most autonomic effectors most of the
time
 Acetylcholine: slows heartbeat and promotes digestion and
elimination

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THE BIG PICTURE: THE PERIPHERAL NERVOUS SYSTEM AND THE WHOLE BODY




The peripheral nervous system is composed 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 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 the nervous
effectors in the body
Every major organ is influenced, directly or indirectly, by
peripheral nervous system output
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