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No. 30
1. Cervical Plexus
2. Brachial Plexus
Chapter 3 The Peripheral Nervous
System
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Introduction:
The peripheral nervous system is the nervous
structures outside the brain and spinal cord. It is
composed of nerves and ganglia, and usually
divided into three portions according to their
origins and distribution:
(1) Spinal nerves (31 pairs):
They attach to the spinal cord and make their
exit through the intervertebral foramina.
(2) Cranial nerves (12pairs):
They attach to the brain and make their exit
through the skull.
(3) Visceral nerves:
 A further part of the peripheral nervous
system which is not a separate
morphological entity but a grouping of
special nerve components which travel
within certain spinal and cranial nerves to
innervate smooth muscle, cardiac muscle
and gland, that is the visceral nervous
system.
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According to the difference of subject
innervated by the nerves, the peripheral
nervous system is divided the somatic
nerves (supply the body surface, bones,
joints, and skeletal muscle) and visceral
nerves (are distributed in the viscera,
heart, vessels, and smooth muscles).
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In the peripheral nerves (the somatic and
visceral nerves), there are two nerve
fibers called afferent nerves (sensory
nerves) and efferent nerves (motor
nerves).
The visceral efferent (motor) nerve is also
called the vegetative nervous system
or autonomic nervous system and is
further divided into sympathetic nerve
and parasympathetic nerve.
Section 1 The Spinal Nerve
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Introduction:
1. The formation, division, fibrous components of
spinal nerves
(1) Formation of the spinal nerves
The spinal nerves, 31 pairs in number, are
formed from the union of ventral (anterior) and
dorsal (posterior) roots that leave or enter the
spinal cord.
The anterior root contains motor fibers arising
from the large motor cells of the anterior and
lateral horns of the gray matter.
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The posterior root contains sensory
fibers which are the central processes of
the pseudounipolar neurons in the spinal
ganglion.
So the spinal nerves are mixed nerves.
Spinal ganglion:
The spinal ganglion contains cell bodies
of pseudounipolar neurons, their
peripheral processes are united with the
anterior root to form the spinal nerve at
the intervertebral foramen.
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(2) The division of spinal nerves
There are 31 pairs of spinal nerves, including 8
pairs of cervical, 12 pairs of thoracic, 5 pairs of
lumbar, 5 pairs of sacral, and 1 pair of coccygeal
nerves.
The spinal nerves leave the vertebral canal by
passing through the intervertebral foramina.
All spinal nerves emerge from the intervertebral
foramina between adjoining vertebrae, but the
1st cervical nerve exits between the occipital
bone and the atlas.
The 8th cervical nerve emerges between the
seventh cervical and the first thoracic vertebrae.
The first four of the sacral nerves make their exit
through foramina in the sacrum, the fifth and the
coccygeal nerves emerge from the sacral hiatus.
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(3) Fibrous components of spinal nerve
The spinal nerves consist of four components:
① The somatic sensory (afferent) fibers
The fibers come from the pseudounipolar neurons
in the spinal ganglion. The central processes of
pseudounipolar neurons constitute the posterior
roots of spinal nerves to go into the spinal cord,
while their peripheral processes enter into the
spinal nerves to distribute the skin, skeletal
muscles, muscular tendon, and the joints of the
trunk and limbs, conducting impulses of the
superficial sensations of skin (sensations of pain,
thermal and tactile) and the deep (proprioceptive)
sensations of muscles, tendon, and joints
(sensations of the body posture, movement,
vibration) into the center.
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② The visceral sensory (afferent) fibers
They are also from the pseudounipolar neurons in
the spinal ganglion. The central processes form
the posterior roots of the spinal nerves to go into
the spinal cord, while their peripheral processes
distribute the viscera, heart, blood vessels, and
glands.
③ The somatic motor (efferent) fibers
The somatic motor fibers coming from the
anterior horn of spinal cord, distribute to the
skeletal muscles to control their voluntary
movement.
④ The visceral motor (efferent) fibers
 The visceral motor fibers coming from the
lateral horn in the thoracolumbar
segments (sympathetic nerve center) and
sacral parasympathetic nucleus
(parasympathetic nerve center) of spinal
cord, distribute the viscera, heart and
blood vessels, and glands to innervate the
movements of cardiac and smooth
muscles, and to control the secretion of
glands.
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2. Branches of the spinal nerves
Immediately after leaving the foramen, the spinal
nerve is divided into:
①The dorsal (posterior) branch: for the supply
of muscles and skin of the posterior part of the
body. The posterior branches are generally
smaller than the anterior, but the posterior
branch of the second cervical nerve is larger, its
cuteneous branch called the greater occipital
nerve, supplies the skin of the occiput.
②The anterior branch: for the supply of
structures of the limbs, the lateral and ventral
trunk.
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③The meningeal branch: supplies the
dura matter.
④The communicating branches:
connects with the sympathetic trunk.
Except for the anterior branches of
thoracic nerves which are quite separated
from each other, the anterior branches of
other spinal nerves form intricate plexus
before supplying a part of body.
They are named the cervical plexus, the
brachial plexus, the lumbar plexus and the
sacral plexus.
Ⅰ. The Cervical Plexus
Ⅰ) The Formation of Cervical Plexus
 It is formed by the anterior branches of
the four upper cervical nerves (C1-4).
Ⅱ) Location of Cervical Plexus
 It is situated deeply in the upper part of
the neck, rested on the levator scapulae
and scalenus medius and is covered by
the sternocleiodmastoid.
Ⅲ) The Branches of Cervical Plexus
 Its branches may be divided into two
groups, superficial and deep.
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1. The superficial branches
They emerge near the middle of the posterior
border of the sternocleidomastoid and have fanshaped distribution.
1) The lesser occipital nerve(C1). It is
distributed to the skin of occiput and behind the
auricle.
2) The greater auricular nerve(C2-3). It
supplies much of the external ear and some skin
around the auricle.
3) The transverse nerve of neck(C2-3). It is
distributed to the skin of the anterior part of the
neck.
4) The supraclavicular nerves(C2-4). They are
distributed to the skin at the upper portion of the
chest, the base of the neck and the shoulder.
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2. The deep branches
The phrenic nerve (C3-5)
It descends to the root of the neck,
running obliquely across the front of the
scalenus anterior, then passes between
the subclavian artery and vein to enter the
chest. Within the chest it descends in front
of the root of the lung between the
pericardium and the mediastinal pleura to
the diaphragm.
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The motor fibers supply the diaphragm,
the sensory fibers are distributed to the
diaphragmatic peritoneum, the
diaphragmatic and mediastinal pleurae
and the pericardium. The sensory fibers of
the right phrenic nerve is also distributed
to the liver, the gallbladder and the biliary
system.
Ⅱ. The Brachial Plexus
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Ⅰ) The Formation and Location of Brachial Plexus
The brachial plexus is formed by the union of the
anterior branches of the four lower cervical
nerves (C5-8) and great part of the anterior
branch of the first thoracic nerve (T1).
It extends from the lower part of the side of the
neck to the axilla.
Three trunks: between the scalenus anterior and
scalenus medium.
Superior trunk (C5-6),
Middle trunk (C7),
Inferior trunk (C8, T1).
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Divisions: beneath the clavicle, each of
these trunks is divided into two divisions—
an anterior and posterior divisions.
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Cords: in the axilla.
Lateral cord: the anterior divisions of the
superior and middle trunks unite to form.
Medial cord: the anterior division of the
inferior trunk.
Posterior cord: the posterior divisions of
all the three trunks to from.
Ⅱ) The Branches of Brachial
Plexus
1. The branches above the clavicle
1) The long thoracic nerve: supplies the
serratus anterior→”winging of the scapula”.
2) The suprascapular nerve: supplies
the supraspinatus and the infraspinatus.
3) The dorsal scapular nerve: supplies
the levator scapulae and rhomboideus.
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2. The branches below the clavicle
1) The subscapular nerve (C5-7)
It originates from the posterior cord of the
brachial plexus, supplies the subscapularis and
teres major.
2) The thoracodorsal nerve (C6-8)
It arises from the posterior cord of the brachial
plexus, supplies the latissimus dorsi.
3) The lateral and medial pectoral nerves
(anterior thoracic nerves) (C5-T1)
They arise from the lateral and medial cords and
supply the pectoralis major and pectoralis minor.
4) The axillary nerve (C5-6)
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Arision:
It arises from the posterior cord.
Route:
It passes backward with the posterior humeral
circumflex vessels through the quadrangular
space and winds round the surgical neck of the
humerus to reach the deltoid muscle.
Branches and their distribution:
Muscular branches supplies the deltoid and teres
major, and cutaneous branch to the skin over the
deltoid.
Clinical significance:
This nerve may be injured in fracture of the
humeral neck or in dislocation of the shoulder
joint.
The deltoid is paralyzed and wasting.
A small patch of anesthesia over this muscle may
also occur.
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5) The musculocutaneous nerve (C5-7)
It arises from the lateral cord of the brachial
plexus and perforates through the
coracobrachialis, then descends between the
biceps brachii and the brachialis and gives off
some branches to supply these three muscles.
A little above the elbow, it pierces the deep fascia
between the tendon of the biceps and the
brachioradialis as the lateral antebrachial
cutaneous nerve which supplies the skin of the
lateral side of the forearm.
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6) The median nerve (C6-8, T1)
Arision:
It arises by two roots, one from the lateral and
another from the medial cord of the brachial
plexus, which embrace the axillary artery and
unite each other in front or on the lateral side of
the vessel.
Route:
It descends along the medial side of the biceps
with the brachial artery.
It is lateral to the brachial artery—in the upper
part of the arm.
It crosses the brachial artery anteriorly and
posteriorly at the middle part of the arm.
It lies on the medial side of the artery to the
cubital fossa.
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It enters the forearm, by passing between
the two heads of the pronator teres, and
descends deep to the flexor digitorum
superficialis, lying on the flexor digitorum
profundus, to the wrist, lying between the
tendons of the flexor carpi radialis and
palmaris longus, then passes beneath the
flexor retinaculum into the palm of the
hand and is covered by the palmar
aponeurosis.
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Branches and their distribution:
①It has no branches in the arm.
②In the forearm: it supply the both
pronators (protonator teres and pronator
quadatus) and most of the flexor muscles
(flexor carpi radialis, palmaris longus,
flexor digiturum superficialis, flexor pollicis
longus and lateral half of the flexor
digitorum) except the brachioradialis, the
flexor carpi ulnaris and the medial half of
the flexor digitorum profundus.
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③Before reaching the flexor retinaculum, it gives
rise to a small palmar cutaneous branch to the
skin of the thenar and central part of the palm.
④In the palm, it divides into a recurrent branch
and three common palmar digital nerves.
The recurrent branch innervates the thenar
muscles except for the adductor pollicis.
Each of common palmar digital nerves divides
into proper palmar digital nerves for the skin
of the palmar aspect of the thumb, second, third
and the lateral half of the fourth fingers, as well
as the skin on the dorsum of the middle and
terminal phalanges.
⑤It also contributes muscular branches to the
first and second lumbricales.
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Clinical significance:
An injury to the median nerve in the forearm can
result in:
① A loss of pronation of the forearm, flexion of
the second, the third fingers and the distal
phalanx of the thumb and opposition of the
thumb.
② Wrist flexion is severely affected.
③ The thenar muscles are paralyzed, so that the
thumb becomes to lie in the same plane as the
other digits, it is just an “ape-like” hand.
④ There is also a loss or weakness of sensation
from the skin of the radial part of the palmar
surface of the thumb, index, middle and radial
half of the ring finger.
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7) The ulnar nerve (C7-T1)
Arision:
It arises from the medial cord of the brachial
plexus.
Route:
At first it lies to the medial side of the brachial
artery, about the middle of the arm it pierces the
medial intermuscular septum and descends to
pass along the groove between the medial
epicondyle of humerus and the olecranon, here
the ulnar nerve is superficial and easily palpable
against the bone, so that, it is easily injured. It
enters the forearm between the heads of the
flexor carpi ulnaris, and runs downward deep to
this muscle on the flexor digitorum profundus
with the ulnar artery at its medial side to the
wrist.
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Branches and their distribution:
① In the arm, the ulnar nerve has no branch.
② At the wrist, it crosses the anterior surface of
the flexor retinaculum and immediately divides
into a superficial branch and a deep palmar
branch.
The superficial branch is cutaneous branch, it
provides:
*A proper digital nerve to the skin of ulnar side
of the little finger and the skin of hypothenar.
*A common palmer digital nerve divides into
two proper palmardigital nerves for the skin of
contiguous sides of fourth and fifth fingers.
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The deep palmar branch is muscular
branch, it supplies the muscles of
hypothenar, the third and the fourth
lumbricales, the adductor pollicis and
interossei.
③ About 5 cm above the wrist, it gives off
a dorsal branch, distribute to the ulnar
half of the dorsum of the hand and the
posterior surface of the ulnar one and half
fingers.
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Clinical significance:
Ulnar nerve may be damaged at the elbow or the
wrist. It results in:
① Interfering with the flexion of the ring and little
fingers, metacarpophalangeal joints.
② The extension of the interphalangeal joints,
adduction of the thumb is lost.
③ There is also a loss of abduction and adduction
of the fingers.
④ The hand is known as “clawhand”.
⑤ The sensation from the skin of the hypothenar
and the little finger is lost.
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8) The radial nerve (C5-T1)
Arision:
It is the continuation of the posterior cord.
Route:
In the axilla, it is placed behind the axillary artery.
Then it runs between the lateral and medial
heads of the triceps brachii, and passes along the
radial groove.
When it reaches the lower one third of the
humerus, the radial nerve pierces the lateral
intermuscular septum and continues inferiorly
between the brachialis and brachioradialis at the
level of the lateral epicondyle of the humerus,
where it divides into deep and superficial
branches.
Branches and their distribution:
① Superficial branch:
 At about the junction of the middle and
lower one third of the forearm, it
continues downwards to the dorsum of the
hand. Here it distributes to the skin on the
radial side of the dorsum of the hand, and
on the proximal digits of the lateral two
and half fingers.
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② Deep branch:
It pierces through the supinator, curves
around the lateral side of the radius to
enter the posterior fascial compartment of
the forearm.
It distributes the brachioradialis and the
extensors of the forearm.
In the arm, the radial nerve also supplies
the triceps brachii and the skin on the
dorsum of the arm.
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Clinical significance:
Injury to the radial nerve in the upper or
middle part of the arm results in:
① The paralysis of the triceps brachii,
brachioradialis, supinator and extensors of
the wrist, thumb and fingers.
② The “Wristdrop”.
③ Loss of sensation in the areas of skin
supplied by this nerve.