506
UNIT 3
Regulation and Integration of the Body
13
Homeostatic Imbalance
13.3
Injuries to the brachial plexus are common. When severe, they
weaken or paralyze the entire upper limb. Such injuries may
occur when the upper limb is pulled hard, stretching the plexus
(as when a football tackler yanks the arm of the running back),
or by blows to the top of the shoulder that force the humerus
inferiorly (as when a cyclist pitches headfirst off a motorcycle
and his shoulder grinds into the pavement).
Te brachial plexus ends in the axilla, where its three cords
wind along the axillary artery and give rise to the main nerves of
the upper limb (Figure 13.10c, d). Five of these nerves are espe-
cially important: the axillary, musculocutaneous, median, ulnar,
and radial nerves. We describe their distribution and targets
briefly here. For more detail, see ±able 13.4 and Appendix G.
Axillary Nerve
Te
axillary nerve
branches off the posterior
cord and runs posterior to the surgical neck of the humerus. It
innervates the deltoid and teres minor muscles and the skin and
joint capsule of the shoulder.
Musculocutaneous Nerve
Te
musculocutaneous nerve
,
the major end branch of the lateral cord, courses inferiorly in
the anterior arm, supplying motor fibers to the biceps brachii,
brachialis, and coracobrachialis muscles. Distal to the elbow, it
provides cutaneous sensation in the lateral forearm.
Median Nerve
Te
median nerve
descends through the arm
to the anterior forearm, where it gives off branches to the skin
and to most flexor muscles. On reaching the hand, it innervates
five intrinsic muscles of the lateral palm. Te median nerve ac-
tivates muscles that pronate the forearm, flex the wrist and fin-
gers, and oppose the thumb.
Homeostatic Imbalance
13.4
Median nerve injury makes it difficult to use the pincer grasp (op-
posed thumb and index finger) to pick up small objects. Because
this nerve runs down the midline of the forearm and wrist, it is
a frequent casualty of wrist-slashing suicide attempts. In carpal
tunnel syndrome (see p. 234), the median nerve is compressed.
Ulnar Nerve
Te
ulnar nerve
branches off the medial cord of
the plexus. It descends along the medial aspect of the arm to-
ward the elbow, swings behind the medial epicondyle, and then
follows the ulna along the medial forearm. Tere it supplies the
flexor carpi ulnaris and the medial part of the flexor digitorum
profundus (the flexors not supplied by the median nerve).
Te ulnar nerve continues into the hand, where it innervates
most intrinsic hand muscles and the skin of the medial aspect
of the hand. It causes the wrist and fingers to flex, and (with the
median nerve) adducts and abducts the medial fingers.
Homeostatic Imbalance
13.5
Where it takes a superficial course, the ulnar nerve is very vul-
nerable to injury. Striking the “funny bone”—the spot where
this nerve rests against the medial epicondyle—makes the little
finger tingle. Severe or chronic damage can lead to sensory loss,
paralysis, and muscle atrophy. Affected individuals have trouble
making a fist and gripping objects. As the little and ring fingers
become hyperextended at the knuckles and flexed at the distal
interphalangeal joints, the hand contorts into a
clawhand
.
Radial Nerve
Te
radial nerve
, the largest branch of the bra-
chial plexus, is a continuation of the posterior cord. Tis nerve
wraps around the humerus (in the radial groove), and then runs
anteriorly around the lateral epicondyle at the elbow. Tere it
divides into a superficial branch that follows the lateral edge of
the radius to the hand and a deep branch (not illustrated) that
runs posteriorly. It supplies the posterior skin of the limb along
its entire course. Its motor branches innervate essentially all
the extensor muscles of the upper limb. Muscles controlled by
the radial nerve extend the elbow, supinate the forearm, extend
the wrist and fingers, and abduct the thumb.
Homeostatic Imbalance
13.6
±rauma to the radial nerve results in
wrist drop
, inability to ex-
tend the hand at the wrist. Improper use of a crutch or “Saturday
night paralysis,” in which an intoxicated person falls asleep with
an arm draped over the back of a chair or sofa edge, can com-
press the radial nerve and impair its blood supply.
Lumbosacral Plexus and Lower Limb
Te sacral and lumbar plexuses overlap substantially. Because
many fibers of the lumbar plexus contribute to the sacral plexus
via the
lumbosacral trunk
, the two plexuses are o²en referred
to as the
lumbosacral plexus
. Although the lumbosacral plexus
serves mainly the lower limb, it also sends some branches to the
abdomen, pelvis, and buttock.
Lumbar Plexus
Te
lumbar plexus
arises from spinal nerves
L
1
–L
4
and lies within the psoas major muscle
(Figure 13.11)
.
Its proximal branches innervate parts of the abdominal wall
muscles and the psoas muscle, but its major branches descend
to innervate the anterior and medial thigh. Appendix G shows
the distribution and targets of the major lumbosacral nerves.
Te
femoral nerve
, the largest terminal nerve of this plexus,
runs deep to the inguinal ligament to enter the thigh and then
divides into several large branches. Te motor branches inner-
vate anterior thigh muscles (quadriceps), which are the princi-
pal thigh flexors and knee extensors. Te cutaneous branches
serve the skin of the anterior thigh and the medial surface of the
leg from knee to foot.
Te
obturator nerve
(ob
0
tu-ra
9
tor) enters the medial thigh
via the obturator foramen and innervates the adductor muscles.
Table 13.5
summarizes the branches of the lumbar plexus.
Homeostatic Imbalance
13.7
When the spinal roots of the lumbar plexus are compressed,
as by a herniated disc, gait problems occur because the femo-
ral nerve serves the prime movers that flex the hip and extend
the knee. Other symptoms are pain or numbness of the ante-
rior thigh and (if the obturator nerve is impaired) of the medial
thigh.
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