Regulation and Integration of the Body
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.
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.
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.
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.
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.
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.
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
, 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.
±rauma to the radial nerve results in
, 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
, the two plexuses are o²en referred
to as the
. Although the lumbosacral plexus
serves mainly the lower limb, it also sends some branches to the
abdomen, pelvis, and buttock.
arises from spinal nerves
and lies within the psoas major muscle
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.
, 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.
tor) enters the medial thigh
via the obturator foramen and innervates the adductor muscles.
summarizes the branches of the lumbar plexus.
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