The Peripheral Nervous System and Reﬂex Activity
(all thigh extensors and knee ﬂexors) and to the adductor
magnus. Immediately above the knee, the two divisions of the
sciatic nerve diverge.
continues through the popliteal fossa (the
region just posterior to the knee joint) and supplies the poste-
rior compartment muscles of the leg and the skin of the poste-
rior calf and sole of the foot.
In the vicinity of the knee, the tibial nerve gives oﬀ the
, which serves the skin of the posterolateral leg.
At the ankle the tibial nerve divides into the
eral plantar nerves
, which serve most of the foot.
common ﬁbular nerve
common peroneal nerve
ﬁbula), descends from its point of origin, wraps around
the neck of the ﬁbula, and then divides into superﬁcial and deep
branches. Tese branches innervate the knee joint, skin of the
anterior and lateral leg and dorsum of the foot, and muscles of
the anterolateral leg (the extensors that dorsiﬂex the foot).
Te next largest sacral plexus branches are the
inferior gluteal nerves
. ±ogether, they innervate the buttock
(gluteal) and tensor fasciae latae muscles. Te
dal; “shameful”) innervates the muscles and skin of the
perineum, and helps stimulate erection and control urination
(see ±able 10.7). Other branches of the sacral plexus supply the
thigh rotators and muscles of the pelvic ﬂoor.
Injury to the proximal part of the sciatic nerve—as might follow
a fall, disc herniation, or badly placed injection into the but-
tock—can impair the lower limbs in a variety of ways depend-
ing on the nerve roots injured.
ĭ-kah), characterized by stabbing pain radiat-
ing over the course of the sciatic nerve, is common. When the
nerve is transected, the leg is nearly useless. Te leg cannot be
Branches of the Sacral Plexus
(See Figure 13.12 and Appendix G)
Composed of two nerves (tibial and common ﬁbular) in a common sheath; they diverge
just proximal to the knee
Tibial (including sural,
medial and lateral
plantar, and medial
Cutaneous branches: to skin of posterior surface of leg and sole of foot
Motor branches: to muscles of back of thigh, leg, and foot [hamstrings (except short
head of biceps femoris), posterior part of adductor magnus, triceps surae, tibialis
posterior, popliteus, ﬂexor digitorum longus, ﬂexor hallucis longus, and intrinsic muscles
(superﬁcial and deep
Cutaneous branches: to skin of anterior and lateral surface of leg and dorsum of foot
Motor branches: to short head of biceps femoris of thigh, ﬁbular muscles of lateral
compartment of leg, tibialis anterior, and extensor muscles of toes (extensor hallucis
longus, extensors digitorum longus and brevis)
Motor branches: to gluteus medius and minimus and tensor fasciae latae
Motor branches: to gluteus maximus
Posterior femoral cutaneous
Skin of buttock, posterior thigh, and popliteal region; length varies; may also innervate
part of skin of calf and heel
Supplies most of skin and muscles of perineum (region encompassing external genitalia
and anus and including clitoris, labia, and vaginal mucosa in females, and scrotum and
penis in males); external anal sphincter
ﬂexed (because the hamstrings are paralyzed), and the foot and
ankle cannot move at all. Te foot drops into plantar ﬂexion
(it dangles), a condition called
. Recovery from sciatic
nerve injury is usually slow and incomplete.
If the lesion occurs below the knee, thigh muscles are spared.
When the tibial nerve is injured, the paralyzed calf muscles can-
not plantar ﬂex the foot and a shuﬄing gait develops. Te com-
mon ﬁbular nerve is susceptible to injury largely because of its
superﬁcial location at the head and neck of the ﬁbula. Even a
tight leg cast, or lying too long on your side on a ﬁrm mattress,
can compress this nerve and cause footdrop.
Anterolateral Thorax and Abdominal Wall
Only in the thorax are the ventral rami arranged in a simple
segmental pattern corresponding to that of the dorsal rami. Te
ventral rami of ±
mostly course anteriorly, deep to each
rib, as the
. Tese nerves supply the intercos-
tal muscles, the muscle and skin of the anterolateral thorax, and
most of the abdominal wall. Along their course, these nerves
to the skin (Figure 13.8b).
±wo thoracic nerves are unusual: the tiny ±
(most ﬁbers en-
ter the brachial plexus) and ±
, which lies inferior to the twel²h
rib, making it a
Te dorsal rami innervate the posterior body trunk in a neat,
segmented pattern. Via its several branches, each dorsal ramus
innervates the narrow strip of muscle (and skin) in line with
where it emerges from the spinal column (Figure 13.8b).
Innervation of Skin: Dermatomes
mah-tōm; “skin segment”) is an area of skin
innervated by the cutaneous branches of a single spinal nerve.
Every spinal nerve except C
innervates dermatomes. In patients