Chapter 13
The Peripheral Nervous System and Reflex Activity
509
13
(all thigh extensors and knee flexors) and to the adductor
magnus. Immediately above the knee, the two divisions of the
sciatic nerve diverge.
Te
tibial nerve
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
sural
nerve
, which serves the skin of the posterolateral leg.
At the ankle the tibial nerve divides into the
medial
and
lat-
eral plantar nerves
, which serve most of the foot.
Te
common fibular nerve
, or
common peroneal nerve
(
per-
one
5
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
superior
and
inferior gluteal nerves
. ±ogether, they innervate the buttock
(gluteal) and tensor fasciae latae muscles. Te
pudendal nerve
(pu-den
9
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.
Homeostatic Imbalance
13.8
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.
Sciatica
(si-at
9
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
Table 13.6
Branches of the Sacral Plexus
(See Figure 13.12 and Appendix G)
NERVES
VENTRAL RAMI
STRUCTURES SERVED
Sciatic nerve
L
4
, L
5
, S
1
–S
3
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
calcaneal branches)
L
4
–S
3
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
of foot]
Common fibular
(superficial and deep
branches)
L
4
–S
2
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)
Superior gluteal
L
4
, L
5
, S
1
Motor branches: to gluteus medius and minimus and tensor fasciae latae
Inferior gluteal
L
5
–S
2
Motor branches: to gluteus maximus
Posterior femoral cutaneous
S
1
–S
3
Skin of buttock, posterior thigh, and popliteal region; length varies; may also innervate
part of skin of calf and heel
Pudendal
S
2
–S
4
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
footdrop
. 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 ±
1
–±
12
mostly course anteriorly, deep to each
rib, as the
intercostal nerves
. 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
give off
cutaneous branches
to the skin (Figure 13.8b).
±wo thoracic nerves are unusual: the tiny ±
1
(most fibers en-
ter the brachial plexus) and ±
12
, which lies inferior to the twel²h
rib, making it a
subcostal nerve
.
Back
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
A
dermatome
(der
9
mah-tōm; “skin segment”) is an area of skin
innervated by the cutaneous branches of a single spinal nerve.
Every spinal nerve except C
1
innervates dermatomes. In patients
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