62 Neuroanatomy: Draw It to Know It
The Leg & Foot
FIGURE 4-5 Tibialis anterior. FIGURE 4-6 Peroneus longus.
FIGURE 4-7 Extensor digitorum brevis.Here, we will draw the innervation of the leg and foot.
To localize each form of peroneal or tibial nerve injury,
learn at least one muscle from each muscle group. Label
the top of the page from left to right as thigh, leg, and
foot. First, show that the innervation of the leg and foot
is derived from the sciatic nerve, supplied by L4–S3.
Th en, defi ne two key anatomic structures. Indicate that
the popliteal fossa is the depression behind the knee and
that the fi bular neck is the continuation of the head of
the fi bula (the top of the lateral leg bone).
Now, proximal to the popliteal fossa, let’s show how
the sciatic nerve unbundles to innervate the anterior, lat-
eral, and posterior leg compartments. Show that the
common peroneal nerve leaves the path of the sciatic
nerve; passes inferolaterally through the popliteal fossa;
wraps around the fi bular neck; and then splits into the
deep peroneal nerve, which innervates the muscles of the
anterior leg and dorsum of the foot, and the superfi cial
peroneal nerve, which innervates the muscles of the lat-
eral leg. Next, show that the tibial nerve continues
straight down the posterior leg to innervate the muscles
of the posterior leg and plantar foot.
Now, show that the deep peroneal nerve innervates
tibialis anterior, supplied by L4, L5. Tibialis anterior
provides foot dorsifl exion and to a lesser extent foot
inversion. Next, show that the deep peroneal nerve
also innervates extensor digitorum longus, extensor hal-
lucis longus, and peroneus tertius, supplied by L5, S1.
Extensor digitorum longus extends the toes (except the
great toe); extensor hallucis longus extends the great toe,
only; and peroneus tertius assists in foot eversion. To a
lesser extent, all three of these muscles also provide foot
dorsifl exion.
Next, show that the superfi cial peroneal nerve inner-
vates peroneus longus and peroneus brevis, supplied by
L5, S1. Th ey provide foot eversion and to a lesser extent
foot plantar fl exion.
Th en, show that in addition to the aforementioned
muscles, the deep peroneal nerve also innervates the
short extensor muscles of the foot: extensor digitorum
brevis and extensor hallucis brevis, supplied by L5, S1.
Extensor digitorum brevis extends the middle three toes
and extensor hallucis brevis extends only the great toe
and only at the proximal phalanx.