64 Neuroanatomy: Draw It to Know It
The Leg & Foot (Cont.)
FIGURE 4-9 Tibialis posterior.FIGURE 4-10 Flexor digitorum longus.FIGURE 4-8 G a s t r o c n e m i u s.
Now, let’s show the tibial nerve-innervated muscles. First,
indicate the superfi cial posterior compartment muscles:
gastrocnemius and soleus, supplied by S1, S2. Both mus-
cles provide foot plantar fl exion: we test gastrocnemius
with the knee extended and soleus with the knee fl exed.
Next, show the deep posterior compartment muscles:
tibialis posterior and fl exor digitorum longus and fl exor
hallucis longus, supplied by L5, S1, primarily. Note that
some texts indicate that L4 also innervates tibialis poste-
rior and some texts indicate that S2 also innervates the
fl exor digitorum and hallucis muscles. Tibialis posterior
provides foot inversion; fl exor digitorum longus fl exes
the toes (except the great toe); and fl exor hallucis longus
fl exes the great toe.
Now, add the lesser muscles that the tibial nerve
innervates: popliteus and plantaris. Popliteus unlocks
the knee at the beginning of knee fl exion and plantaris
acts in concert with gastrocnemius.
Finally, let’s begin to address the sensory innervation
of the leg and foot. First, show that the common peroneal
nerve derives a common sensory trunk that produces
both the lateral sural cutaneous nerve and also the sural
communicating branch. Show that the tibial nerve pro-
duces the medial sural cutaneous nerve, which joins the
sural communicating branch to form the sural nerve.
Th en, show that when the sural nerve passes through the
ankle, it produces both the lateral calcaneal nerve branch
and also the lateral dorsal cutaneous nerve. Th e lateral
calcaneal branch is the lateral corollary of the medial cal-
caneal branch, which we will draw in a moment. But
fi rst, show another important anatomic region, the tarsal
tunnel, which is the medial entry zone of the tibial nerve
through the ankle into the foot.
Th e medial malleolus and medial calcaneus form the
superior and inferior boundaries of the tarsal tunnel,
respectively, and the fl exor retinaculum forms its roof.
Show that within the tarsal tunnel, the tibial nerve divides
into the plantar nerves (medial and lateral) and also the
medial calcaneal sensory nerve. Th e plantar nerves inner-
vate the plantar intrinsic foot muscles, supplied by S1–S3,
and the plantar nerves and medial calcaneal nerve pro-
vide sensory coverage to the sole of the foot.^1 – 4 , 6 – 8