Notes
Leg & Foot
Here, we will draw the innervation of the leg and foot.
Label the top of the page from left to right as: thigh, leg, and foot.
Within the thigh, draw the sciatic nerve, supplied by L4–S3.
Next, draw the popliteal fossa (the depression behind the knee).
Then draw the fibular neck.
It is the continuation of the head of the fibula (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, lateral, and posterior leg compartments.
Show that the common peroneal nerve leaves the sciatic nerve, passes inferolaterally through the popliteal fossa, and wraps around the fibular neck.
Indicate that it divides into the deep peroneal nerve and the superficial peroneal nerve, which innervate the anterior and lateral leg and dorsum of the foot.
Injury to the common peroneal nerve or either of its divisions is an important cause of foot drop.
Now, draw the tarsal tunnel at the medial aspect of the ankle.
Show that the tibial nerve passes through the posterior leg and divides within the tarsal tunnel.
The medial malleolus and medial calcaneus form the superior and inferior boundaries of the tarsal tunnel and the flexor retinaculum forms its roof.
Next, start a table for the innervation of the lower extremity.
Label the header as Muscle and Nerve Root.
We will begin with key clinical musculature.
First, show that the deep peroneal nerve innervates the following leg muscle: tibialis anterior (L4-5), which provides foot dorsiflexion.
Next, show that the superficial peroneal nerve innervates the following leg muscles: peroneus longus and peroneus brevis, supplied by L5, S1. They primarily provide foot eversion.
To a lesser extent, they also provide foot plantar flexion.
Now, within the foot, indicate that the deep peroneal nerve innervates the short extensor muscles: extensor digitorum brevis and extensor hallucis brevis, supplied by L5, S1.
Extensor digitorum brevis extends the middle three toes.
Extensor hallucis brevis extends only the great toe and only at the proximal phalanx.
Next, indicate that the tibial nerve innervates the following superficial posterior compartment leg muscles: gastrocnemius and soleus, supplied by S1, S2.
Both muscles provide foot plantar flexion: we test gastrocnemius with the knee extended and soleus with the knee flexed.
This completes the essential innervation of the lower extremity.
END OF ESSENTIAL / START OF ADVANCED
Now, let’s include some advanced musculature of the lower extremity.
Return to the deep peroneal nerve. Indicate that within the leg, it also innervates extensor digitorum longus, extensor hallucis 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 dorsiflexion.
Now, show that the tibial nerve innervates the following deep posterior compartment leg muscles: tibialis posterior and flexor digitorum longus and flexor hallucis longus, supplied by L5, S1, primarily.
Note that some texts indicate that L4 also innervates tibialis posterior and some texts indicate that S2 also innervates the flexor digitorum and hallucis muscles.
Tibialis posterior provides foot inversion; flexor digitorum longus flexes the toes (except the great toe); and flexor hallucis longus flexes 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 flexion 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 produces the medial sural cutaneous nerve, which joins the sural communicating branch to form the sural nerve.
Then, 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.
The lateral calcaneal branch is the lateral corollary of the medial calcaneal branch, which we will draw in a moment.
Finally, show that within the tarsal tunnel, the tibial nerve divides into the plantar nerves (medial and lateral), which innervate the plantar intrinsic foot muscles, supplied by S1/S3, and also the medial calcaneal sensory nerve, which provides sensory coverage to the sole of the foot.
Tarsal tunnel syndrome is a form of distal nerve entrapment.