The tibial nerve passes under the tendinous arch formed between the fibular and tibial heads of the soleus muscle and passes vertically through the deep region of the posterior compartment of the leg on the surface of the tibialis posterior muscle with the posterior tibial vessels.
The tibial nerve leaves the posterior compartment of the leg at the ankle by passing through the tarsal tunnel behind the medial malleolus. It enters the foot to supply most intrinsic muscles and skin.
In the leg, the tibial nerve gives rise to: branches that supply all the muscles in the posterior compartment of the leg, and two cutaneous branches, the sural nerve and medial calcaneal nerve.
Branches of the tibial nerve that innervate the superficial group of muscles of the posterior compartment and popliteus muscle of the deep group originate high in the leg between the two heads of the gastrocnemius muscle in the distal region of the popliteal fossa (Fig. 6.91). Branches innervate the gastrocnemius, plantaris, and soleus muscles, and pass more deeply into the popliteus muscle.
Branches to the deep muscles of the posterior compartment originate from the tibial nerve deep to the soleus muscle in the upper half of the leg and innervate the tibialis posterior, flexor hallucis longus, and flexor digitorum longus muscles.
The sural nerve originates high in the leg between the two heads of the gastrocnemius muscle (Fig. 6.90). It descends superficial to the belly of the gastrocnemius muscle and penetrates through the deep fascia approximately in the middle of the leg where it is joined by a sural communicating branch from the common fibular nerve. It passes down the leg, around the lateral malleolus, and into the foot.
The sural nerve supplies skin on the lower posterolateral surface of the leg and the lateral side of the foot and little toe.
The medial calcaneal nerve is often multiple and originates from the tibial nerve low in the leg near the ankle and descends onto the medial side of the heel.
The medial calcaneal nerve innervates skin on the medial surface and sole of the heel (Fig. 6.90).
Lateral compartment of leg
There are two muscles in the lateral compartment of the leg—the fibularis longus and fibularis brevis (Fig. 6.91 and Table 6.8). Both evert the foot (turn the sole outward) and are innervated by the superficial fibular nerve, which is a branch of the common fibular nerve.
The fibularis longus muscle arises in the lateral compartment of the leg, but its tendon crosses under the foot to attach to bones on the medial side (Fig. 6.91). It originates from both the upper lateral surface of the fibula and from the anterior aspect of the fibular head and occasionally up onto the adjacent region of the lateral tibial condyle.
The common fibular nerve passes anteriorly around the fibular neck between the attachments of the fibularis longus to the fibular head and shaft.
Distally, the fibularis longus descends in the leg to form a tendon, which, in order: passes posterior to the lateral malleolus in a shallow bony groove, swings forward to enter the lateral side of the foot, descends obliquely down the lateral side of the foot where it curves forward under a bony tubercle (fibular trochlea) of the calcaneus, enters a deep groove on the inferior surface of one of the other tarsal bones (the cuboid), and swings under the foot to cross the sole and attach to the inferior surfaces of bones on the medial side of the foot (lateral sides of the base of metatarsal I and the distal end of the medial cuneiform).
The fibularis longus everts and plantarflexes the foot.