Tibial nerve

Origin and course

The tibial nerve typically arises in the lower third of the posterior thigh as the larger terminal  branch of the sciatic nerve, close to the apex of the popliteal fossa. From its origin, the tibial nerve descends through the popliteal fossa, lateral to the popliteal vessels. As the nerve approaches the distal aspect of the popliteal fossa, it becomes medial to the popliteal vessels  and passes deep to heads of the gastrocnemius muscle. Then it runs below the tendinous arch  formed by the heads of the soleus muscle to enter the deep posterior compartment of the leg. Here it descends on the surface of the tibialis posterior muscle, along with the posterior tibial vessels, as a neurovascular bundle.

The tibial nerve exits the posterior compartment of the leg at the ankle. It passes posterior to the medial malleolus and through the tarsal tunnel (a fibro-osseous canal on the posteromedial aspect of the ankle), accompanied by deep flexor tendons, to enter the sole of the foot where it gives rise to branches providing both muscular and cutaneous innervation.

There is a useful mnemonic to remember the order of structures that pass through the tarsal tunnel (from anterior to posterior):

Tom, Dick And Very Nervous Harry:

1. Tibialis posterior flexor
2. Digitorum longus
3. Artery (posterior tibial)
4. Vein (posterior tibial)
5. Nerve (tibial)
6. Flexor Hallucis longus

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Branches and innervation

As the tibial nerve passes through the posterior compartment of the leg, it gives rise to muscular and articular branches. The muscular branches supply all the muscles in the posterior compartment of the leg, both superficial and deep:

• Superficial muscle group: gastrocnemius, plantaris, soleus and popliteus
• Deep muscle group: tibialis posterior, flexor hallucis longus, and flexor digitorum longus

The articular branches provide innervation to the knee joint, superior and inferior tibiofibular joints and the ankle joint.

In the leg, the tibial nerve also gives rise to two cutaneous branches: sural nerve and medial calcaneal nerve.

• The sural nerve arises in the region of the popliteal fossa between the two heads of the gastrocnemius muscle. It runs distally, on the belly of the gastrocnemius, piercing the deep fascia midway down the leg to enter the subcutaneous layer, where it is joined by the sural communicating branch of the common fibular nerve. It is important to note here that some textbooks refer to the main branch from the tibial nerve as the medial sural cutaneous nerve and the sural communicating branch as the lateral sural cutaneous nerve. These unite to form the sural nerve.
  The sural nerve passes between the lateral malleolus and the calcaneus and into the foot. Here, it gives rise to lateral calcaneal branches and the lateral dorsal cutaneous nerve which provide cutaneous innervation to the lower posterolateral surface of the leg and the lateral side of the foot and little toe.
• The medial calcaneal nerve arises from the tibial nerve close to the ankle and runs onto the medial aspect of the calcaneus. This nerve supplies the skin on the medial surface and sole of the heel.

In the foot, the tibial nerve divides into two terminal branches, the larger medial plantar nerve and the smaller lateral plantar nerve.

• The medial plantar nerve runs deep to the abductor hallucis muscle. Distally, it gives rise to the medial proper digital nerve to the great toe and terminates near the metatarsal bases as three common plantar digital nerves, which further divide into proper plantar digital nerves to toes I-IV. The medial plantar nerve supplies four intrinsic muscles of the foot: abductor hallucis, the flexor digitorum brevis, the flexor hallucis brevis, and the first lumbrical. The cutaneous distribution of the medial plantar nerve is to the anterior two-thirds of the medial sole and medial three and one-half toes, including the nail beds on the dorsum (similar to the cutaneous distribution of the median nerve in the hand). 
  
  A good way of remembering the muscles supplied by the medial plantar nerve is with the mnemonic “LAFF muscles” (First Lumbrical, Abductor Hallucis, Flexor digitorum brevis, Flexor hallucis brevis).
• The lateral plantar nerve runs deep to the abductor hallucis muscle as well. It crosses the sole of the foot between the flexor digitorum brevis and quadratus plantae muscles to reach the lateral compartment of the sole, where it divides into the superficial and deep branches. The lateral plantar nerve gives motor supply for the quadratus plantae, the flexor digiti minimi brevis, the adductor hallucis, the dorsal and plantar interossei, three lumbricals, and abductor digiti minimi. The nerve provides cutaneous innervation to the anterior two-thirds of the lateral sole and lateral one and one-half toes (similar to the cutaneous distribution of the ulnar nerve in the hand).