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Sciatic nerve

Anatomy, course, function and clinical significance of the sciatic nerve.

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Hello everyone! This is Matt from Kenhub, and in this tutorial, we will be looking at the sciatic nerve. The sciatic nerve highlighted here in green is the largest and longest of the peripheral nerves. It arises from the lumbosacral plexus and is formed by the anterior and posterior divisions of the fourth and fifth lumbar nerve roots - L4 and L5 – and the first, second and third sacral nerve roots – S1, S2 and S3. The sciatic nerve is considered the nerve of the lower limb since it supplies most muscles and skin of the posterior compartment of the thigh, the leg and the foot.

The sciatic nerve travels inferiorly coursing along the anterior or pelvic surface of the coccygeus muscle – one of the muscles contributing to the pelvic diaphragm – to exit the lesser pelvis through the greater sciatic foramen inferior to the piriformis muscle here. It then passes deep to the gluteus maximus muscle to enter the posterior compartment of the thigh. There, the sciatic nerve continues to course inferiorly deep to the three hamstring muscles – the semimembranosus muscle, the semitendinosus muscle and the long head of the biceps femoris muscle. Note that it lies superficially to the hamstring part of the adductor magnus muscle.

At the apex of the popliteal fossa, the sciatic nerve divides into two main branches – the tibial nerve in yellow and the common fibular nerve in green. The two branches travel laterally to the popliteal vessels. The common fibular nerve also known as the common peroneal nerve contains posterior nerve fibers from L4 and L5 of the lumbosacral trunk and the first two sacral nerves S1 and S2. On its course, the common fibular nerve follows the medial border of the biceps femoris down to the head of the fibula where it enters the anterior compartment of the leg by crossing over the plantaris muscle and the lateral head of the gastrocnemius muscle. It then wraps around the neck of the fibula and pierces the fibularis longus muscle. At this point, the common fibular nerve divides into two terminal branches. The first being the superficial fibular nerve which provides motor innervation to the fibularis longus and fibularis brevis muscles as well as cutaneous supply to dorsum of the foot and the inferior third of the leg on its medial and lateral aspects. The other branch called the deep fibular nerve provides motor innervation to the extensor digitorum longus, extensor hallucis longus, fibularis tertius, tibialis anterior and extensor digitorum brevis muscles as well as cutaneous innervation to the first interdigital cleft.

The common fibular nerve is the most commonly injured nerve in the lower limb. Because of its subcutaneous position around the head of the fibula, it is vulnerable to direct trauma. Damage to the common fibular nerve at the neck of the fibula can cause weakness or paralysis of the muscles in the anterior and lateral compartments of the lower limb. This can cause a loss of dorsiflexion which often results in foot drop – a type of gait abnormality in which the toes cannot clear the ground during walking. If damage to the common fibular nerve occurs after it divides into its terminal branches, symptoms are dependent on which of the terminal branch was affected. For example, an isolated lesion of the superficial fibular nerve generally affects sensory innervation which can cause pain in the distal leg and the dorsum of the foot. However, in the case of an isolated lesion of the deep fibular nerve, muscular innervation is generally affected preventing dorsiflexion and resulting in foot drop and a steppage gait. A steppage gait is a gait abnormality that is often adopted to counteract the effects of foot drop by exaggerating the flexion at the hip and knee during walking to allow the toes to clear the ground.

The other terminal branch of the sciatic nerve – the tibial nerve – contains anterior nerve fibers from L4 and L5 of the lumbosacral trunk and the first three sacral nerves S1, S2 and S3. After the bifurcation of the sciatic nerve, the tibial nerve travels inferiorly straight through the popliteal fossa giving off genicular branches to the knee joint; muscular branches to the soleus, plantaris, popliteus and gastrocnemius muscles as well as the medial sural cutaneous nerve which travels between the gastrocnemius muscle behind the lateral malleolus and terminates laterally on the fifth metatarsal.

As the tibial nerve travels inferiorly accompanied by the tibial vessels, it pierces the tendinous arch of the soleus and continues between the flexor digitorum longus, tibialis posterior and flexor hallucis longus muscles innervating them all. As the tibial nerve passes through the tarsal tunnel of the medial ankle here, it divides into two terminal branches – the medial plantar nerve and the lateral plantar nerve. These two terminal branches of the tibial nerve supply all the muscles on the plantar aspect of the foot. Compression of the tibial nerve or either of its terminal branches in the tarsal tunnel leads to a condition known as tarsal tunnel syndrome. With tarsal tunnel syndrome, a patient may experience plantar pain and sensory disturbances. This syndrome can also result in palsies of the intrinsic foot muscles.

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