The Sciatic Nerve Anatomy USMLE Step 1- Origin , Course , Relations , Branches , Clinical application
The sciatic nerve is a major nerve of the lower limb. It is a thick flat band, approximately 2cm wide – the largest nerve in the body. In this article, we shall look at the anatomy of the sciatic nerve – its anatomical course, motor and sensory functions, and its clinical correlations.
Nerve Roots: L4-S3.
Motor: Innervates the muscles of the posterior thigh and the hamstring portion of the adductor magnus. Indirectly innervates (via its terminal branches) the muscles of the leg and foot.
Sensory: No direct sensory functions. Indirectly innervates (via its terminal branches) the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot.
The sciatic nerve is derived from the lumbosacral plexus. After its formation, it leaves the pelvis and enters the gluteal region via greater sciatic foramen. It emerges inferiorly to the piriformis muscle and descends in an inferolateral direction.
As the nerve moves through the gluteal region, it crosses the posterior surface of the superior gemellus, obturator internus, inferior gemellus and quadratus femoris muscles. It then enters the posterior thigh by passing deep to the long head of the biceps femoris.
Within the posterior thigh, the nerve gives rise to branches to the hamstring muscles and adductor magnus. When the sciatic nerve reaches the apex of the popliteal fossa, it terminates by bifurcating into the tibial and common fibular nerves.
Note: the sciatic nerve can be described as two individual nerves bundled together in the same connective tissue sheath – the tibial and common fibular nerves. These usually separate at the apex of the popliteal fossa, however in approximately 12% of people they separate as they leave the pelvis.
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