The sciatic nerve is the largest nerve in the human body and is derived from the sacral plexus. It originates from the ventral rami of the lumbar (L4-L5) and sacral spinal nerves (S1, S2, S3). It contains fibers from both the posterior and anterior divisions of these spinal nerves.
The sciatic nerve arises in the lumbosacral region. It descends through the posterior aspect of the thigh. Before entering the popliteal fossa, the nerve terminates by splitting into two large terminal branches: the tibial nerve and common fibular (peroneal) nerve.
This article will discuss the anatomy and function of the sciatic nerve.
|Origin||L4, L5, S1, S2, S3|
|Branches||Muscular branches of sciatic nerve, tibial nerve, common fibular (peroneal) nerve|
Motor: Muscles of the posterior thigh, ischial portion of adductor magnus, muscles of the posterior, anterior and lateral compartments of the leg, foot muscles.
Sensory: Posterior thigh, lateral and posterior leg, dorsum and sole of the foot.
- Origin and course
- Clinical relations
Origin and course
The sciatic nerve is a terminal branch of the sacral plexus. It is formed from both anterior and posterior divisions of the anterior (ventral) rami of spinal nerves L4 through S3. The anterior branches of these five spinal nerves meet and converge in the posterior pelvic region to form a single large nerve. The sciatic nerve then descends posteriorly and leaves the pelvis through the greater sciatic foramen. It passes inferior to the piriformis muscle, accompanied by the posterior femoral cutaneous nerve, pudendal nerve, internal pudendal artery and vein, inferior gluteal nerve, inferior gluteal artery and vein.
The sciatic nerve then continues its course through the posterior thigh. It runs between the long head of the biceps femoris muscle and the adductor magnus muscle, and laterally to the semitendinosus and semimembranosus muscles.
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On its course through the posterior thigh, the sciatic nerve gives off several small motor muscular branches that innervate the several muscles of the thigh.
At the apex of the popliteal fossa, the sciatic nerve terminates by dividing into two terminal branches:
- The tibial nerve
- The common fibular (peroneal) nerve
The tibial nerve continues the course of the sciatic nerve and descends down through the posterior aspect of the leg as far as the heel of the foot. More specifically, the tibial nerve passes through the center of the popliteal fossa and runs below the tendinous arch of the soleus muscle. It continues its course in a neurovascular bundle through the posterior leg compartment and passes through the tarsal tunnel. When it reaches the foot, the tibial nerve divides into two terminal branches: medial and lateral plantar nerves that innervate the majority of the foot muscles.
In contrast to the tibial nerve, the common fibular (peroneal) nerve courses laterally towards the head of the fibula. When it reaches the anterior compartment of the leg, the nerve divides underneath the fibularis longus muscle into the superficial fibular (peroneal) nerve and deep fibular (peroneal) nerve. The superficial branch supplies the lateral compartment of the leg, while the deep branch supplies the anterior compartment of the leg and medial aspect of the foot.
The deep fibular (peroneal) nerve descends between the fibula and the superior part of fibularis (peroneus) longus, runs deep to extensor digitorum longus and anterior to the interosseous membrane.
The sciatic nerve is a mixed nerve that provides numerous branches for sensory and motor supply for the skin and muscles of the lower limb. Additionally, it provides articular branches for the innervation of the lower limb joints.
- The sciatic nerve provides a direct motor supply for the muscles of the posterior thigh via small muscular branches. These muscles include biceps femoris, semimembranosus, semitendinosus, and the ischial portion of the adductor magnus
- The tibial nerve provides motor supply for the muscles in the posterior portion of the leg and foot. These muscles include the gastrocnemius, soleus, plantaris, popliteus, flexor hallucis longus, flexor digitorum longus, and tibialis posterior.
- The common peroneal nerve supplies the muscles of the anterior and lateral sections of the leg and foot. The muscles of the anterior aspect include: tibialis anterior, extensor hallucis longus, extensor digitorum longus, and peroneus tertius. The lateral leg muscles include peroneus longus and peroneus brevis.
The sciatic nerve gives off sensory branches that provide sensory supply for the largest portion of the lower limb. First, on its course, the sciatic nerve innervates the skin of the posterior thigh. Then the sensory supply continues through its terminal branches;
- The tibial nerve innervates the sole of the foot.
- The branches of the common peroneal nerve innervate the lateral aspect of the leg and dorsum of the foot, as well as the skin between the first two toes.
- The tibial nerve and common peroneal nerve make up the medial and lateral sural nerves. These nerves provide sensation to the calf and a small lateral portion of the foot.
Sciatica refers to the compression or irritation of the sciatic nerve. It is characterized by pain in the ipsilateral buttock area that radiates down the lower extremity. It is usually caused by herniation in the lumbosacral spinal region or direct or indirect trauma to the nerve.
Due to its characteristic clinical presentation, the diagnosis is usually straightforward. In some cases, clinicians would perform an imaging procedure, usually an MRI. The majority of cases of sciatica are treated conservatively with rest, pain medication and some physical therapy. More serious injuries to the nerve would require surgery.