Sciatic Nerve – Anatomy

The Sciatic nerve is the largest nerve in the human body. The nerve originates in the lower spine and travels all the way down to the feet in both legs. Throughout its course, the nerve gives numerous branches that supply the muscles and carry sensations from different regions of the legs. The compression of the sciatic nerve roots may cause symptoms of low back pain that radiates down to the leg known as sciatica.

Sciatic Nerve Origin 

The Sciatic nerve originates from the nerve roots of the lower lumbar and upper sacral spine. The L4, L5, S1, S2, and S3 nerve roots join together to form the Sciatic nerve. The nerve roots exit the neural foramen in the vertebral column. The roots originate from the spinal cord carrying transmissions to and from the brain.

At its maximum width, the Sciatic nerve is about 2cms in width. The rope-like nerve is protected by a fatty sheath that runs throughout its course. The nerve receives its blood supply both from outside the nerve and through a network of blood vessels running in its outer layer.

Sciatic Nerve Course

The Sciatic nerve exits the pelvis along with some blood vessels through the greater sciatic notch. The greater sciatic opening lies in the back of the pelvis under the piriformis muscle.

The nerve courses under the piriformis muscle and lies to rest on a small group of muscles responsible for external rotation of the hip joint. The nerve lies under the large buttock muscle (gluteus maximus).

The nerve after crossing the gluteus maximus enters the upper thigh and comes to lie under the large muscle in the back of the thigh (bicep femoris).

The nerve continues down the back of the thigh and divides into two branches in the back of the knee. The back of the knee joint known as the popliteal fossa is a conduit for various structures from the thigh to the lower leg.

The two divisions of the Sciatic nerve, the tibial nerve, and the common peroneal nerve continue down the leg. The tibial nerve continues in the back of the leg and supplies the calf and the soles of the feet.

The common peroneal nerve travels to the outer leg and the feet. The nerve divides into two branches, the deep peroneal nerve, and the superficial peroneal nerve. Together they supply the muscles in the front and the outer side of the leg.

Although the largest nerve, the Sciatic nerve throughout its course lies deep in the thigh and the buttock and cannot be felt on palpation.

Motor Function

The Sciatic nerve controls the major muscle groups in the thigh, leg, and feet. The muscle groups help the human body to perform activities such as walking, running, sitting, climbing stairs, etc.

The Sciatic nerve supplies the four hamstring muscles namely the semitendinosus, semimembranosus, bicep femoris – long head, and bicep femoris – short head. The Sciatic nerve also supplies a part of adductor magnus muscles.

The hamstring muscles help the body to bend the knee and coordinate activities such as walking, climbing stairs, etc.

The tibial division of the Sciatic nerve supplies the muscles of the calf. The muscles include gastrocnemius, plantaris, soleus, flexor digitorum longus, and hallucis longus. Together these muscles help us to stand on our toes and move the toes down.

The peroneal part of the Sciatic nerve divides to supply the major muscles on the outer side and the front of the leg. The muscles include tibialis anterior, peroneus longus, peroneus brevis, extensor digitorum longus, and hallucis longus.

The muscles on the front and the side of the leg help us to move the foot and the toes up and outwards. The muscles of the calf and the front of the legs together function to help us to walk in a coordinated manner.

Besides the major muscles, both the tibial and the peroneal division supply a number of small muscles in the feet. The small muscles in the feet help us to walk with stability.

Sensory function

Besides the motor function, the Sciatic nerve carries sensory signals from the legs and feet through its branches. The sensation from the back of the leg (calf) and the outer sole is carried by the tibial nerve.

The sensory input from the front and the outer aspect of the leg is carried by the common peroneal division. The sensation from the top of the feet is carried by the superficial peroneal nerve, whereas the sensation from the 1st and the 2nd webspace is carried by the deep peroneal nerve.

Blood Supply

The blood supply of the nerve is vital in maintaining the Sciatic nerve’s function. The blood supply may be compromised in systemic diseases such as diabetes mellitus and in cases of external pressure over the nerve.