Sciatic nerve

At a glance

English

Sciatic nerve

Latin (TA2)

Nervus ischiadicus

System

Nerve

Region

Lower limb

Side

Paired, left and right

About the sciatic nerve

The sciatic (ischiadic) nerve is the sacral plexus's biggest branch and the widest single nerve in the body. It carries fibres from L4 to S3. The L4 and L5 contributions come down as the lumbosacral trunk over the ala of the sacrum, and the S1 to S3 rami emerge from the anterior sacral foramina; they merge on the front of piriformis. In truth the nerve is two nerves bound in one sheath: a medial tibial part built from anterior divisions of the rami and a lateral common fibular part built from posterior divisions.

It leaves the pelvis through the greater sciatic foramen, normally below piriformis, though the fibular part sometimes passes through or above that muscle. In the buttock it lies deep to gluteus maximus, about midway between the ischial tuberosity and the greater trochanter. It enters the back of the thigh behind adductor magnus and beneath the long head of biceps femoris, and usually divides at the apex of the popliteal fossa into the tibial and common fibular nerves, though the split can occur anywhere up to the pelvis.

In the thigh it gives no skin branches but supplies the hamstrings: through its tibial part the long head of biceps, semitendinosus, semimembranosus and the hamstring part of adductor magnus, and through its fibular part the short head of biceps. Its two terminal branches then serve every muscle below the knee and the skin of the leg and foot, apart from the inner strip supplied by the saphenous nerve. The tibial nerve covers the calf and sole, and the common fibular nerve the front and side of the leg and the top of the foot.

Sciatica is pain spreading down the back of the leg from irritation of the nerve or, more often, its roots, as in a lumbar disc prolapse, spinal stenosis or spondylolisthesis. Piriformis syndrome and other deep gluteal entrapments can compress the nerve in the buttock. It may be injured in posterior hip dislocation, hip replacement, pelvic fractures and by injections placed too low and medially in the buttock, which is why intramuscular injections go into the upper outer quadrant. A complete lesion paralyses knee flexion and everything below the knee. A sciatic nerve block gives anaesthesia for lower leg and foot surgery.

Course and supply

Arises from

The sacral plexus (L4-S3), formed on the anterior surface of piriformis from the lumbosacral trunk and the S1-S3 ventral rami.

Ends in

Divides, usually at the top of the popliteal fossa, into the tibial nerve and the common fibular nerve.

Supplies

Motor to the hamstrings and part of adductor magnus, and through its branches to all muscles of the leg and foot; sensory to the leg and foot except the medial strip served by the saphenous nerve.