Sciatic nerve
Nervus ischiadicus
Also known as: ischiadic nerve
The largest and thickest nerve in the body (L4–S3, about 2 cm wide). It leaves the pelvis below piriformis and descends through the buttock and posterior thigh, supplying the hamstrings. It then divides into the tibial and common fibular nerves, which supply every muscle below the knee.
Overview
The sciatic nerve is really two nerves in one connective-tissue sheath:
• a medial tibial division, from anterior divisions of L4–S3;
• a lateral common fibular division, from posterior divisions of L4–S2.
It leaves the pelvis through the greater sciatic foramen below piriformis. In about 15% of people the common fibular division passes through or above the muscle. The nerve descends in the gluteal region deep to gluteus maximus, about midway between the greater trochanter and the ischial tuberosity. It lies on the posterior surface of the ischium, the gemelli, obturator internus and quadratus femoris. In the thigh it lies on adductor magnus, crossed posteriorly by the long head of biceps femoris.
It usually divides at the apex of the popliteal fossa, although the level is very variable and it may divide as high as the pelvis. The tibial division supplies the true hamstrings. The common fibular division supplies only the short head of biceps femoris in the thigh, which is a useful EMG landmark. The sciatic nerve has no cutaneous branches in the thigh: the skin of the posterior thigh is supplied by the posterior femoral cutaneous nerve.
- Location
Greater sciatic foramen (infrapiriform) → gluteal region → posterior thigh → apex of the popliteal fossa.
Anatomy
- Innervation
- Directly: the hamstrings and the ischial part of adductor magnus.
• Through its terminal branches: all muscles of the leg and foot, and the skin of the leg (except the medial side) and foot.- Nerve roots
L4, L5, S1, S2, S3.
- Course
Sacral plexus → greater sciatic foramen below piriformis → descends deep to gluteus maximus, midway between the greater trochanter and the ischial tuberosity, over the short rotators → posterior thigh on adductor magnus, deep to the long head of biceps femoris → divides (usually at the apex of the popliteal fossa) into the tibial and common fibular nerves.
- Branches
- Tibial division: semitendinosus, semimembranosus, long head of biceps femoris, and the hamstring (ischial) part of adductor magnus.
• Common fibular division: the short head of biceps femoris.
• Articular branches to the hip.
• Terminal branches: the tibial and common fibular nerves.
Function
- Flexion of the knee and extension of the hip (hamstrings)
- Through the tibial and fibular nerves, all movements of the ankle, foot and toes
- Sensation of the lateral and posterior leg and the foot (via branches)
- Afferent and efferent limbs of the ankle reflex (S1, via the tibial nerve)
Clinical significance
Sciatic nerve injury
Causes include posterior hip dislocation and acetabular fracture (about 10% have sciatic injury, mainly of the fibular division), hip arthroplasty, misplaced gluteal injections and deep buttock trauma. A complete lesion gives:
• weak knee flexion;
• paralysis of all the muscles below the knee (a flail foot with foot drop);
• sensory loss below the knee except over the medial leg (saphenous);
• an absent ankle reflex.
The fibular division is more often affected because it is more tethered and lies more laterally.
"Sciatica"
Pain radiating from the buttock down the back of the leg is usually a lumbosacral radiculopathy, not a lesion of the sciatic nerve itself. The commonest cause is posterolateral disc herniation at L4/5 (an L5 root) or L5/S1 (an S1 root). The straight-leg raise (Lasègue) test stretches these roots. Piriformis syndrome is a debated cause of true sciatic compression at the buttock.
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.