Anatomy · Brain & nerves

Sciatic nerve

The thickest nerve in the body

Peripheral nerves · lower limb

The largest and longest nerve in the human body, as thick as an adult's thumb where it begins. Formed from the lower lumbar and upper sacral roots (L4–S3), it leaves the pelvis through the greater sciatic foramen, runs down the back of the thigh and divides above the knee into the tibial and common fibular nerves. It supplies the hamstrings and, through its branches, every muscle below the knee.

How to spot them

▸A flat, thick, white band emerging from the pelvis under the piriformis muscle, deep to the gluteus maximus
▸Runs down the middle of the back of the thigh, on the adductor magnus and under the hamstrings
▸Splits above the back of the knee into two large branches, one continuing down the calf, the other winding round the fibular neck
▸Formed from roots L4 to S3 of the lumbar and sacral plexuses

What to know

The thickest and longest nerve in the human body, as broad as a thumb

Sciatic nerve — roughly 2 cm wide at its origin.

Formed from spinal roots L4 to S3, it leaves the pelvis through the greater sciatic foramen, usually beneath the piriformis

Sciatic nerve — the piriformis is its landmark.

Divides in the lower thigh into the tibial and common fibular (peroneal) nerves

Sciatic nerve — the two terminal branches.

Pain shooting from the lower back or buttock down the back of the leg, often from a slipped disc pressing on its roots

Sciatic nerve — sciatica.

Injections into the buttock are given in the upper outer quadrant to avoid injuring it

Sciatic nerve — it runs through the lower and inner part of the buttock.

It supplies the hamstring muscles at the back of the thigh, and through its branches all muscles below the knee

Sciatic nerve — the quadriceps are supplied by the femoral nerve, not this one.

Its name comes, by corruption, from the Latin for the hip bone, ischium

Sciatic nerve — ischiadicus became sciaticus.

Key ideas

It is really two nerves — tibial and common fibular — bound in one sheath, and can divide anywhere between the pelvis and the knee.
'Sciatica' describes the pain pattern; it is a symptom, not a diagnosis, and is usually caused by a herniated disc compressing a nerve root.

Quick-fire facts

•Roughly 2 cm wide at its origin, it is the longest and thickest nerve in the body
•Formed from ventral rami of L4, L5, S1, S2 and S3
•Leaves the pelvis via the greater sciatic foramen, usually below the piriformis muscle
•In a minority of people it splits within the pelvis and passes through or above the piriformis, a variation linked to piriformis syndrome
•Supplies the hamstrings and adductor magnus (ischial part) in the thigh, and all muscles of the leg and foot via its branches
•Gives sensation to most of the skin of the leg and foot, except the medial side supplied by the saphenous nerve
•Sciatica affects an estimated 10–40 per cent of people at some point in life, depending on the study
•Buttock injections are given in the upper outer quadrant to avoid it
•The word sciatic derives via medieval Latin from Greek ischion, 'hip joint'

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Sources

NHS — Sciatica
TeachMeAnatomy — Sciatic nerve