Sciatic nerve roots, exit and two components
The sciatic nerve is two nerves in one tough covering: tibial (calf and sole) and common peroneal (front and side of the leg).
Remember
The sciatic nerve and its tibial part draw on the roots L4 to S3 (the parts of nerves that leave the spinal cord), while the common peroneal part draws on L4 to S2, the whole sciatic set with S3 removed.
The sciatic nerve is the largest nerve in the body, about 2 cm wide. It is best thought of as two nerves wrapped together in a fibrous covering (a tough layer of connective tissue): the tibial nerve and the common peroneal nerve, which is also called the common fibular nerve.
Its roots are L4, L5, S1, S2 and S3, written L4 to S3. They meet in the sacral plexus (a network of nerves lying beside the sacrum). The tibial part draws on the same set, L4 to S3. The common peroneal part receives L4, L5, S1 and S2, written L4 to S2, so it has no contribution from S3.
The nerve passes out of the pelvis by way of the greater sciatic foramen (the large opening at the back of the pelvis), usually below the piriformis muscle (a small, pear-shaped muscle deep in the buttock). The superior gluteal nerve passes above piriformis and supplies muscles in the buttock and hip.
The sciatic nerve then passes over the deep muscles of the buttock and enters the back of the thigh, deep to the long head of biceps femoris (one of the hamstrings). In the thigh it gives branches to the hamstring muscles (the group of muscles that extend the hip and flex the knee) and to the hamstring part of adductor magnus (the part of that muscle that sits at the back of the thigh and helps to extend the hip).
In most people the two parts separate just above the popliteal fossa (the hollow behind the knee), although they may separate higher up. One hamstring muscle, the short head of biceps femoris, is supplied by the common peroneal part, whereas the other hamstring muscles are supplied by the tibial part.
Key facts
- Roots of the whole nerve
- L4, L5, S1, S2 and S3 form the sciatic nerve, written L4 to S3.
- Tibial part
- it draws on the same roots as the whole nerve, L4 to S3.
- Common peroneal part
- it receives L4, L5, S1 and S2, written L4 to S2, so it has no contribution from S3.
- Usual exit
- it passes out of the pelvis by way of the greater sciatic foramen, usually below piriformis.
- Where the parts separate
- in most people they separate just above the popliteal fossa, but they may separate higher up.
- Hamstring muscle with a different supply
- the short head of biceps femoris is supplied by the common peroneal part, while the other hamstring muscles (semitendinosus, semimembranosus and the long head of biceps femoris) are supplied by the tibial part.
Exam traps
- L4 to S3 is the root set of the whole sciatic nerve and of its tibial part, so it is too wide for the common peroneal part, which does not receive S3.
- Passing above piriformis is the route of the superior gluteal nerve. The sciatic nerve uses the same greater sciatic foramen but, in most people, emerges below the muscle.
- Passing through piriformis itself is a variant course seen in a minority of people, so it is not the usual relationship of the sciatic nerve to the muscle.
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