The sciatic nerve, also known as the ischiatic nerve, is the longest, widest single nerve in the body. It’s the yellow nerve in the diagram on the left. At its largest point, it’s as big around as an index finger. Take a moment to look at your index finger. It’s pretty incredible to have a nerve this large moving through us. It deserves respect.
The nerve roots that will form the sciatic nerve begin in the lower back, coming out of the right and left hand side of the lower spine. The lumbar spine includes five vertebrae (L1 – L5). Below the lumbar spine is the sacrum. The nerves that make up the sciatic nerve come out of the bottom two lumbar vertebrae (L4 and L5 and the top three sacral vertebrae S1, S2, and S3).
These five nerves come together out of L4, L5, S1, S2, and S3 to make the sciatic nerve. They come together right on the front of the piriformis muscle. First two nerves are formed, the tibial and the peroneal. Then those nerves are encased in the sheath that will be referred to as the sciatic nerve. Basically the sciatic nerve is born at the piriformis muscle. But things can get even closer between the sciatic nerve and the piriformis. In about 15% of the population the two nerves – tibial and peroneal – that live in one sheath until they split at the back of the knee, don’t join up until lower than the piriformis.
If this is the case the peroneal nerve segment will pass through the piriformis muscle while the tibial nerve runs in front of it, and then they will join up together to form the sciatic nerve. For some, this will never be an issue. But if the piriformis goes into spasm, then those individuals will feel sciatic pain. Because the peroneal segment is going through that muscle, there’s no real “cure” for that, only a management of the pain by calming down the piriformis. There is a surgery available but it doesn’t often work.