The sciatic nerve originates in the lower spine with nerve roots that exit the spinal cord, and it extends down the back of the body from the legs to the toes, notes Spine-Health.com. It exits the spinal cord through gaps in the bones found in the lower spine.
The diameter of the sciatic nerve is about the same as that of the largest point of a man’s thumb and is the longest and largest single nerve in the body, reports Spine-Health.com. Five nerves comprise the sciatic nerve, which forms on the left- and right-hand side of the lower spine. The first three sacral spine nerves and the fourth and fifth lumbar nerves combine to form the sciatic nerve.
The segment above where each individual nerve exits the spine determines the name of the nerve, according to Spine-Health.com. Two vertebral segments provide an exit for each nerve. Nerves emerging from the sacral foramen are the S1, S2 and S3 nerves, while the nerve exiting between lumbar segments 4 and 5 is the L3 nerve root, and the nerve exiting between the sacral segment and lumbar segment 5 is the L5 nerve root. The nerves emerge on the piriformis muscle to form one large nerve that travels down the backs of the legs to provide sensory and motor functions.