The sciatic nerve is the largest and longest single nerve in the human body, about as big around as a thumb at its largest point. The nerve originates in the lower spine as nerve roots exit the spinal cord (through the bones in the spine), and extends all the way down the back of the leg to the toes.
The sciatic nerve can be very sensitive and may become irritated or compressed by various conditions of the back, including herniated discs, spinal stenosis, degenerative disc disease and spondylolisthesis.
If the sciatic nerve is irritated, a series of symptoms known as sciatica may develop. Sciatica is typically characterized by low back pain, and referred leg pain and foot pain.
The sciatic nerve is comprised of five nerves. It is formed on the right and left hand side of the lower spine by the combination of the fourth and fifth lumbar nerves and the first three nerves in the sacral spine. Each nerve exits the spine between two vertebral segments and is named for the segment above it.
The nerve that exits between lumbar segment 4 and lumbar segment 5 (L4 and L5) is called the L4 nerve root, and the nerve that exits between the L5 and Sacral segment 1 is called the L5.
The nerves that emerge from the sacral foramen are called the S1, S2 and S3 nerves.
The five nerves group together on the front surface of the piriformis muscle (in the rear) and become one large nerve - the sciatic nerve. This nerve then travels down the back of each leg, branching out to innervate specific regions of the leg and foot.
In the lower thigh/above the back of the knee, the sciatic nerve divides into two nerves, the tibial and peroneal nerves, which innervate different parts of the lower leg:
The peroneal nerves travel laterally (sideways) along the outer aspect of the knee to the upper foot.
The tibial nerves continue to travel downward towards the feet and innervate the heel and sole of the foot.
The sciatic nerve supplies sensation and strength to the leg as well as the reflexes of the leg. It connects the spinal cord with the outside of the thigh, the hamstring muscles in the back of the thighs, and muscles in the lower leg and feet. As such, when the sciatic nerve is impaired, it can lead to muscle weakness in the leg and/or numbness or tingling.