The sciatic nerve is the largest and longest single nerve in the human body, about as big around as a man's thumb at its largest point.

Watch: Sciatic Nerve Anatomy Video

The nerve originates in the lower spine as nerve roots exit the spinal cord (through gaps in the bones at the back of the spine), and extends all the way down the back of the leg to the toes.

See What You Need to Know About Sciatica

Any problem in the lower spine can affect one of the nerves that feeds into the sciatic nerve, causing pain to radiate along that part of the nerve.

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 and/or numbness or tingling in the leg, ankle, foot, and/or toes.

See Leg Pain and Numbness: What Might These Symptoms Mean?

Article continues below

Sciatic Nerve Anatomy: A Combination of Nerves

The sciatic nerve is actually made up 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.

See Spinal Anatomy and Back Pain

Each nerve exits the spine between two vertebral segments and is named for the segment above it.

  • The nerve that exits the spine 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 include S1, S2, and S3.

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 provide motor and sensory functions to specific regions of the leg and foot.

See Spinal Cord and Spinal Nerve Roots

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:

  • Peroneal nerves. The peroneal nerves travel laterally (sideways) along the outer part of the knee and down into the upper foot.
  • Tibial nerves. The tibial nerves continue to travel downward toward the feet and innervate the heel and sole of the foot.

Because of the different nerve pathways, symptoms may present in different parts of the leg or foot depending on where the nerve is affected.

See Sciatica Symptoms