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.

The sciatic nerve starts as a collection of nerve roots in the lumbar spine. These nerve roots exit the spinal canal through a number of openings in the bones called foramen and then combine to form one large nerve.
Sciatic Nerve Anatomy Video

The sciatic nerve supplies sensation, strength, and reflex functions to 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?

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Sciatic Nerve Anatomy: A Combination of Nerves

The sciatic nerve is actually made up of 5 nerves that originate in the lower spine.

  • At their roots, nerves branch off the spinal cord and exit the vertebral column through gaps between the vertebral bones.
  • Each nerve is named for the vertebral bone above it. The sciatic nerve is formed by:
    • The nerve that exits the spine between lumbar segment 4 and lumbar segment 5 (L4 and L5) called the L4 nerve root.
    • The nerve that exits between the L5 and sacral segment 1, called the L5.
  • The L4, L5, S1, S2, and S3 nerves come together on the front surface of the piriformis muscle (in the buttocks) and become one large nerve, the sciatic nerve.
  • The sciatic nerve then travels down the back of each leg, branching out near the knee to provide motor and sensory functions to specific regions of the leg and foot.

See Lumbar Spine Anatomy and Pain

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.

See Spinal Cord and Spinal Nerve Roots

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Peroneal and tibial nerves
In the lower thigh/above the back of the knee, the sciatic nerve divides into 2 nerves, the tibial and peroneal nerves. These nerves 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