The sciatic nerve is the largest and longest nerve in the human body.1,10 Every person has 2 sciatic nerves, the right and left nerves—supplying each lower limb.

The sciatic nerve originates in the lower spine and is formed by the combination of spinal nerves L4 to S3. The sciatic nerve is responsible for motor and sensory functions of the lower body.
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Sciatic Nerve Anatomy Video

Origin and Branches of the Sciatic Nerve

The sciatic nerve starts in the lower spine and follows a long path through the buttock, down the back of the thigh and leg, and finally ends in the foot.

Origin
The sciatic nerve is formed by the combination of 5 nerves in the lumbar (lower) and sacral spine—L4, L5, S1, S2, and S3. These nerve fibers are typically responsible for motor and sensory functions of the lower body. The 5 nerves group together near the front surface of the piriformis muscle deep in the buttock and form the large, thick sciatic nerve. At its thickest portion, the nerve measures around 2 cm in diameter.10,17

Watch: Lumbar Spine Anatomy Video

The nerve leaves the pelvis along with its surrounding nerves and blood vessels through the greater sciatic foramen below the piriformis muscle. It progresses downward between the muscles of the thigh and is surrounded by a single long fatty sheath from the pelvis to the knee, where it divides. In around 10% of the population, the nerve may divide above the knee.17

Watch: Piriformis Syndrome Video

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Branches
At the popliteal fossa near the back of the knee, the sciatic nerve divides into 2 main branches: the tibial nerve and the common peroneal nerve. The popliteal fossa is a rhomboid-shaped space that serves as a conduit for blood vessels and nerves in the leg.

  • The tibial nerve continues down the back of the leg to the heel and sole of the foot.
  • The common peroneal travels sideways along the outer part of the knee to the outer border of the leg and foot.

Both these nerves finally terminate into sural nerves.

The sciatic nerve is usually undivided and leaves the pelvis through the greater sciatic foramen below the piriformis muscle (left). Sometimes, the nerve may be divided, with one part passing through the piriformis and the other below the muscle (right).

6 Anatomic Variants of the Sciatic Nerve

It is estimated that around 16% of the population may have variations in the anatomical structure of the sciatic nerve.1,18

These variations include1:

  • The sciatic nerve divides above the piriformis muscle; one portion passes through the piriformis, the other leaves the pelvis below the muscle. This variant is the most common among others.
  • The sciatic nerve divides above the piriformis muscle; one portion passes through the piriformis, the other leaves the pelvic area above the muscle.
  • The sciatic nerve divides above piriformis, one portion travels in front of it, the other travels behind it.
  • An undivided sciatic nerve exits through the piriformis muscle.
  • An undivided sciatic nerve exits from behind the top part of the piriformis.

In cases where the sciatic nerve divides, both portions of the nerve immediately merge again and course downward as a single nerve.

See Spinal Cord and Spinal Nerve Roots

While the variants are considered normal, these individuals may be at a higher risk of developing sciatica pain due to impingement, entrapment, or irritation of the nerve.18

See Sciatica Causes

Functions of the Sciatic Nerve

The sciatic nerve supplies major parts of the thighs, legs, and feet and has both motor and sensory functions.

Motor functions of the sciatic nerve
The motor functions include:

  • Knee flexion: Bending the knee
  • Hip adduction: Bringing the thighs together/movement of the leg toward the midline of the body
  • Plantar flexion: Pointing the foot downward
  • Flexion of toes: Pointing the toes downward
  • Dorsiflexion of the foot: Pointing the foot upward
  • Extension of toes: Pointing the toes upward

When the sciatic nerve is compressed, it is common to experience reduced motor functions and weakness in the affected leg.

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

Sensory functions of the sciatic nerve
The sciatic nerve provides sensations to the skin (dermatome) over the following areas:

  • Front, back, and outer part of the thigh
  • Front, back, and outer part of the lower leg
  • The top and outer side of the foot
  • Sole of foot
  • The web between the first and second toes

Sensory symptoms such as burning, tingling, and/or numbness may be experienced when the sciatic nerve is inflamed or irritated.

Watch: Spinal Causes of Foot Pain Video

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Sciatica

When the sciatic nerve is irritated, inflamed, or compressed, it causes sciatica. Sciatica is the term used to describe pain and/or neurological symptoms that are typically felt along the path of the sciatic nerve. An estimated 10% to 40% of the population is affected by sciatica at some point in their lives.10

See What You Need to Know About Sciatica

Sciatica nerve pain may vary depending on the type and severity of the underlying cause and may be accompanied by other symptoms such as numbness and weakness.

See Sciatica Symptoms

References:

  1. Giuffre BA, Jeanmonod R. Anatomy, Sciatic Nerve. [Updated 2018 Dec 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482431/.
  1. Davis D, Vasudevan A. Sciatica. [Updated 2019 Feb 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507908/.
  1. Ryan MM, Jones HR Jr. Mononeuropathies. In: Neuromuscular Disorders of Infancy, Childhood, and Adolescence. Elsevier; 2015:243-273. doi:10.1016/b978-0-12-417044-5.00014-7.

Complete Listing of References

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