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.
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.2,3
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.3
Watch: Piriformis Syndrome Video
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.
5 Anatomic Variants of the Sciatic Nerve
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.
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.4
See Sciatica Causes
In This Article:
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.
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.
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.2
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.