Sciatic Nerve: Muscle Innervation and Function

The sciatic nerve supplies major parts of the skin and muscles in the thigh, leg, and foot. This mixed innervation is responsible for vital motor and sensory functions throughout each leg. These functions include the sensation of pain and/or numbness and the facilitation of leg movements, such as while walking, running, and climbing, ankle movement, and toe raises.

If the sciatic nerve is affected due to inflammation or compression, the function of the muscles that it supplies may be reduced. This reduced muscle function results in leg and/or foot weakness. Similarly, trauma to the sciatic nerve can cause sensory problems in the leg, which typically include a burning, searing, icy, or numb feeling in one or more parts.

Muscles Controlled by the Sciatic Nerve

After traveling through the greater sciatic foramen in the pelvis, the sciatic nerve innervates a series of muscles along its course. An undivided, thick nerve extends from the pelvis to the knee. Near the back of the knee, the nerve divides into the tibial and common peroneal (fibular) branches. Each of these branches contains fibers from the nerve roots in the lower spine.

• The undivided sciatic nerve innervates the 4 hamstring muscles and the short head of the biceps femoris muscle along the back of the thigh. The nerve also partially supplies the adductor magnus muscle along the inner front side of the thigh.¹Katirji B, Wilbourn AJ. Mononeuropathies of the Lower Limb. In: Peripheral Neuropathy. Elsevier; 2005:1487-1510. doi:10.1016/b978-0-7216-9491-7.50064-8
• The tibial branch of the sciatic nerve is derived from the L5, S1, S2, and S3 spinal nerves. This nerve supplies the calf muscles along the back of the leg. The sensory component of this nerve continues down as the sural nerve and supplies parts of the outer side of the ankle and heel.²Kaufmann P. Tibial Nerve. In: Encyclopedia of the Neurological Sciences. Elsevier; 2003:537-539. doi:10.1016/b0-12-226870-9/02054-2
• The common peroneal branch of the sciatic nerve is derived from the L4, L5, S1, and S2 spinal nerves. The nerve supplies several muscles along the front and outer side of the leg. The sensory component of this nerve supplies the skin over the first webspace of the toes.³Bouche P. Compression and entrapment neuropathies. In: Handbook of Clinical Neurology. Elsevier; 2013:311-366. doi:10.1016/b978-0-444-52902-2.00019-9

Read more about Lumbar Spinal Nerves

The Role of the Sciatic Nerve in the Lower Limb: Movement and Function

The sciatic nerve provides a wide range of motor functions, including powering movement to the muscles that move each thigh, leg, and foot. Important motor functions include:

• Inversion of the foot—Movement of the foot toward the midline of the body
• Eversion of the foot—Movement of the foot away from the midline of the body
• Dorsiflexion of the foot—Pointing the foot upward
• Plantarflexion—Pointing the foot downward
• Flexion of toes—Pointing the toes downward
• Extension of toes—Pointing the toes upward
• Knee flexion—Bending the knee
• Hip adduction—Bringing the thighs together/movement of the leg toward the midline of the body

If the sciatic nerve is impaired in any way, the affected portion of the nerve root or sciatic nerve determines which motor functions are affected.

• When the nerve root is involved, a set of symptoms called sciatica occurs in the areas of the leg supplied by the affected nerve. An example of a sciatic nerve root-related motor dysfunction is foot drop (difficulty in lifting the front part of the foot) and steppage gait, which occurs when the L5 nerve root in the lumbosacral joint is affected.

• When the main body of the sciatic nerve is affected, sciatic neuropathy occurs. This condition has more widespread symptoms and signs, such as extensive leg weakness, which may occur in several areas of the thigh and leg, including the knee, ankle, and/or foot.

General motor impairment signs of a sciatic nerve problem include difficulty or inability to walk, bend the leg, and/or stand on tiptoe or heel.³Bouche P. Compression and entrapment neuropathies. In: Handbook of Clinical Neurology. Elsevier; 2013:311-366. doi:10.1016/b978-0-444-52902-2.00019-9

See Nerve Pain in the Leg

Sensory Functions of the Sciatic Nerve

The sciatic nerve provides sensation to different areas of skin (dermatome) over the lower limb(s). When the nerve is affected, the corresponding area of skin may feel abnormal with varying degrees of pain, numbness, and/or a pins-and-needles sensation.

Commonly affected areas are:

• 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 the foot
• The web between the first and second toes

Similar to the motor symptoms, sensory impairments are also determined by the location of sciatic nerve involvement.

• When a nerve root is affected, the sciatica symptoms are limited to the dermatomal areas supplied by that nerve. For example, sensory symptoms of the L5 nerve root may be experienced as pain or numbness along the outer side of the leg. There may also be a loss of sensation in the area of skin between the great toe and the second toe. It is usually easy to pinpoint an area of pain or numbness when the nerve root is involved.
• When sciatic neuropathy occurs, depending on the severity, more widespread symptoms and signs may be experienced. These symptoms may include burning, tingling, pins-and-needles, and/or numbness, in different parts of the thigh, leg, and/or foot.

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

The sensory symptoms of sciatic nerve impairment may be described as a sensation of warm water running down the thigh, foot sitting in a bucket of ice, or inability to lift the leg or foot due to a feeling of heaviness.

The lengthy course of the sciatic nerve in the lower limbs helps provide several motor and sensory functions. The complex anatomical features of this nerve in the lower back and pelvis predisposes it to trauma. The symptoms and signs produced by this nerve may be motor and/or sensory and are usually dictated by the severity of the underlying cause.