The Anatomy of the Sciatic Nerve

The sciatic nerve controls sensation and movement of the leg and foot

In This Article

The sciatic nerves, located in the legs, are the largest nerves in the body. Each sciatic nerve stimulates movement of leg muscles and carries sensory messages from the leg to the spine. The right and left sciatic nerves each control functions on the ipsilateral (same) side of the body. 

Sciatic nerve pain, which is often referred to as sciatica, is very common and has many causes. A pinched nerve and a herniated disc are the most common causes of sciatica. Since the sciatic nerve mediates sensation and movement, an injury or compression of the nerve can cause you to have pain, decreased sensation, and/or weakness. 

Anatomy

The sciatic nerve is a peripheral nerve. Its nerve roots emerge from the lower spine and combine to form the sciatic nerve. As the sciatic nerve runs down the leg, it divides into a number of smaller branches along the way.

Many of its branches provide nerve stimulation to the muscles in the legs. Sensory nerves throughout the leg and foot travel up the leg to merge with the sciatic nerve.

Structure 

The spinal cord, composed of nerve fibers, runs through the spine—also called the backbone and the vertebral column. Nerve roots, also referred to as spinal nerves, exit the spine at each vertebral level through foramen (openings). 

The sciatic nerve is formed by a total of five spinal nerves that combine with each other.

Two of these nerves—the L4 and L5 nerve roots—emerge from the lower lumbar sections of the spine. The other three nerve roots—the S1, S2, and the S3 nerve roots—emerge from the sacral spine, which is the lowest part of the spine.

These nerve roots have fibers that originate from the ventral region (also called anterior or front) of the spine and the posterior (also called dorsal or back) region of the spine. The anterior fibers of the spine carry motor function and the posterior fibers of the spine have a sensory function.

The sciatic nerve has sensory and motor functions because nerves from the anterior and posterior parts of the spine combine to form the nerve roots that merge into the sciatic nerve.

The sciatic nerve varies in width and diameter as it travels down the leg, and it is at its largest width at the region where the five nerves combine before it begins to divide into its various branches as it travels down the leg. 

Location

The sciatic nerve travels through the greater sciatic foramen, an opening formed by bones of the pelvis. The nerve passes through the greater sciatic foramen and travels down the back of the leg along the front of the piriformis muscle, which runs deep in the upper leg. 

As the sciatic nerve travels down the upper part of the leg (behind the thigh), several branches separate off from it, providing motor stimulation to muscles in the upper part of the leg.

Once the sciatic nerve nears the back of the knee, it divides into two main branches, the tibial nerve and the fibular nerve. The tibial and fibular nerves travel down the leg to the foot, dividing into smaller motor and sensory branches along the way. The tibial nerve is primarily a motor nerve, and the fibular nerve is primarily a sensory nerve.

Function 

The sciatic nerve controls most movement and sensation throughout the leg and foot.

The motor branches of the sciatic nerve receive messages from the anterior fibers in the spine and the spinal roots. They travel down the leg, with smaller nerve branches extending to muscles in the leg and foot throughout the course of the nerve. 

Sensation is detected by small sensory nerves located in the feet and legs. These nerves merge as they travel up the sciatic nerve to the spinal nerve roots before they enter the posterior fibers in the spinal cord, eventually sending the sensory input up to the brain.

Motor 

Motor branches of the sciatic nerve stimulate muscles of the leg. These motor nerves stimulate several movements, including extension of the hip, flexion of the leg at the knee, and flexion of the foot and toes.

Muscles stimulated by the sciatic nerve in the thigh include:

  • Semitendinosus muscle
  • Semimembranosus muscle
  • Short head of the biceps femoris
  • Long head of the biceps femoris
  • Adductor magnus (often described as the hamstring muscles)

Branches of the tibial branch of the sciatic nerve stimulate muscles in the lower leg, including the:

  • Lateral and medial gastrocnemius
  • Soleus
  • Flexor digitorum longus
  • Popliteus
  • Tibialis posterior
  • Flexor hallucis longus

In the foot, the tibial nerve branches out into the lateral plantar nerve, medial plantar nerve, lateral dorsal cutaneous nerve, lateral calcaneal, and medial calcaneal branches, and plantar digital nerves, which supply stimulation for movement of the muscles in the foot.

Sensory 

Sensory nerves in the leg carry messages of light touch, temperature, pain, position sense, and vibration. These small nerves merge into the main branch of the sciatic nerve as they travel up the leg.

The branches of the sciatic nerve control sensation of the whole entire foot and most of the leg below the knee.

Sensory branches of the sciatic nerve include: 

  • Sural nerve: Detects sensation in the back of the leg and lateral (towards the small toe) areas of the foot, and merges with the fibular and tibial nerves
  • Deep fibular nerve: Detects sensation in the lateral side of the lower leg, and merges with the fibular nerve
  • Superficial fibular nerve: Detects sensations in the lateral side of the leg right above the foot and the back of the foot, and merges with the fibular nerve
  • Medial calcaneal branches: Detect sensation in the heel of the foot, and merges with the tibial nerve

Associated Conditions 

Several conditions can affect the sciatic nerve, causing pain, weakness and/or loss of sensation of the whole area supplied by the nerve or one or more of its branches. 

Herniated Disc

The spine is a column of bones with cartilaginous discs in between them. When a disc herniates (moves out of place), it can compress a sciatic nerve root. This can cause symptoms of sciatic nerve dysfunction such as pain, weakness, or diminished sensation in areas of the foot and leg that are supplied by the sciatic nerve. 

A herniated disc can be repaired surgically, and sometimes therapy can relieve the pressure on the nerve. Other treatments include anti-inflammatory medication and injection or steroids or pain medication near the affected area.

Spinal Foramen Disease

Problems such as arthritis, inflammation, and bone deterioration can affect the size and shape of the foramen through which the spinal roots travel, resulting in compression of the nerve root (pinched nerve), with resulting symptoms of sciatic dysfunction or pain. Some women experience symptoms of a pinched nerve during pregnancy, and it usually resolves after the baby is born.

Treatments include rest, anti-inflammatory medication, and therapy.

Nerve Injury

The sciatic nerve or any of its roots or branches can be damaged due to trauma. In some instances, nerve injuries can occur during surgery, particularly if there is a major disease in the pelvic region, such as cancer. An injury to the distal (lower) branches if the nerve can cause a foot drop, which results in “slapping” down of the foot when you walk.

Neuropathy

Nerve disease can occur due to chronic alcohol use, vitamin deficiency, medications, or inflammatory disease. This type of disease is described as neuropathy, and it can affect nerves throughout the body. Neuropathy typically begins distally (at the tips of the fingers and toes) and can start in multiple areas of the body before it spreads.

Neuropathy often causes symptoms such as pain, tingling, or a burning sensation. As it worsens and advances, it causes sensory loss. Late stage advanced neuropathy may also cause weakness.

Muscle Spasm

If a muscle suddenly spasms (involuntarily tightens), it can cause pressure on nearby nerves, resulting in symptoms. Since the sciatic nerve travels along the piriformis muscle, spasms of this muscle may cause symptoms of sciatica. Muscle spasms typically do not damage a nerve. The symptoms should resolve once the muscle relaxes, either on its own or with the help of muscle relaxants. 

Cancer

The sciatic nerve can be infiltrated or compressed by a tumor, a cancerous mass, or metastatic cancer from elsewhere in the body. These growths can be removed surgically or treated with chemotherapy or radiation. 

Infection

An infection of the pelvic region can involve the sciatic nerve. Meningitis, an infection of the fluid and lining that surrounds the spine and brain, may also cause inflammation and disease in or near the sciatic nerve. Infections that are treated with antimicrobials such as antibiotics can improve before causing permanent nerve damage. 

Rehabilitation

A pinched nerve or a herniated disc are fairly common conditions, and other conditions involving the sciatic nerve— such as cancer and infections— are less common. 

Physical therapy is a useful method of managing mild to moderate sciatic nerve compression and irritation. Anti-inflammatory medications can be useful as well. 

Surgical removal of structures that impinge on the sciatic nerve is possible, but many people experience recurrent symptoms after having surgery for a herniated disc or for narrowing of the foramen.

In some instances, surgical repair of the sciatic nerve is possible, especially if an injury is fairly recent. New techniques including tissue grafts and stem cells are currently being studied as possible regenerative techniques for a damaged sciatic nerve.

The decision about how to manage your sciatic nerve disease requires a thorough medical evaluation and is personalized to your situation.

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Article Sources
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