Dominic Stone
The sciatic nerve is the largest nerve in the human body, originating in the lower back and travelling through the pelvis, buttocks, and down the back of each leg. It is responsible for vital motor and sensory functions throughout each leg. The sciatic nerve is surrounded by a single long fatty sheath from the pelvis to the knee, where it divides into two branches: the tibial nerve and the common peroneal nerve. The sciatic nerve innervates a series of muscles along its course, including the hamstring muscles, the adductor magnus, and the short head of the biceps femoris. It also indirectly innervates several other muscles via its two terminal branches.
| Characteristics | Values |
|---|---|
| Size | 2cm wide (largest nerve in the body) |
| Anatomy | The nerve is surrounded by a long fatty sheath from the pelvis to the knee. At the knee, it divides into two branches: the tibial nerve and the common peroneal nerve. |
| Muscles | The sciatic nerve surrounds the muscles of the thigh, including the hamstring group and the adductor magnus. It also surrounds the muscles of the calf and foot. |
| Functions | Motor function: helps the muscles in the leg and feet move. Sensory function: helps feel sensations in the legs. |
| Conditions | Sciatica: pain or discomfort caused by the sciatic nerve being compressed or pinched. Piriformis syndrome: caused by the piriformis muscle compressing the sciatic nerve. |
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What You'll Learn
The sciatic nerve and piriformis muscle
The sciatic nerve is one of the major nerves of the lower limb and is the largest nerve in the body, measuring 2cm wide. It is surrounded by a single long fatty sheath from the pelvis to the knee. At the knee, the nerve divides into two branches: the tibial nerve and the common fibular nerve.
The sciatic nerve runs adjacent to the piriformis muscle, which is located in the buttock region and functions as an external rotator of the hip. The piriformis muscle connects the lowermost vertebrae with the upper part of the leg, travelling through the "sciatic notch", an opening in the pelvic bone that allows the sciatic nerve to travel into the leg. The proximity of the sciatic nerve and the piriformis muscle means that irritation or compression of the nerve near the piriformis muscle can result in piriformis syndrome, a painful condition that causes sciatica-like pain. Piriformis syndrome is diagnosed based on a patient's medical history, a physical examination, and diagnostic tests such as X-rays, MRIs, or nerve conduction tests. Treatment for piriformis syndrome includes anti-inflammatory medications and carefully progressing stretching of the muscle.
In most cases, the sciatic nerve divides above the piriformis muscle, with one portion passing through the muscle and the other leaving the pelvis below the muscle. However, in some individuals, the sciatic nerve exits through the piriformis muscle without dividing. This proximity between the sciatic nerve and the piriformis muscle means that the nerve can be affected by irritation or inflammation of the muscle.
The sciatic nerve indirectly innervates all the muscles of the leg and foot via its terminal branches: the tibial nerve and the common fibular nerve. It also indirectly innervates the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot. The nerve gives rise to smaller branches, called collaterals, which supply the muscles in the thigh, including the hamstring group and the adductor magnus.
The piriformis muscle receives innervation from nerve branches coming off L5, S1, and S2. When the piriformis muscle is overused, irritated, or inflamed, it can lead to irritation of the adjacent sciatic nerve, resulting in symptoms such as pain, numbness, and tingling along the back of the leg and into the foot.
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Sciatic nerve branches
The sciatic nerve is the longest and largest nerve in the human body. It is a thick, flat band about 2cm wide, which starts just outside the base of the spine and runs through the gluteal muscles, down the back of the legs. The nerve is surrounded by a single long fatty sheath from the pelvis to the knee.
The sciatic nerve gives off smaller branches, known as collaterals, which include muscle branches that supply the muscles in the thigh, such as the hamstring group at the back of the thigh, and the adductor magnus muscles along the inner thigh. Other small branches supply the leg and foot muscles.
The sciatic nerve also gives rise to articular branches that supply the back of the hip joint and the back and side of the knee joint. While the nerve passes through the gluteal region, it does not innervate any muscles there.
The sciatic nerve divides above the piriformis muscle, with one portion passing through the muscle, and the other leaving the pelvis below the muscle. In most cases, the two portions merge again and continue downward as a single nerve. However, in some individuals, the nerve may remain divided and course downward in two separate branches.
At the knee, the sciatic nerve divides into two main branches: the tibial nerve and the common fibular nerve (also known as the common peroneal nerve). The tibial nerve runs down the back of the calf to the heel and sole of the foot. The common fibular nerve travels sideways along the outer part of the knee to the outer border of the lower leg and foot. These nerves terminate into small sensory nerves in the calf that innervate the outer side of each foot.
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Sciatic nerve and gluteal trauma
The sciatic nerve is the largest and longest nerve in the human body. It is 2cm wide and emerges through the greater sciatic foramen, inferior to the piriformis muscle. It supplies the muscles of the leg and foot and provides sensory innervation to the skin of the lateral leg, heel, and dorsal and plantar surfaces of the foot.
The nerve is surrounded by a single long fatty sheath from the pelvis to the knee, where it divides into two main branches: the tibial nerve and the common peroneal nerve. The sciatic nerve does not innervate any muscles in the gluteal region, but it does innervate the muscles in the posterior compartment of the thigh, including the hamstring muscles and the adductor magnus.
Sciatic nerve injury can occur due to trauma, pressure, stretching, or cutting of the nerve. This can cause symptoms such as paresthesias, loss of muscle power, and pain. Injection injuries, also known as injection palsy, can occur when an intramuscular injection is given in the gluteal region. This can result in a loss of movement or sensation in the affected lower extremity, with or without pain. The sciatic nerve must be considered when administering intramuscular injections into the gluteal region to avoid injury.
Piriformis syndrome is a condition that occurs when the piriformis muscle presses on the sciatic nerve, causing pain or numbness in the butt, hip, or upper leg. It can be caused by injury, swelling, muscle spasms, or scar tissue in the piriformis. It is important to note that the sciatic nerve passes through the gluteal region, but it does not innervate any muscles in this area. However, the nerve can be compressed as it passes through the deep gluteal space, resulting in a condition called deep gluteal syndrome (DGS). DGS is a less well-known cause of sciatica, and it can be challenging to diagnose as standard investigations such as MRI may not visualise the deep gluteal space.
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Sciatic nerve innervation
The sciatic nerve is the largest nerve in the human body, a thick, flat band approximately 2 cm wide. It originates in the lower back and travels posteriorly through the lower limb as far down as the heel of the foot.
The nerve originates from the ventral rami of spinal nerves L4 through S3 and contains fibres from both the posterior and anterior divisions of the lumbosacral plexus. After leaving the lower vertebrae, the nerve fibres converge to form a single nerve. It exits the pelvis through the greater sciatic foramen, inferior to the piriformis muscle, and descends between the greater trochanter of the femur and the ischial tuberosity.
The sciatic nerve provides motor innervation to the posterior compartment of the thigh, including the biceps femoris, semimembranosus, semitendinosus, and the hamstring portion of the adductor magnus, providing knee flexion and hip adduction.
The sciatic nerve also indirectly innervates several other muscles in the leg and foot via its two terminal branches: the tibial nerve and the common fibular nerve. The tibial nerve innervates the muscles of the posterior leg (calf muscles) and some of the intrinsic muscles of the foot, while the common fibular nerve innervates the muscles of the anterior and lateral leg and the remaining intrinsic foot muscles.
In total, the sciatic nerve innervates the muscles of the posterior thigh, entire leg, and entire foot. It does not have any direct cutaneous functions but does provide indirect sensory innervation via its terminal branches, supplying the skin of the posterolateral leg, lateral foot, heel, and both the dorsal and plantar surfaces of the foot.
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Sciatic nerve injury
The sciatic nerve is the longest and largest nerve in the human body. It is a thick, flat band, about 2cm wide, that runs from the lower back, through the pelvis, buttocks, and down the back of each leg. It is surrounded by a single long fatty sheath from the pelvis to the knee, where it divides into two branches: the tibial nerve and the common fibular nerve. The sciatic nerve is responsible for two types of functions: motor function, which helps the muscles in the leg and feet move, and sensory function, which helps people feel sensations in their legs.
Sciatic nerve injuries can also be caused by misplaced intramuscular injections in the gluteal region, which is the most common cause of injury. It is attributed to either frequent injections or poor techniques resulting from inadequately trained or unqualified staff. Other causes of sciatic nerve injuries include total hip replacement, where nerve compression and stretching during surgery damage the nerve, and bone spurs, degenerative disc disease, herniated discs, piriformis syndrome, lumbar spinal stenosis, and spondylolisthesis.
Treatment options for sciatic nerve injuries include transsacral blocks with methylprednisolone, and exercise, which has been shown to enhance motor axon regeneration and peripheral nerve regeneration.
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Frequently asked questions
The sciatic nerve is the largest nerve in the human body, a thick, flat band 2cm wide. It is formed from five spinal nerves and provides sensory and motor supply to the skin and muscles of the thigh, leg, and foot.
The sciatic nerve directly innervates the muscles in the posterior compartment of the thigh, including the hamstring muscles, the biceps femoris, semimembranosus, semitendinosus, and the adductor magnus.
The sciatic nerve indirectly innervates the muscles of the entire leg and foot via its two terminal branches: the tibial nerve and the common fibular nerve.
In most cases, the sciatic nerve divides above the piriformis muscle, with one portion passing through the muscle and the other passing above it. In some individuals, the piriformis muscle splits the common fibular and tibial nerves. Piriformis Syndrome is a condition where the sciatic nerve is compressed by the piriformis muscle, causing pain in the buttock and hip.
