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The sacral plexus is a network of nerve fibres that supplies the skin and muscles of the pelvis and lower limb. It is formed by the anterior rami of the sacral spinal nerves S1, S2, S3, and S4, and the lumbar spinal nerves L4 and L5. The sacral plexus provides motor and sensory nerves for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis. The S2 nerve supplies many muscles, either directly or through nerves originating from S2, and is most commonly known to govern the toes.
| Characteristics | Values |
|---|---|
| Definition | The sacral spinal nerve 2 (S2) is a spinal nerve of the sacral segment. |
| Origin | The spinal column from below the 2nd body of the sacrum. |
| Function | S2 supplies many muscles, either directly or through nerves originating from S2. It is known to govern the toes. |
| Muscles | The muscles supplied by S2 are not innervated with S2 as a single origin but partly by S2 and partly by other spinal nerves. |
| Motor Functions | Innervates the skeletal muscles in the perineum, the external urethral sphincter, the external anal sphincter, levator ani, and gluteus maximus. |
| Sensory Functions | Innervates the penis, the clitoris, and most of the skin of the perineum. |
| Roots | L4, L5, S1, S2, S3, and S4. |
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What You'll Learn
The sacral plexus and its branches
The sacral plexus is a network of nerve fibres that supply the skin and muscles of the pelvis and lower limb. It is located on the surface of the posterior pelvic wall, anterior to the piriformis muscle. The plexus is formed by the anterior rami (divisions) of the sacral spinal nerves S1, S2, S3, and S4. It also receives contributions from the lumbar spinal nerves L4 and L5.
The sacral plexus provides motor and sensory nerves for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis. The nerves forming the sacral plexus converge towards the lower part of the greater sciatic foramen and unite to form a flattened band, from which several branches arise. These branches can be categorized into anterior and posterior divisions. The anterior branches supply the flexor muscles of the lower limb, while the posterior branches supply the extensor and abductor muscles.
The anterior divisions of the fourth and fifth lumbar and first sacral nerve roots give rise to the nerve to the quadratus femoris and gemellus inferior. This nerve leaves the pelvis through the greater sciatic foramen and runs down in front of the sciatic nerve, supplying the anterior surfaces of the quadratus femoris and gemellus inferior muscles. The nerve to the obturator internus and gemellus superior arises from the anterior division of the fifth lumbar and first and second sacral nerve roots. It also leaves the pelvis through the greater sciatic foramen and gives off a branch to the gemellus superior, entering the upper part of the posterior surface of this muscle.
The posterior cutaneous nerve of the thigh is another branch of the sacral plexus. It leaves the pelvis via the greater sciatic foramen and enters the gluteal region inferiorly to the piriformis muscle. It descends deep into the gluteus maximus and runs down the back of the thigh to the knee, innervating the skin on the posterior surface of the thigh and leg. The sciatic nerve is the largest nerve in the body and is a continuation of the flattened back of the sacral plexus. It supplies nearly the whole of the skin of the leg and the muscles of the back of the thigh, leg, and foot.
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The pudendal nerve
The sensory function of the pudendal nerve provides information about touch, pleasure, pain, and temperature to the penis, vagina, perineum, anal canal, and anus. It is crucial for sensation and function in the pelvic region, including the genitals and anus. The nerve innervates the external genitalia of both sexes and the skin around the anus, anal canal, and perineum through its branches: the inferior rectal nerve, perineal nerve, and dorsal nerve.
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The obturator internus and gemellus superior
The obturator internus is a deep gluteal muscle that originates from the anterior divisions of the sacral plexus, specifically the L5, S1, and S2 nerve roots. It exits the pelvis through the greater sciatic foramen, located below the piriformis muscle. This muscle is innervated by the nerve to the obturator internus, which also supplies the fascia of the lateral sidewall of the ischioanal fossa.
The gemellus superior, or superior gemellus, is a small muscle located deep in the posterior pelvis. It is part of the group of deep gluteal muscles, including the piriformis, obturator internus, obturator externus, gemellus inferior, and quadratus femoris. These muscles work together to stabilise the hip joint and enable external rotation and abduction of the thigh. The superior gemellus is supplied by the sacral plexus, specifically the L4, L5, S1, and S2 nerve roots, and receives blood supply from branches of the internal iliac artery.
Both the obturator internus and gemellus superior muscles contribute to the triceps coxae, or three-headed muscle of the pelvis. The tendons of these muscles, along with the gemellus inferior, fuse together and insert onto the medial surface of the greater trochanter of the femur. This unique muscle group is responsible for the external rotation and abduction of the thigh, with the specific movement depending on the position of the leg.
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The piriformis
Piriformis syndrome is a condition in which the piriformis muscle, located in the buttock region, spasms and causes buttock pain. The piriformis muscle can also irritate the nearby sciatic nerve, causing pain, numbness, and tingling along the back of the leg and into the foot (similar to sciatic pain). Most patients describe symptoms of acute tenderness in the buttock and sciatica-like pain down the back of the thigh, calf, and foot. These symptoms often become worse after prolonged sitting, walking, or running and may feel better after lying down on the back. The exact causes of piriformis syndrome are unknown. Suspected causes include muscle spasm in the piriformis muscle, either because of irritation in the muscle itself or irritation of a nearby structure.
The sacral plexus is a network of nerve fibres that supplies the skin and muscles of the pelvis and lower limb. It is located on the surface of the posterior pelvic wall, anterior to the piriformis muscle. The plexus is formed by the anterior rami of the sacral spinal nerves S1, S2, S3, and S4. The inferior gluteal nerve leaves the pelvis via the greater sciatic foramen, entering the gluteal region inferiorly to the piriformis muscle. It is accompanied by the inferior gluteal artery and vein for much of its course.
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Myotome testing
The cervical and thoracic myotomes (C1-T12) are tested with the patient in a seated position, using movements of the neck and upper limb through the shoulder, elbow, wrist, metacarpophalangeal, and interphalangeal joints. The lumbar and sacral myotomes (L1-S3) are tested with the patient lying supine, using movements of the hip, knee, ankle, intertarsal, and metatarsophangeal joints.
For example, the C6 myotome can be tested by evaluating the strength of lower arm flexion. The patient's wrist is held from above, and they are instructed to "flex their hand up to their shoulder" while resistance is provided at the wrist. This tests the biceps muscle. The C7 myotome can be tested by asking the patient to extend their forearm against the examiner's resistance, testing the triceps.
In the case of S2, the myotome can be tested by evaluating knee flexion. The test is performed by holding the knee from the side and applying resistance under the ankle. The patient is instructed to pull their lower leg towards their buttock as hard as possible. This tests the hamstrings.
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Frequently asked questions
The sacral plexus is a network of nerve fibres that supplies the skin and muscles of the pelvis and lower limb.
S2 is a spinal nerve of the sacral segment that originates from the spinal column from below the 2nd body of the sacrum. It supplies many muscles, either directly or through nerves originating from S2.
S2 supplies muscles that govern the toes. It also innervates the hamstrings, the gluteus maximus, the obturator internus, and the superior gemellus.
S2 is involved in knee flexion. It also provides sensory innervation to the hip joint.
