Sacral plexus

A “plexus” is a place where multiple nerve roots intermingle in a network-like arrangement. The Korean terms for “sacral plexus” and “sacral nerve network,” and the English term “sacral plexus,” all refer to the same structure. The character used for “plexus” in the Sino-Korean term means “a gathering,” highlighting that this is a collection of roots and branches rather than a single thick nerve.

It is generally formed by the anterior rami of the fourth and fifth lumbar nerves and the first through fourth sacral nerves, which come together inside the pelvis. Part of the fourth lumbar nerve and the fifth lumbar nerve join to form the lumbosacral trunk, which contributes to this network. It lies anterior to the piriformis muscle, deep along the lateral wall of the pelvis, and sends out several branches toward the greater sciatic foramen.

These branches control muscles that move the hip and carry skin sensation from the back of the leg and the foot. Some of the nerves that regulate organs inside the pelvis also arise in this region.

Within a plexus, fibers from one nerve root divide among several peripheral nerves, and each peripheral nerve contains a mixture of fibers from several roots. Rather than picturing a single continuous electrical wire, it is easier to think of a three-dimensional network in which routes split and rejoin, like a highway interchange. So, rather than dividing the body into simple map-like sections, such as “S1 supplies only this area,” it is more accurate to understand that these distributions overlap.

The branching pattern also varies somewhat from person to person. Anatomical studies examining the lumbosacral plexus have also found that variations in how nerve roots divide and join are common. These differences are often individual variations within the normal range.

Numbness or tingling, pulling sensations, and electric-shock-like sensations in the buttocks and back of the leg can arise anywhere along a nerve’s path. Possible sites include where the nerves emerge from the spine, inside the pelvis, near the piriformis muscle deep in the buttock, and along the route down the leg. The fact that these nerve pathways pass deep through the pelvis also explains why many people experience numbness or tingling in the buttocks and legs when sitting for long periods, with pelvic pressure in late pregnancy, or in a posture that shifts their weight to one side.

• Change positions. If you have been sitting in the same position for a long time, stand up, walk, and gently shake your legs.
• Loosen the buttock muscles. Try stretching the deep buttock muscles by lying down and gently pulling one knee toward the opposite side of your chest. Stop immediately if numbness or tingling worsens.
• Avoid sitting on hard surfaces for long periods. Using a cushion or taking your wallet out of your back pocket reduces pressure deep in the buttock.
• Walk and do light exercise. Increased blood flow makes the tissues around the nerve pathways more supple.

If you also develop leg weakness or a feeling that your foot is dropping, changes in sensation around the groin or anus, or changes in bladder or bowel control, have your condition assessed by a professional who treats the lower back and pelvis. The same applies if numbness or tingling continues for more than a few weeks.