Sacral Spinal Roots & Ganglia (Left)

Description

Five of the 31 pairs of spinal nerves emerge from the sacral spine. These five pairs of spinal nerves are known as the sacral nerves and are numbered from the first to fifth sacral nerves.

The sacral nerves are mixed nerves, as each is formed by the union of its posterior root (sensory nerve fibers) and its anterior root (motor nerve fibers). Located at the proximal end of each posterior root is a spinal ganglion. The ganglia contain the cell bodies of sensory nerve fibers. Each of the five sacral nerves terminates by dividing into anterior and posterior rami.

The posterior rami of the first to third sacral nerves innervate the multifidus muscle and skin of the buttocks close to the midline. The posterior rami of the fourth and fifth sacral nerves combine with that of the coccygeal nerve to innervate the skin over the coccyx.

The anterior rami of the first to fourth sacral nerves, together with the lumbosacral trunk (fourth and fifth lumbar nerves), form the sacral plexus. There are five major branches of the sacral plexus, each of which combine fibers from two or more rami. The branches are the superior gluteal, inferior gluteal, sciatic, posterior femoral, and pudendal nerves. Additionally, there are minor branches that serve smaller territories. These include the nerve to the obturator internus, nerve to piriformis, nerve to quadratus femoris, and the perforating cutaneous nerve.

The nerves of the sacral plexus innervate muscles of the pelvis and perineum, as well as the skin overlying the buttocks, posterior lower limbs, anal region, and genitals. In addition, the anterior rami of the second, third, and fourth sacral nerves carry parasympathetic fibers. These target the hindgut, pelvic viscera, and genitals.