Anterior Root of Third Sacral Nerve

Origin

The anterior root of the third sacral nerve originates as a series of rootlets that emerge from the ventrolateral surface of the spinal cord. These quickly merge to form the anterior root.

Course

The anterior root runs inferiorly. At lumbar and sacral levels, the anterior root descends through the vertebral column until it reaches the third sacral foramen. Just before entering this foramen, the anterior and posterior roots merge to form the spinal nerve.

Branches

There are no named branches; however, fibers do contribute to the sacral plexus. Specifically, the sciatic and pudendal nerves. Parasympathetic fibers exit the spinal cord via the second, third, and fourth sacral nerves, and combine to form the pelvic splanchnic nerve.

Supplied Structures

The anterior root of the third sacral nerve supplies the multifidus muscle via the posterior ramus. It also contributes to the sacral plexus, and the following nerves and targets.

- The sciatic nerve is the largest nerve in the body. It gives rise to the common fibular, tibial, and sural nerves. The third sacral nerve specifically contributes to the tibial portion of the sciatic nerve. It arises from the fourth and fifth lumbar nerves, as well as the first, second, and third sacral nerves. The sciatic nerve innervates all the muscles of the posterior thigh, anterior and posterior leg, and foot. The tibial branch specifically innervates the muscles of the posterior thigh, leg, and foot.

- The pudendal nerve arises from the second, third, and fourth sacral nerves and transmits motor and sensory innervation to the abdomen and pelvis. Motor innervation is to the muscles of the perineum (bulbospongiosus, ischiocavernousus, superficial transverse perineal, deep transverse perineal, external anal, and urethral sphincter muscles).

Muscles significantly innervated by the anterior ramus of the third sacral nerve include the perineal muscles, and the external anal and urethral sphincter muscles.

The pelvic splanchnic nerve carries parasympathetic innervation to the hindgut organs, kidneys, reproductive organs, and genitals. Parasympathetic fibers exit the spinal cord via the second, third, and fourth sacral nerves, and combine to form the pelvic splanchnic nerve.