Anterior Ramus of First Sacral Nerve (Right)

Origin

The anterior ramus of the fist sacral nerve originates as one of two branches of the first sacral nerve, the other being the posterior ramus.

Course

The anterior ramus of the first sacral nerve passes out from its respective anterior sacral foramen on the anterior surface of the sacrum. It then passes behind the lateral sacral artery and comes to lie in front of the piriformis muscle, as it courses laterally and inferiorly on the pelvic wall.

The anterior ramus of the first sacral nerve further divides into anterior and posterior divisions, which contribute to the sacral plexus. The plexus is situated on the posterolateral pelvic wall, on the anterior surface of piriformis muscle.

Branches

The anterior ramus of the first sacral nerve is a mixed nerve as it contains both somatic efferent (motor) and afferent (sensory) neurons.

The somatic efferent neurons emerge from the anterior gray horn of the first sacral segment of the spinal cord. These are lower motor neurons which exit the cord through the anterolateral sulcus as they travel inside the anterior motor rootlets and root of first sacral spinal segment. They subsequently travel through the first sacral nerve to enter the anterior ramus before reaching the sacral plexus. The efferent neurons travel through the branches of sacral plexus to provide motor innervation to the gluteal region and lower limb. For instance, the efferent neurons enter the gluteal region via the superior and inferior gluteal nerves, nerve to piriformis, nerve to obturator internus, and nerve to quadratus femoris. They also enter the hamstring compartment via the sciatic nerve; into the posterior compartment of the leg and the foot via the tibial nerve and its branches; into the lateral compartment of the leg via the superficial fibular nerve; into the anterior compartment of the leg via the deep fibular nerve.

The somatic afferent neurons travel through branches of the sciatic nerve to enter the sacral plexus. These include the cutaneous branches of the deep fibular, superficial fibular, sural, and medial and lateral calcaneal nerves. The somatic afferent neurons also travel through the posterior cutaneous nerve of the thigh to enter the sacral plexus.

From the plexus onwards, these neurons travel through the sensory root and rootlets of the first sacral nerve. The cell bodies of these neurons are located inside the spinal ganglion of the first sacral nerve. The axons then travel through the posterolateral sulcus to enter the right posterior sensory horn of the first sacral spinal cord segment.

The anterior ramus of the first sacral nerve is also connected to the sympathetic trunk through the gray communicating branch (ramus communicans), which serves as a conduit for the postganglionic sympathetic neurons.

Supplied Structures

The anterior ramus of first sacral nerve supplies motor innervation to the gluteal muscles (gluteus maximus, medius and minimus, and tensor fasciae latae muscles) by the somatic efferent neurons traveling through the superior and inferior gluteal nerves. The efferent neurons also travel through various muscular branches to innervate muscles such as piriformis, obturator internus, superior and inferior gemelli, quadratus femoris, and the hamstrings (hamstring part of adductor magnus, biceps femoris, semimembranosus, and semitendinosus muscles).

The efferent neurons from the anterior ramus of first sacral nerve travel further to supply the following muscles in the leg, namely extensor hallucis longus, extensor digitorum longus, and peroneus tertius muscles via the deep fibular nerve; peroneus longus and brevis muscles via the superficial fibular nerve; popliteus muscle via the tibial nerve.

In addition, the anterior ramus of first sacral nerve serves as the motor innervation of the following muscles in the foot: extensor digitorum brevis, abductor hallucis, abductor digiti minimi, flexor digitorum brevis, quadratus plantae, and flexor hallucis brevis muscles.

Somatic afferent neurons from the deep and superficial fibular, sural, and medial and lateral calcaneal nerves transmit general sensations from the lower anterior part of the leg, dorsum of the foot, heel, and the lateral side of the sole of the foot.