Quick Facts
Origin: Second sacral nerve (S2).
Course: Emerges from the anterior sacral foramen and passes laterally to lie on the piriformis muscle.
Branches: Superior and inferior gluteal, deep and superficial fibular, sciatic and tibial nerves, nerves to piriformis and obturator externus, posterior cutaneous nerve of the thigh, and perforating cutaneous nerve.
Supply: Motor innervation to the lower limb. Sensory innervation from the lower anterior part of the leg, dorsum of the foot, heel region, and the area on the lateral side of the sole of the foot.
Origin
The anterior (ventral) ramus of second sacral nerve originates as one of two branches of the second sacral nerve, the other branch being the posterior ramus.
Course
The anterior ramus 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 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, deep to internal iliac vessels.
Branches
The anterior ramus of second 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 second sacral segment of the spinal cord. These are lower motor neurons which exit the spinal cord through the anterolateral sulcus, as they travel inside the anterior motor rootlets and root of second sacral spinal segment. They subsequently travel through the second sacral nerve to enter the anterior ramus before reaching the sacral plexus.
The efferent neurons travel through the branches of sacral plexus to supply motor innervation to the gluteal region and the lower limb. For instance, the efferent neurons enter the gluteal region through the following nerves: inferior gluteal nerve, nerve to piriformis, and nerve to obturator internus. They also enter the hamstring compartment via the sciatic nerve, the posterior compartment of the leg and the foot via the tibial nerve and its branches, and into the lateral compartment of the leg via the superficial fibular nerve.
The dendrites of the somatic afferent neurons travel through branches of the sciatic nerve to enter the sacral plexus. These include cutaneous branches of the superficial fibular and sural nerves, and the medial and lateral calcaneal nerves (branches of the tibial nerve). The somatic afferent neurons also travel through the posterior cutaneous nerve of the thigh and the perforating cutaneous nerve to enter the sacral plexus.
From the plexus onwards, these neurons travel through the sensory root and rootlets of the second sacral nerve. The cell bodies of these neurons are located inside the posterior root ganglion of the second sacral nerve. The axons then travel through the posterolateral sulcus to enter the right posterior sensory horn of the second sacral spinal cord segment.
The anterior ramus of second 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.
The pelvic splanchnic nerves contain preganglionic parasympathetic neurons. These travel towards the inferior hypogastric plexus to synapse with the cell bodies of the postganglionic neurons to innervate the pelvic viscera and genitalia.
Supplied Structures
The anterior ramus of the second sacral nerve supplies motor innervation to the gluteus maximus by the somatic efferent neurons traveling through the superior gluteal nerve. The efferent neurons also travel through various muscular branches of the sciatic nerve to innervate piriformis, obturator internus, superior gemellus, and the hamstring muscles (hamstring part of adductor magnus, biceps femoris, semimembranosus, and semitendinosus).
The efferent neurons from the same anterior ramus of the second sacral nerve travel further downwards to supply the following muscles in the leg region; peroneus longus and brevis muscles via the superficial fibular nerve, and flexor hallucis longus, flexor digitorum longus, gastrocnemius, plantaris, and soleus muscles via the tibial nerve.
Lastly, the efferent neurons from the anterior ramus of the second sacral nerve serve as the motor innervation of muscles in the foot region (extensor digitorum brevis, abductor hallucis, abductor digiti minimi, flexor digitorum brevis, quadratus plantae, and flexor hallucis brevis muscles).
The somatic afferent neurons from the deep fibular, 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 region, and the area on the lateral side of the sole of the foot.
