Anterior Ramus of Fourth Sacral Nerve (Left)

Origin

The anterior or anterior ramus of fourth sacral nerve originates as one of two branches of the fourth sacral nerve, the other branch being the posterior ramus.

Course

The anterior ramus of fourth 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 fourth sacral nerve (unlike other rami of the lumbosacral plexus) does not divide into anterior and posterior divisions. It contributes to the sacral plexus.

Branches

The anterior ramus of fourth 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 fourth 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 fourth sacral spinal segment. They subsequently travel through the fourth sacral nerve to enter the anterior ramus before reaching the sacral plexus.

The efferent neurons eventually innervate structures in the pelvic cavity and the perineum through direct muscular branches of fourth sacral anterior ramus and the inferior rectal and perineal branches of the pudendal nerve.

The somatic afferent neurons travel through the anococcygeal nerve and the cutaneous branches of the pudendal nerve (inferior rectal nerve, posterior scrotal nerve (males) or posterior labial nerve (females), posterior nerve of the penis (males) or clitoris (females)) to enter the sacral plexus. From the plexus onwards, these neurons travel through the sensory root and rootlets of the fourth sacral nerve. The cell bodies of these neurons are located inside the posterior root ganglion of the fourth sacral nerve. The axons then travel through the posterolateral sulcus to enter the right posterior sensory horn of the fourth sacral spinal cord segment.

The anterior ramus of fourth 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, which emerge from the S4 (and S2, S3) roots, 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 fourth sacral nerve supplies motor innervation, through the somatic efferent neurons, to the levator ani and coccygeus muscles via direct muscular branches, as well as through the inferior rectal nerve branch of the pudendal nerve. The efferent neurons in the latter nerve, also provide innervation to the external anal sphincter.

Some efferent neurons travel through the perineal branches of pudendal nerve to supply muscles inside the deep perineal pouch (deep transverse perineal muscle, external urethral sphincter, compressor urethrae, and sphincter urethrovaginalis muscles) and superficial perineal pouch (superficial transverse perineal muscle, ischiocavernosus, and bulbospongiosus muscles).

The somatic afferent neurons conduct cutaneous general sensory information from:

- the perianal skin over the anal triangle via the anococcygeal and inferior rectal nerves;

- skin on the posterior surface of scrotum (males) or labium majus (females) via the posterior scrotal or labial nerve;

- the body and glans of the penis (males) or clitoris (females) via the posterior nerve of the penis (males) or clitoris (females).