The CNS is composed of the spinal cord and brain. There are twelve paired cranial nerves and 31 paired spinal nerves that innervate the skeletal muscle. The spinal cord processes all the sensory information, controls body movements, and regulates visceral functions. It serves as a passageway for sensory information to the brain.

The PNS is divided into the somatic and autonomic divisions, each containing both afferent and efferent elements. The somatic division provides sensory neurons to the skin, muscles, and joints and handles voluntary control of body movements. The motor nerves control skeletal muscle movement and the sensory nerves return information to the CNS involving touch, proprioception, pain, and temperature. The autonomic division components are involved in the involuntary control system, and in the pelvis this relates to the function of the pelvic viscera.

In the somatic division, the primary motor neurons are α, β, and γ. The α neurons innervate the extrafusal muscle fibers (muscles that contract) and the γ neurons innervate the intrafusal muscle (proprioception) fibers [1]. The somatic efferent fibers originate from motor neurons in the anterior cell column of the spinal cord and exit the spinal cord at the dorsal root ganglion to form the spinal nerve [3]. In the pelvis, these fibers make up the sacral plexus, which is located on the posterior wall of the pelvis anterior to the piriformis muscle (Fig. 2.3). The nerves are formed by the ventral rami of the first through fourth sacral nerve roots (S1–S4) and the fourth and fifth lumbar nerve roots (L4–L5). Somatic afferent fibers supply the peritoneum, urethra, vagina, and rectum. The dorsal horn of the spinal cord and the dorsal column nuclei are the places where the information from the body surface and tissue travel to reach the brain [4].

The pudendal nerve is composed of the ventral roots of the second, third, and fourth sacral nerve roots and originates from the ventral portion of the anterior horn. It carries somatic motor, general sensory, and visceral sympathetic motor nerve fibers, contributed from the sacral plexus and pelvic plexus, correspondingly [5]. The pudendal nerve exits the pelvic cavity below the piriformis through the greater sciatic foramen. It reenters the pelvic cavity by curving around the ischial spine and passing through the lesser sciatic foramen. It continues down the lateral wall of the ischiorectal fossa into Alcock’s canal, which is caudal to the levator ani muscles. The pudendal nerve separates into three branches: the dorsal clitoral nerve, the inferior rectal nerve, and the perineal nerve which innervate the clitoris, the external anal sphincter, and the perineal musculature and inner perineal skin, respectively [6].

Several other somatic nerves are encountered in the pelvis during pelvic surgery. The ilioinguinal nerve departs from the first lumbar nerve root and is responsible for sensation of the skin covering the upper and medial part of the thigh, the mons pubis, and labia majora. The genitofemoral nerve originates from the first and second lumbar nerve roots and also innervates the skin of the mons pubis and labia majora.

The autonomic nervous system in the pelvis is composed of the thoracolumbar outflow which is sympathetic and the sacral outflow which is parasympathetic. The efferent fibers carry information from the CNS to the organs and afferent fibers carry sensory information away from the internal organs back to the CNS. This system is distinctive in that the preganglionic and postganglionic neurons are in the PNS [1]. The sympathetic system of the autonomic nerves controls the “fight or flight” response, while the parasympathetic system controls the “rest response.” The nerves generally travel in large bundles referred to as plexuses that contain both sympathetic and parasympathetic nerves fibers (Fig. 2.3). For example, the pelvic plexus or inferior hypogastric plexus is primarily located on the pelvic sidewall but stretches out over the rectum and vaginal sidewalls to the base of the bladder. The plexus lies internal to the internal iliac arteries and travels with the arteries to the visceral organs. It is composed of both sympathetic and parasympathetic fibers from the hypogastric nerves, sacral splanchnic nerves, and the pelvic splanchnic nerves. The pelvic plexus can be further subdivided into the rectal plexus, uterovaginal plexus, and the vesical plexus.

The parasympathetic nerves in the pelvis arise from S2–S4 and are also referred to as the pelvic splanchnic nerves. Their cell bodies are in the lateral gray horn of the spinal cord and the axons extend within the pelvic nerve to the ganglia adjacent to the end organ [7]. The afferent fibers control primarily unconscious visceral motor reflex sensations, like bladder distension, rectal fullness, and sexual sensation. The spinothalamic and spinoreticular tracts receive afferent fibers from the vagina and clitoris and are linked to arousal and pleasure sensation [4]. Efferent fibers originate in the intermediolateral cell column at S2–S4 [3]. The preganglionic and postganglionic synapses primarily use ACh as the neurotransmitter.