Pelvic Nerves Anatomy
The pelvic organs receive their autonomic innervation via the right and left pelvic plexuses, which lie adjacent to the internal iliac arteries and their branches (Fig. 5.30). Nerves pass from the plexuses to the bladder, reproductive organs and the rectum by accompanying the arteries to these organs. The plexuses and their branches contain efferent fibres from both the parasympathetic and sympathetic systems, which reach the pelvis from different parts of the spinal cord.
The parasympathetic component of the pelvic plexuses is provided by the pelvic splanchnic nerves (nervi erigentes), which leave the spinal cord in the second, third and fourth sacral nerves. The parasympathetic fibres control micturition, dilation of the erectile tissues in both sexes, and defecation. The pelvic plexuses also provide the parasympathetic innervation of the descending and sigmoid parts of the colon. These fibres ascend into the abdomen in the hypogastric plexus and are distributed with the branches of the inferior mesenteric artery. Extensive dissections during surgical excision of the prostate gland or the rectum may damage the parasympathetic innervation (cavernous nerves) to the erectile tissues, resulting in erectile dysfunction (impotence).
The sympathetic fibres destined for the pelvic autonomic plexuses arise from the lower thoracic and upper lumbar segments of the spinal cord and pass through the lumbar portions of the sympathetic trunks on the posterior abdominal wall. From here, they descend in the hypogastric plexus to reach the pelvis. Sympathetic fibres innervate the smooth muscle of the reproductive organs in both sexes, and in the male are responsible for coordinating ejaculation (p. 231).
The two sympathetic trunks pass from the posterior abdominal wall (p. 198) into the pelvis by crossing behind the common iliac vessels. Descending in front of the sacrum and piriformis muscles (Fig. 5.31), they incline towards the midline and fuse on the anterior surface of the coccyx. Each trunk bears three or four ganglia and provides grey rami communicantes, consisting of postganglionic fibres, to the sacral nerves. These fibres supply blood vessels and sweat glands in the areas innervated by the appropriate sacral nerves.
The lower lumbar and upper sacral spinal nerves are predominantly concerned with the innervation of the lower limb. However, a few fibres derived from these spinal nerves are distributed to the pelvic walls and floor and to the perineum.
This branch of the lumbar plexus (Fig. 4.101) emerges from the medial border of psoas major and enters the pelvis by crossing in front of the ala of the sacrum. It descends lateral to the common and internal iliac vessels and the ureter (Fig. 5.30) and reaches the medial surface of obturator internus. The nerve approaches the obturator vessels from above and continues with them through the obturator canal (Fig. 5.31) into the medial compartment of the thigh (p. 266). On the lateral pelvic wall, the obturator nerve may be closely related to the ovary (p. 221 and Fig. 5.13). Here, ovarian disease may spread to involve the pelvic peritoneum or the nerve itself, producing pain referred to the medial side of the thigh.
Sacral and coccygeal nerves
The anterior rami of the first four sacral nerves emerge through the anterior sacral foramina and merge to form the sacral plexus (Fig. 5.31). The fifth sacral nerve and the coccygeal nerves are small and do not contribute to the plexus. All the sacral and coccygeal nerves receive grey rami communicantes from the sympathetic trunk.
This plexus lies on the posterior pelvic wall in front of piriformis (Fig. 5.31), covered anteriorly by the pelvic fascia. The plexus is formed by the anterior rami of the upper four sacral nerves and is supplemented by the lumbosacral trunk, which carries fibres from the fourth and fifth lumbar nerves (p. 203). The branches of the sacral plexus are distributed to the lower limb, pelvic walls and floor, and perineum.
Those branches that leave the pelvis accompany piriformis through the greater sciatic foramen to enter the buttock (pp 270, 271). The nerve to obturator internus and the pudendal nerve then pass forwards through the lesser sciatic foramen to gain the perineum (Fig. 5.28).