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.
Sacral plexus
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).
