Nerves of the Pelvis

Although the muscles of the pelvic floor were initially thought to have innervation both from direct branches of the sacral nerves on the pelvic surface and via the puden-dal nerve on the perineal surface, recent anatomic, neuro-physiologic, and experimental evidence indicates that these standard descriptions are inaccurate and that the levator ani muscles are innervated solely by a nerve traveling on the superior (intrapelvic) surface of the muscles without contribution of the pudendal nerve1,2 (Figure 4-2.5).

Barber et al.1 performed systematic cadaver dissections to characterize the nerve supply to the pelvic floor muscles. The nerve supplying the coccygeus muscle and the levator ani muscles (all three) originates from S3,S4, and/or S5. The nerve exits the foramina and travels 2 to 3 cm medial to the ischial spine and arcus tendineus levator ani across the coccygeus, iliococcygeus, pubococcygeus, and puborectalis muscles. The nerve is sometimes firmly embedded in the fascia of the muscles or may be loosely attached during its course. There are small branches that penetrate the body of each muscle as the nerve traverses them. Occasionally, a separate nerve comes directly from S5 to innervate the pub-orectalis muscle. The piriformis receives innervation directly from sacral nerves (motor efferent) from L5-S2.

In the pelvis, the sympathetic nerves to the pelvis originate at the T5 to L2 spinal level and act to promote storage by causing relaxation of the bladder and rectum and contraction of the smooth muscle components of the urethral and anal sphincter. The parasympathetic nerve supply to the pelvic viscera originates from the second, third, and fourth sacral nerves. The parasympathetic nerves combine

Piriformis Levator

Muscle Plate

Figure 4-2.4. Muscles of the pelvis (abdominal view). (Reprinted with the permission of The Cleveland Clinic Foundation.)

Piriformis Levator

Muscle Plate

Figure 4-2.4. Muscles of the pelvis (abdominal view). (Reprinted with the permission of The Cleveland Clinic Foundation.)

Ischial Spine

Ischial Spine

Figure 4-2.3. Muscles of the pelvis (sagittal view). (Reprinted with the permission of The Cleveland Clinic Foundation.)

Figure 4-2.5. Nerves and vessels of the pelvis. (Reprinted with the permission of The Cleveland Clinic Foundation.)

with the hypogastric plexus and pelvic sympathetic nerves to form the pelvic nerve plexus. This plexus of nerves leaves the sacral surface to fan out on either side of the rectum approximately 3 to 4 cm superior to the pelvic floor muscles, then disperse throughout the pelvis through the endopelvic fascia.

Pelvic floor muscles have constant tone except during voiding, defecation, and during the Valsalva maneuver. This activity serves as a constant support for the pelvic viscera. The levator muscles and the skeletal components of the urethral and anal sphincters all have the ability to contract quickly at the time of an acute stress, such as a cough or sneeze, to maintain continence.

Because of the nerves' undefined course and small size, iatrogenic damage during pelvic surgery is possible. This may produce a range of effects both sensory and motor in nature. Radical hysterectomy and rectal resection are common causes of pelvic plexus injury resulting in bowel and/or bladder dysfunction.

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