What is the neuroanatomy of pelvic floor relevant to the urologic management of neurologic disease?

Updated: Aug 08, 2019
  • Author: Frances M Dyro, MD; Chief Editor: Robert A Egan, MD  more...
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The striated muscle forming the urethral rhabdosphincter and the periurethral striated muscle (part of the pelvic diaphragm) together make up the external urethral sphincter mechanism in humans. In women, the rhabdosphincter forms a 1.5-cm long circular ring around the middle third of the urethra, extending cranially as far as the posterior bladder base. [2] In men, the rhabdosphincter has 3 sections extending over a greater length of urethra. On cystoscopy, this striated muscle can be seen to contract with electrical stimulation of the pudendal nerve.

Spinal cord nuclei supplying the vesicourethral smooth muscle and rhabdosphincter are in the lumbosacral region. The sympathetic autonomic nucleus is in the anteromediolateral gray matter at T10-T12, and the parasympathetic nucleus is at S2-S4. Motor neurons of the urethral rhabdosphincter are in the nucleus of Onufrowicz (Onuf) in the sacral ventral horns. The neurons are smaller, more spherical, and more closely packed than other anterior horn cells.

The nucleus of Onuf and the sacral parasympathetic nucleus are at slightly different levels. This can be of clinical significance in lesions of the conus. The neurons of the Onuf nucleus are relatively spared in many neuromuscular disorders.

The somatomotor supply to the rhabdosphincter is via the pudendal nerves. This has been documented by direct stimulation studies and in horseradish peroxidase tracings; however, some studies have shown contributions from some branches of the pelvic plexus. The role of these branches is in question; an autonomic function has been postulated.

In dogs, complete silence of the rhabdosphincter is seen only if the pudendal and pelvic efferents are sectioned. The Onuf nucleus shows changes after injection of horseradish peroxidase into either the pudendal or the pelvic nerve. Some evidence suggests that the rhabdosphincter in cats has triple innervation, with cholinergic, adrenergic, possibly copeptidergic, and traditional motor endplates.

The pudendal nerve leaves the pelvis through the lower part of the greater sciatic notch beneath the lower border of the piriformis (see the images below). It bifurcates just before the sacrospinal ligament, with one branch going to the anal sphincter and the other to the urethral sphincter. Studies on children with meningomyelocele have suggested that different sacral levels supply the 2 sphincters, so that absence of activity in one is not invariably accompanied by a deficit in the other.

Pelvic neuroanatomy. Pelvic neuroanatomy.
Pelvic muscles and fascia. Pelvic muscles and fascia.

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