Pelvic Floor Anatomy and Applied Physiology

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Pelvic floor muscles have two major functions: they provide support or act as a floor for the abdominal viscera including the rectum; and they provide a constrictor or continence mechanism to the urethral, anal, and vaginal orifices (in females). This article discusses the relevance of pelvic floor to the anal opening and closure function, and discusses new findings with regards to the role of these muscles in the vaginal and urethra closure mechanisms.

Section snippets

Internal Anal Sphincter

Circular muscle layer of the rectum expands caudally into the anal canal and becomes the IAS. The circular muscles in the sphincteric region are thicker than those of the rectal circular smooth muscle with discrete septa in between the muscle bundles. Similarly, the longitudinal muscles of the rectum extend into the anal canal and end up as thin septa that penetrate into the puborectalis and EAS muscles. Longitudinal muscle of the anal canal is also referred to as the “conjoined tendon”

Puborectalis and deep pelvic floor muscles

In 1555, Vesalius14 wrote an account of the pelvic floor muscles, which he named “musculus sedem attollens.” This was later replaced by the more definitive name of “levator ani” by Von Behr and coworkers.15 The pelvic diaphragm, first so named in 1861 by Meyer,16 included primitive flexors and abductors of the caudal part of the vertebral column. These muscles included coccygeus (also referred to as “ischiococcygeus”), ileococcygeus, and pubococcygeus and these three muscles were believed to

Pelvic floor imaging

Advances in MRI, CT scanning, and three-dimensional US imaging have provided novel insights into the anatomy and function of the pelvic floor muscles. Ultrafast CT scanning can image dynamic changes in the pelvic floor muscle during contraction and defecation.25 These studies reveal that the levator hiatus becomes smaller during pelvic floor contraction and larger during the act of defecation (Fig. 5). The changes in the pelvic floor hiatus size are predominantly related to the puborectalis

Sensory function of the rectum and anal canal

Similar to other viscera, colonic distention results in nondescript discomfort and at higher degrees of distention one feels pain that is poorly localized. Rectal distention, however, is perceived as rectal fullness that is more localized and somewhat defined (ie, as a desire to defecate). In addition to mucosal nerve endings, there are also low threshold, slowly adapting mechanoreceptors in the muscularis propria of the rectum. These intraganglionic laminar endings detect mechanical

Applied physiology

Ideally speaking, one should describe the function of each component of the pelvic floor muscle individually; however, no such information is available. Broadly, the pelvic floor muscles can be considered to have two important functions. They provide support or “floor” to the pelvic viscera; and they provide constrictor functions to the urethra, vagina, and anal canal. Described next is the role of the pelvic floor muscles on the rectum and anal canal, touching on their role in the closure

Anal canal pressure

Anal canal pressure is a major determinant of the strength of anal continence mechanism and its brief discussion is extremely relevant. Anal canal pressure can be measured with perfusion manometry (using either side hole or sleeve sensor); solid-state transducers; or more recently with a large number of closely spaced array of pressure sensors (high-resolution manometry).29 Furthermore, the pressures can be displayed in the form of colored topographic (contour) plots, which are convenient to

Vaginal high-pressure zone

How does puborectalis, a U-shaped muscle, increase the anal canal pressure? The two anterior limbs of puborectalis muscle are attached to the two pubic rami and posteriorly they join each other behind the anal canal. Contraction of the puborectalis muscle lifts up the anal canal in the ventral or anterior direction and in so doing causes compression of the anal canal, vagina, and urethra against the back of pubic symphysis (Fig. 11). It then follows that there would be a high-pressure zone in

Summary

Pelvic floor muscles have two important functions: they provide physical support to the pelvic viscera; and they provide constrictor mechanism to the anal canal, vagina, and urethra. Newer imaging and physiologic studies strongly suggest that these two functions of the pelvic floor are quite distinct and are likely related to different components of the pelvic floor muscles. The pubococcygeus, ileococcygeus, and ischiococcygeus most likely provide the physical support or act as a floor for the

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    Supported by National Institutes of Health grant RO1-DK60733.

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