Pelvic floor muscles have two major functions; they provide 1; support or act as a “ floor” for the abdominal viscera including the rectum and 2; constrictor or continence mechanism to the urethral, anal and vaginal orifices (in females). Here, we will discuss the relevance of pelvic floor to the anal opening and closure function, and discuss new findings with regards to the role of these muscles in the vaginal and urethra closure mechanisms.
The bony pelvis is composed of sacrum, ileum, ischium, and pubis. It is divided into the false (greater) and true (lesser) pelvis by the pelvic brim. The sacral promontory, the anterior ala of the sacrum, the arcuate line of the ilium, the pectineal line of the pubis and the pubic crest that culminates in the symphsis pubis. The shape of the female bony pelvis can be classified into four broad categories: gynecoid, anthropoid, android, and platypelloid. The pelvic diaphragm is a wide but thin muscular layer of tissue that forms the inferior border of the abdominopelvic cavity. Composed of a broad, funnel-shaped sling of fascia and muscle, it extends from the symphysis pubis to the coccyx and from one lateral sidewall to the other. The urogenital diaphragm, also called the triangular ligament, is a strong, muscular membrane that occupies the area between the symphysis pubis and ischial tuberosities and stretches across the triangular anterior portion of the pelvic outlet. The urogenital diaphragm is external and inferior to the pelvic diaphragm. The pelvic ligaments are not classic ligaments but are thickenings of retroperitoneal fascia and consist primarily of blood and lymphatic vessels, nerves, and fatty connective tissue. Anatomists call the retroperitoneal fascia subserous fascia, whereas surgeons refer to this fascial layer as endopelvic fascia. The connective tissue is denser immediately adjacent to the lateral walls of the cervix and the vagina. The broad ligaments are a thin, mesenteric-like double reflection of peritoneum stretching from the lateral pelvic sidewalls to the uterus. The cardinal, or Mackenrodt's, ligaments extend from the lateral aspects of the upper part of the cervix and the vagina to the pelvic wall. The uterosacral ligaments extend from the upper portion of the cervix posteriorly to the third sacral vertebra.
The pelvic floor is comprised of number of muscles and they are organized into superficial and deep muscle layers. There is significant controversy with regards to the nomenclature, but generally speaking, the superficial muscle layer and the muscles relevant to the anal canal function are the external anal sphincter, perineal body and possibly the puboperineal (or transverse perinei) muscles (Figures 1A and 1B). The deep pelvic floor muscles consist of pubococcygeus, ileococcygeuys, coccygeus and puborectalis muscles. In fact, puborectalis muscle is located in between the superficial and deep muscle layers, and it is better to view this as the middle muscle layer of the pelvic floor. In addition to the skeletal muscles of the pelvic floor, caudal extension of the circular and longitudinal smooth muscles from the rectum into the anal canal constitutes the internal anal sphincter and external anal sphincter of the anal canal respectively. First, we discuss the salient as well as some of the controversial aspects of anatomy of the pelvic floor and anal sphincter muscles, followed by a discussion of the function of each of component of the pelvic floor muscles and to their role in anal sphincter closure and opening.