Muscles of the pelvic floor

Gross anatomy

The pelvis marks an important transition point between the thoracoabdominal region and the lower limbs. Not only is it important for walking, but it also houses organs of the urogenital and distal digestive systems and acts as a conduit for arteries, veins, lymphatic vessels, and nerves necessary for daily functioning. The pelvis is a musculoskeletal structure that is made up of hip and sacrococcygeal bones, along with several muscular layers. It is further divided into the greater (false) and lesser (true) pelvis. The false pelvis is the wide-area just above the inlet between the ala of the ilia while the true pelvis is the area between the inlet and the outlet. It has two lateral walls, a posterior wall (sacrococcygeal bones), and a muscular floor.

The lower part of the pelvis is sealed off by a muscular diaphragm and perineal membrane known as the pelvic floor. There are two (males) or three (females) openings that allow passage of the outlet components of the pelvic viscera in the pelvic floor. The muscles of the pelvic floor contribute to maintaining continence and help prevent the contents of the pelvic cavity from falling through its outlet. 

Muscles

The muscles of the pelvic floor are collectively referred to as the levator ani and coccygeus muscles. They form a large sheet of skeletal muscle that is thicker in some areas than in others. The muscles are attached along the inner walls of the true pelvis to a condensed area of the obturator fascia known as the tendinous arch of levator ani muscle. They can be subdivided based on their points of attachment as well as the pelvic organs with which they are associated. Note that the levator ani is made up of the puborectalis, pubococcygeus, and iliococcygeus muscles. The coccygeus (also referred to as ischiococcygeus) is not part of the levator ani. 

The pelvic surface of the levator ani is separated from the visceral organs by their associated fascia. The perineal surface functions as the medial and superior walls of the ischioanal fossa and its associated anterior recess respectively. There is loose connective tissue between the posterior border of the muscle and the coccyx. Finally, the outlets of the visceral organs separate the medial border of the two muscles. 

Blood supply

The anterior division of the internal iliac artery is responsible for supplying the levator ani group of muscles with oxygenated, nutrient-rich blood. Its three terminal branches – the pudendal, inferior gluteal, and inferior vesical arteries – access and pierce these muscles in order to supply them. Venous drainage is achieved by the similarly named veins.

Learn more about the arterial blood supply and venous drainage of the pelvic floor and pelvis with these study units!

Innervation

Branches from the sacral plexus contribute to the innervation of the levator ani group of muscles. The pudendal nerve, which originates from the second to fourth sacral segments (S2-S4) directly innervate the pubococcygeus muscle. Direct branches arising from the fourth sacral segment form the nerve to levator ani (S4), which also innervates the pubococcygeus. The remaining coccygeus and iliococcygeus are innervated by direct branches from the fourth and fifth segments (S4 and S5) of the sacral plexus.

Check out these resources to solidify your knowledge about the sacral plexus and innervation of the pelvic floor muscles.

Function

The muscles of the pelvic floor are primarily supportive structures. They help keep the pelvic viscera in place and prevent them from being pushed through the pelvis during strain. It achieves this task by being unconsciously contracted at rest and can be consciously contracted during times of raised intra-abdominal pressure (vomiting, sneezing, coughing, lifting a heavy object, or forced expiration). 

Contraction of the levator ani muscles also provides additional occlusion to the outlet segments of the pelvic viscera. In other words, the muscles aid in maintaining both urinary and fecal continence until it is convenient to void. The puborectalis muscle best demonstrates this function. Recall that it is a muscular sling U-shaped muscle that arches around the anorectal junction. When this part of the muscle is contracted, it pulls the anorectal junction anteriorly, forming a 90 degrees angle between the rectum and anus. Therefore fecal matter cannot flow freely from the rectum. In order for micturition (urination) and defecation to occur, the levator ani muscles must be relaxed.

Want to learn more about the muscles of the pelvic floor? With attachments, innervations and functions clearly detailed in handy revision tables, our trunk wall muscle chart has you covered. 

The pelvic floor muscles also provide additional support during childbirth to the presenting fetal part – the part closest to the uterine outlet. It holds the fetus in place while the uterine cervix dilates and contracts. It also keeps the presenting part of the fetus in the anteroposterior plane of the pelvic outlet to further support the process of delivery.

Histologically, the majority of the pelvic floor muscles are made up of slow-twitch or type I muscle fibers. The prevalence of type I fibers is important given the function of the pelvic floor muscles outlined above. Recall that type I fibers are ideal for long periods of contraction, while type II fibers are needed for quick response to physiological changes. 

Embryology

The muscles of the pelvic floor can be contracted under voluntary control and are therefore skeletal muscles. This means that these muscle fibers arise from the dermomyotomes of somites and undergo an epithelial to mesenchymal transition. Multiple transformation factors promote the conversion of mesenchymal cells to myoblasts (primitive muscle cells).

Myoblasts are characterized by longer nuclei and cell bodies in comparison to their precursor cells. Some of these myoblasts fuse to form myotubes – multinucleated cylindrical cells. As fusion takes place, myofibrils and other skeletal muscle organelles begin to appear in the cell cytoplasm. As the myoblasts continue to form they migrate away from their myotomes to form non-segmented muscle groups. Those of the pelvic floor arises from the hypaxial division of the sacrococcygeal myotomes.