The Physiology of the Rectosigmoid Region Anal Canal and Pelvic Floor Musculature Maintains Fecal Continence

The sigmoid colon and rectum are reservoirs with a capacity of up to 500 mL in humans. Distensibility in this region is an adaptation for temporarily accommodating the mass movements of feces. The rectum begins at the level of the third sacral vertebra and follows the curvature of the sacrum and coccyx for its entire length. It connects to the anal canal surrounded by the internal and external anal sphincters. The pelvic floor is formed by overlapping sheets of striated fibers called levator ani muscles. This muscle group, which includes the puborectalis muscle and the striated external anal sphincter, comprise a functional unit that maintains continence. These skeletal muscles behave in many respects like the somatic muscles that maintain posture elsewhere in the body (see Chapter 5).

The pelvic floor musculature can be imagined as an inverted funnel consisting of the levator ani and external sphincter muscles in a continuous sheet from the bottom margins of the pelvis to the anal verge (the transition zone between mucosal epithelium and stratified squamous epithelium of the skin). After defecation, the levator ani contract to restore the perineum to its normal position. Fibers of the puborectalis join behind the anorectum and pass around it on both sides to insert on the pubis. This forms a U-shaped sling that pulls the anorectal tube anteriorly, such that the long axis of the anal canal lies at nearly a right angle to that of the rectum (Fig. 26.38). Tonic pull of the puborectalis narrows the anorectal tube from side to side at the bend of the angle, resulting in a physiological valve that is important in the mechanisms that control continence.

The puborectalis sling and the upper margins of the internal and external sphincters form the anorectal ring, which marks the boundary of the anal canal and rectum. Surrounding the anal canal for a length of about 2 cm are the internal and external anal sphincters. The external anal sphincter is skeletal muscle attached to the coccyx posteriorly and the perineum anteriorly. When contracted, it compresses the anus into a slit, closing the orifice. The internal anal sphincter is a modified extension of the circular muscle layer of the rectum. It is comprised of smooth muscle that, like other sphincteric muscles in the digestive tract, contracts tonically to sustain closure of the anal canal.

Sensory Innervation and Continence. Mechanorecep-tors in the rectum detect distension and supply the enteric neural circuits with sensory information, similar to the innervation of the upper portions of the GI tract. Unlike the rectum, the anal canal in the region of skin at the anal verge is innervated by somatosensory nerves that transmit signals to the CNS. This region has sensory receptors that detect touch, pain, and temperature with high sensitivity. Processing of information from these receptors allows the in-

Rectum Puborectalis Muscle

Structural relationship of the anorectum ^iÈIÉIIHBMÊFancI puborectalis muscle. One end of the puborectalis muscle inserts on the left pubic tubercle, and the other inserts on the right pubic tubercle, forming a loop around the junction of the rectum and anal canal. Contraction of the pub-orectalis muscle helps form the anorectal angle, believed to be important in the maintenance of fecal continence.

Structural relationship of the anorectum ^iÈIÉIIHBMÊFancI puborectalis muscle. One end of the puborectalis muscle inserts on the left pubic tubercle, and the other inserts on the right pubic tubercle, forming a loop around the junction of the rectum and anal canal. Contraction of the pub-orectalis muscle helps form the anorectal angle, believed to be important in the maintenance of fecal continence.

dividual to discriminate consciously between the presence of gas, liquid, and solids in the anal canal. In addition, stretch receptors in the muscles of the pelvic floor detect changes in the orientation of the anorectum as feces are propelled into the region.

Contraction of the internal anal sphincter and the pub-orectalis muscles blocks the passage of feces and maintains continence with small volumes in the rectum (see Clinical Focus Box 26.3). When the rectum is distended, the rec-toanal reflex or rectosphincteric reflex is activated to relax the internal sphincter. Like other enteric reflexes, this one involves a stretch receptor, enteric interneurons, and excitation of inhibitory motor neurons to the smooth muscle sphincter. Distension also results in the sensation of rectal fullness, mediated by the central processing of information from mechanoreceptors in the pelvic floor musculature.

Relaxation of the internal sphincter allows contact of the rectal contents with the sensory receptors in the lining of the anal canal, providing an early warning of the possibility of a breakdown of the continence mechanisms. When this occurs, continence is maintained by voluntary contraction of the external anal sphincter and the puborectalis muscle. The external sphincter closes the anal canal, and the puborectalis sharpens the anorectal angle. An increase in the anorectal angle works in concert with increases in intra-ab-dominal pressure to create a "flap" valve. The flap valve is formed by the collapse of the anterior rectal wall onto the upper end of the anal canal, occluding the lumen.

Whereas the rectoanal reflex is mediated by the ENS, synaptic circuits for the neural reflexes of the external anal sphincter and other pelvic floor muscles reside in the sacral portion of the spinal cord. The mechanosensory receptors are muscle spindles and Golgi tendon organs similar to those found in skeletal muscles elsewhere in the body. Sensory input from the anorectum and pelvic floor is transmitted over dorsal roots to the sacral cord, and motor outflow to these areas is in sacral root motor nerve fibers. The spinal circuits account for the reflex increases in contraction of the external sphincter and pelvic floor muscles by behaviors that raise intra-abdominal pressure, such as coughing, sneezing, and lifting weights.

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Responses

  • PROSPERO
    What maintains rectal continence?
    8 years ago
  • Martin
    Is the puborectalis muscles connected to the tailbone?
    8 years ago

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