Reflex mechanisms involved in defecation process
The internal sphincter, the external sphincter, and the levator ani constitute the motor components. The internal sphincter unconsciously maintains continence by its sustained contraction. Any damage to the internal sphincter will result into the partial loss of control. The puborectalis part of the levator ani gets its attachment to the upper part of the external anal sphincter (Nicholas and Lindsay 2013). This constitutes the puborectalis sling. It gives strength to the sphincters and prevents retraction of their cut edges when they are divided during anal operations. If puborectalis sling is also divided, large gaping occurs, resulting in anal incontinence.
2.1.1 Mechanical Factors of Continence and Defecation
The puborectalis arises from the pubic bone and passes horizontally and posteriorly around the anorectum as the most medial portion of the levator ani muscle. This forms a U-shaped sling around the anorectum near its anatomic junction with the anus, pulling the rectum anteriorly and giving rise to the so-called anorectal angle (Fig. 2.2). Parks postulated a mechanism by which this takes place (Parks et al. 1966). The intra-abdominal pressure increases with sneezing, coughing, or straining. This force is transmitted across the anterior wall of the rectum at the anorectal angle. The underlying mucosa is opposed against the upper anal canal, creating a flap valve mechanism that prevents stool from passing to the lower anal canal, thereby preserving continence. This is now disputed as many multiparous women have this angle obliterated and are still fully continent.