Neuroanatomy and physiology of colorectal function and defaecation: from basic science to human clinical studies

Abstract

Colorectal physiology is complex and involves programmed, coordinated interaction between muscular and neuronal elements. Whilst a detailed understanding remains elusive, novel information has emerged from recent basic science and human clinical studies concerning normal sensorimotor mechanisms and the organization and function of the key elements involved in the control of motility. This chapter summarizes these observations to provide a contemporary review of the neuroanatomy and physiology of colorectal function and defaecation.

The Physiology of Human Defecation

“…As a caveat, it should be noted however, that the majority of colonic manometries have been performed with recording sites spaced 10 cm or more apart. A recent study using high resolution manometry with 1 cm sensor spacing indicates that manometric pressure patterns often propagate for less than 10 cm [144], which indicates propagated activity may have been previously ''mislabeled'' as non-propagated [141]. Although the role of non-propagated activity in luminal transport is not fully understood [141], it is thought to aid mixing of intraluminal contents by local propulsion [145,146] and retropulsion [147] of the fecal bolus.…”

“…This is facilitated by contraction of the pubococcygeus muscle that ''splints'' the perineal body, effectively tensing the anterior wall of the anal canal, allowing only the posterior wall to move backwards [284]. Contraction of the conjoint longitudinal muscles of the anus also causes flattening of the anal vascular cushions [285] and shortening of the anal canal [141]. The incoming fecal bolus possibly further flattens the vascular cushions by direct compression [285].…”

“…Straining during evacuation raises intra-pelvic and hence intra-rectal pressure. Intuitively, simultaneous rectal contractions would likely augment evacuation, but whether this is true or not has not been clearly demonstrated [141]. One group reported no appreciable rise in intra-rectal pressure in relation to intra-pelvic pressure during evacuation [271] and suggested that evacuation is not accompanied with rectal contraction.…”

“…Non-propagated contractions appear to be the most common event and can occur as isolated, seemingly random contractions or in ''bursts'' [116,118]. They have a frequency of between 2 and 4 cycles per minute [141] and an amplitude of between 5 and 50 mm Hg [117]. The duration of these contractions can either be short (\15 s) or long (15-60 s) [116,142,143].…”

“…The frequency of rectal MCs appears unaffected by meal intake [117]. The role of these motor activities is not fully understood [141]. However, rectal MCs are seen to propagate in a retrograde direction [167]; it has thus been postulated they help to keep the rectum empty by acting as a ''braking mechanism'' to untimely flow of colonic contents [167].…”