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The rectoanal inhibitory reflex (RAIR), also known as the anal sampling mechanism, anal sampling reflex, rectosphincteric reflex, or anorectal sampling reflex, is a reflex characterized by a transient involuntary relaxation of the internal anal sphincter in response to distention of the rectum. [1]
The rectum receives fecal material from the descending colon, transmitted through regular muscle contractions called peristalsis. [11] As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to pass feces, a process called defecation. [11]
In humans, the anal canal is approximately 2.5 to 4 cm (0.98 to 1.57 in) long, from the anorectal junction to the anus. [2] [3] [4] It is directed downwards and backwards.. It is surrounded by inner involuntary and outer voluntary sphincters which keep the lumen closed in the form of an anteroposterior sl
In humans, the anus (pl.: anuses or ani; from Latin ānus, "ring", "circle") [1] [2] is the external opening of the rectum located inside the intergluteal cleft.Two sphincters control the exit of feces from the body during an act of defecation, which is the primary function of the anus.
It is an important anatomical landmark in humans, and forms the boundary between the anal canal and the rectum according to the anatomic definition. [1] Colorectal surgeons instead define the anal canal as the zone from the anal verge to the anorectal ring (palpable structure formed by the external anal sphincter and the puborectalis muscle ...
The internal anal sphincter, IAS, or sphincter ani internus is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal.It is about 5 mm thick, and is formed by an aggregation of the smooth (involuntary) circular muscle fibers of the rectum.
The external anal sphincter is far more substantial than the internal anal sphincter. The proximal portion of external anal sphincter overlaps the internal anal sphincter (which terminates distally a little distance proximal to the anal orifice) superficially; where the two overlap, they are separated by the intervening conjoint longitudinal muscle.
The normal thickness of the small intestinal wall is 3–5 mm, [6] and 1–5 mm in the large intestine. [7] Focal, irregular and asymmetrical gastrointestinal wall thickening suggests a malignancy. [7] Segmental or diffuse gastrointestinal wall thickening is most often due to ischemic, inflammatory or infectious disease. [7]