To see if the rectum contains flatus or faeces, the recto-anal inhibitory
reflex results in relaxation of the internal anal sphincter and tightening
of the external anal sphincter. The top end of the anal canal opens into
the rectum and the sensitive epithelium above the dentate line checks to
see if the cause of the distension was flatus or faeces.
As the rectum distends there is a reflex relaxation of the internal anal
sphincter. This is termed the recto-anal inhibitory reflex. As part of this
reflex the external sphincter is tightened to maintain continence. Relaxation
of the internal sphincter permits opening of the top of the anal canal and
`sampling' of the rectal content by the mucosa above the dentate line. This
permits descrimination of flatus from faeces.
Adrian P. Ireland