Rectoanal inhibitory reflex

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 RAIR provides the upper anal canal with the ability to discriminate between flatus and fecal material.

The ability of the rectum to discriminate between gaseous, liquid and solid contents is essential to the ability to voluntarily control defecation. The RAIR allows for voluntary flatulation to occur without also eliminating solid waste, irrespective of the presence of fecal material in the anal canal.^[2]

Reflex arc

The physiological basis for the RAIR is poorly understood,^[3] but it is thought to involve a coordinated response by the internal anal sphincter to rectal distention with recovery of anal pressure from the distal to the proximal sphincter.^[1] Mediated by the autonomic nervous system, the afferent limb of this reflex depends upon an intact network of interstitial cells of Cajal in the internal anal sphincter. These cells, which are mediated at least in part by nitric oxide, provide inhibitory innervation of the internal anal sphincter.^[4]

Clinical significance

Impairment of this reflex can result in fecal incontinence.^[5][6] The absence of a RAIR is pathognomonic for Hirschsprung's disease.^[7]

See also

• External anal sphincter
• Levator ani

References

1. Zbar AP, Aslam M, Gold DM, Gatzen C, Gosling A, Kmiot WA (February 1998). "Parameters of the rectoanal inhibitory reflex in patients with idiopathic fecal incontinence and chronic constipation". Diseases of the Colon and Rectum. 41 (2): 200–8. doi:10.1007/bf02238249. PMID 9556245. S2CID 9535999.
2. Barrett KE (2014). "Intestinal Motility". Gastrointestinal Physiology (2nd ed.). New York: Lange Medical Books/McGraw-Hill. p. 180. ISBN 978-0071774017.
3. Rao SS (January 2004). "Pathophysiology of adult fecal incontinence". Gastroenterology. 126 (1 Suppl 1): S14–22. doi:10.1053/j.gastro.2003.10.013. PMID 14978634.
4. de Lorijn F, de Jonge WJ, Wedel T, Vanderwinden JM, Benninga MA, Boeckxstaens GE (August 2005). "Interstitial cells of Cajal are involved in the afferent limb of the rectoanal inhibitory reflex". Gut. 54 (8): 1107–13. doi:10.1136/gut.2004.051045. PMC 1774907. PMID 16009682.
5. Roberts PL (2005). "Rectoanal Inhibition" (PDF). In Wexner SD, Zbar AP, Pescatori M (eds.). Complex Anorectal Disorders: Investigation and Management. London: Springer Science+Business Media. p. 39. ISBN 978-1-85233-690-5.
6. Deen KI, Premaratna R, Fonseka MM, De Silva HJ (November 1998). "The recto-anal inhibitory reflex: abnormal response in diabetics suggests an intrinsic neuroenteropathy". Journal of Gastroenterology and Hepatology. 13 (11): 1107–10. doi:10.1111/j.1440-1746.1998.tb00584.x. PMID 9870796. S2CID 23202851.
7. Remes-Troche JM, Rao SS (June 2008). "Neurophysiological testing in anorectal disorders". Expert Review of Gastroenterology & Hepatology. 2 (3): 323–35. doi:10.1586/17474124.2.3.323. PMC 3764614. PMID 19072383.