Fecal propulsion in the colon: what a long, strange trip it is!

Abstract

The peristaltic reflex, first described in detail by Bayliss & Starling (1899), is a fundamental reflex that propels gastrointestinal content in an oral to anal direction. The peristaltic reflex contributes aboral propulsion of content through the small and large intestines. The peristaltic reflex is mediated entirely by the enteric nervous system (ENS) which is intrinsic to the gut wall. The peristaltic reflex is initiated by circumferential distention of the gut wall by a bolus of luminal content which activates tension-sensitive intrinsic primary afferent neurons (IPANs) and stretch sensitive interneurons (Furness et al. 2004; Smith et al. 2007). The tension- and stretch-sensitive neurons have orally and anally directed axons that synapse with interneurons. On the oral side of the site of distention, interneurons synapse with motoneurons which release acetylcholine to cause muscle contraction; this is the ascending excitatory component of the peristaltic reflex. On the anal side of the distention, interneurons synapse with motorneurons which release nitric oxide (NO) to cause muscle relaxation; this is the descending inhibitory component of the peristaltic reflex. The polarized reflex produces a pressure gradient which propels intestinal content in an oral to anal direction (Brookes, 2001).