The ileocolonic sphincter.

The human ileocolonic sphincter (ICS) develops a sustained tone mainly due to propagated and not propagated phasic motor activity. The ileocaecocolonic segment is also able to behave, yet uncommonly, as a synchronized segment involving propagated contractions originating from the ileum and migrating to the proximal colon. The ICS motor activity alone has a limited role towards forward flow. On the contrary, the functional entity corresponding to the distal ileum and the ICS provides a clearance mechanism for reflux of colonic contents into the small intestine. The presence of short chain fatty acids (SCFA) in the distal ileum, sensed either by endocrine cells or chemo-sensitive vagal afferents, is an important actor in triggering this clearance mechanism. The ICS tone is in part myogenic but a neuronal nitrergic component is also involved. Reflex excitatory and inhibitory responses of the ICS originating from ileal or colonic distension involve primarily spinal nitrergic and adrenergic pathways.