Gastrointestinal (Gl) motility patterns depend, in part, on coordination of contractile activity by enteric motor neurons. Enteric excitatory and inhibitory motor neurons contribute to development of appropriate motor patterns in Gl organs. In the colon a prominent motor pattern, known as colonic migrating motor complexes (CMMC) provides propulsive forces for movement of fecal material in the anal direction. At present CMMC are thought to be based on the peristaltic reflex in which local stimuli generate ascending contractions and descending inhibition. This concept has become canonical in neurogastroenterology. In fact there is a 3rd phase of the peristaltic reflex that is poorly understood and infrequently even mentioned in scholariy reviews of peristalsis. Enteric motor neurons activate excitatory and inhibitory neurons and elicit region-dependent contraction and relaxation responses. In most regions of the gut, including the colon, the response to nerve stimulation is followed by a post-stimulus excitatory response (PSR also known as rebound excitation). We have discovered that PSR is actually an extremely important component, if not the main propulsive force, in propagating contractions (i.e. CMMC). CMMC are reduced by only an extent by muscarinic antagonists and not dependent upon excitatory peptides. CMMC are totally eliminated by inhibition of nitric oxide synthase in proximal colon or P2Y1 receptors in distal colon. Thus, CMMC are not dependent upon migrating responses to excitatory nerves but on migrating inhibitory responses coupled to PSR. There are striking similarities in PSR between species, and such a high degree of conservation tends to indicate the importance of PSR in colonic motility. Modifying canons, like the peristaltic reflex, requires careful investigation and determination of the mechanisms driving PSR. Our experiments will reveal the cell-specific steps and signaling pathways responsible for PSR and these findings will provide remarkable new ideas about managing colonic propulsive disorders. We will use reporter strains of transgenic mice for cell studies of PSR, and comparative studies on colonic muscles and whole colonic segments from mice, non-human primates, and human patients to develop a unified and mechanistic concept of PSR and its importance in propulsive movements. RELEVANCE (See instructions): Slow transit constipation is a prevalent health care problem that has been assoicated with loss of interstitial cells of Cajal. In this project we will explore the role of ICC in generating propulsive contraction in the colon and show the consequences of a defect in these cells for nrormal colonic transit.