We discuss the multiple cell types involved in rhythmic motor patterns in the large intestine that include tonic inhibition of the muscle layers interrupted by rhythmic colonic migrating motor complexes (CMMCs), and secretomotor activity. We propose a model that assumes these motor patterns are dependent upon myenteric descending 5-Hydroxytrpamine (5-HT, serotonin) interneurons. Asynchronous firing in 5-HT neurons excite inhibitory motor neurons (IMNs) to generate tonic inhibition occurring between CMMCs. IMNs release mainly nitric oxide (NO) to inhibit the muscle, IPANs, glia cells and pacemaker ICC-MY. Mucosal release of 5-HT from enterochromaffin (EC) cells excites the mucosal endings of IPANs that synapse with 5-HT descending interneurons and perhaps ascending interneurons (AINs), thereby coupling EC cell 5-HT to myenteric 5-HT neurons, synchronizing their activity. Synchronized 5-HT neurons generate a slow EPSP in IPANs via 5-HT7 receptors and excite glial cells and ascending excitatory nerve pathways that are normally inhibited by NO. Excited glial cells release prostaglandins to inhibit IMNs (disinhibition) to allow full excitation of ICC-MY and muscle by excitatory motor neurons (EMNs). EMNs release ACh and tachykinins to excite pacemaker ICC-MY and muscle leading to contraction of both the longitudinal and circular muscle layers simultaneously. Myenteric 5-HT neurons also project to the submucous plexus to couple motility with secretion, especially during the CMMC. Glial cells are necessary for switching between different colonic motor behaviors. This model emphasizes the importance of myenteric 5-HT neurons and the consequence of their coupling and uncoupling to mucosal 5-HT during colonic motor behaviors.
- Copyright © 2016, American Journal of Physiology-Gastrointestinal and Liver Physiology