The enteric nervous system and regulation of intestinal motility

The enteric nervous system exerts local control over mixing and propulsive movements in the small intestine. When digestion is in progress, intrinsic primary afferent neurons (IPANs) are activated by the contents of the intes tine. The IPANs that have been physiologically characterized are in the int rinsic myenteric ganglia. They are numerous, about 650/mm length of small i ntestine in the guinea pig, and communicate with each other through slow ex citatory transmission to form self-reinforcing assemblies. High proportions of these neurons respond to chemicals in the lumen or to tension in the mu scle; physiological stimuli activate assemblies of hundreds or thousands of IPANs. The IPANs make direct connections with muscle motor neurons and wit h ascending and descending interneurons. The circular muscle contracts as a n annulus, about 2-3 mm in minimum oral-to-anal extent in the guinea pig sm all intestine. The smooth muscle cells form an electrical syncytium that is innervated by about 300 excitatory and 400 inhibitory motor neurons per mm length. The intrinsic nerve circuits that control mixing and propulsion in the small intestine are now known, but it remains to be determined how the y are programmed to generate the motility patterns that are observed.