Enteric nervous system: reflexes, pattern generators and motility

Abstract

Advances in understanding of the enteric nervous system (ENS) support the brain-in-the-gut concept. Progress relative to sensory neurons, reflex circuits and central pattern generators is summarized. A 'hardwired' polysynaptic circuit in the ENS evokes descending inhibition of the intestinal circular muscle below an activation point, and contraction of the muscle above the activation point. This circuit occupies the lowest complexity level of the hierarchical organization of neural motility control. Networks in the ENS contain central pattern generators, which activate the 'hardwired' basic circuit in recurrent fashion to generate motility patterns linked with intestinal secretion. The dogma that muscle responses seen when the investigator stretches the intestinal wall or stimulates the mucosa reflect a classic reflex can be challenged. No limb equivalent to the afferent limb of a spinal motor reflex has been identified unequivocally in the ENS. Central pattern generators are neural circuits, which generate organized and repetitive motor patterns independent of their sensory input. Central pattern generators rather than afferent-evoked reflexes are postulated to be responsible for the patterns of propulsive contractile behavior and secretion, which recur rhythmically when distension is maintained above threshold or during mucosal application of nutrients or paracrine mediators.