Recently, a third component of the autonomic nervous system, known as the enteric nervous system (ENS), has been described and elucidated. This neural network innervates the gut continuously from esophagus to anus. It is composed of enteric
neurons, and the processes of extrinsic efferent and afferent neurons of the traditional autonomic system. This system regulates the motor and secretory function of the gut. A notable feature of the ENS is the diversity of chemical messengers which
enteric neurons contain and release. In addition to acetylcholine and norepinephrine, various peptide and non-peptide substances have been identified which appear to function as neurotransmitters in the ENS. Inhibitory non-adrenergic non-cholinergic
(NANC) nerves are thought to be important therein.
More recently, nitric oxide (NO) has been identified as an inhibitory transmitter to muscle. It has been shown that NO mediates the anorectal inhibitory reflex in animals and man.