The ENS of most adult mammals is comprised of two major interconnected ganglionated plexuses, the submuco-sal and the myenteric [3, 4]. The submucosal plexus is the smaller of the two. In larger animals, including humans, the submucosal plexus can be divided into separate plexuses of Schabadasch (external) and Meissner (internal) ; however, these plexuses interconnect extensively and clear functional distinctions are not yet known. The submucosal plexus is thus usually treated a single entity , although this practice will probably have to be changed in the future as new information accumulates that suggests a significant segregation of function to the subplexuses of Schabadasch and Meissner . Submu-
cosal plexus neurons project to one another, to the mucosa, and to the myenteric plexus. The neurons that project to the mucosa include intrinsic sensory [31-33] and secretomotor neurons [22, 34, 35]. Some submucosal neurons are bipolar or pseudounipolar in shape and also project to the myenteric plexus; these have been postulated to be sensory in function . A newly discovered subset of submucosal neurons, which evoke vasomotor responses when activated by mucosal stimuli, project both to the mucosa and to blood vessels . These cells may actually function as a unicellular reflex arc, which if true would be a structure that, in vertebrates, is unique to the bowel.
Both the submucosal and the myenteric plexuses contain many interneurons involved in interganglionic projections and the formation of complex microcircuits that are just beginning to be mapped. Motor neurons that excite or relax the muscularis externa are located exclusively in the myenteric plexus [3, 4]. The myenteric plexus of rodents, but not that of humans , probably also contains intrinsic sensory neurons that project to the mucosa as well. The extreme complexity of the ENS and the behaviors of the gut that it regulates have only recently been appreciated. Certainly, the ENS is not, as used to be thought, a system of "relay ganglia" interposed between the brain and effector in the bowel. Because the ENS is so different from the other components of the PNS, it stands to reason that the factors and/or processes that dictate the development of the ENS are likely to be different from those of other peripheral ganglia.
The search for the developmental basis of Hirsch-sprung's disease is likely to be a long one, not simply because of the complexity of the system, but also because it is unlikely that the multitude of neuronal developmental dysganglionoses, of which classical Hirschsprung's disease is but one, are a single disease entity.
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