Chemical coding of neurons in the gastrointestinal tract

The gastrointestinal tract, unlike most of the other internal organs, contains a rich and complex neuronal system embedded within its wall. The first description of this system dates back to the middle of the last century when Meissner and Auerbach first described the rich neuronal network made of microganglia and nerve bundles forming the myenteric and submucous plexuses (Meissner, 1857; Auerbach, 1864). By the turn of the century Bayliss and Starling (1889) first, followed by Langley (1921), provided evidence that this system, named then the Enteric Nervous System (ENS) controls coordinated motor functions in isolation from the central nervous system. In addition the intestine receives inputs from the central nervous system via sympathetic and parasympathetic nerves and sends sensory information via vagal and spinal afferent neurons. Between the beginning of this century and around twenty years ago the attempts to identify neuroanatomically the different classes of enteric neurons involved in the control of intestinal functions were hampered by the inability of the neurohistological techniques to distinguish specific neuronal groups within the apparently homogeneous maze of nerve cells and nerve fibres which form the ENS.