Background

The enteric nervous system (ENS) of the gastrointestinal tract has traditionally been viewed as a system of ganglia that operates largely independently of the brain and spinal cord. Because of this postulated autonomy, the ENS has even been characterised as the “little brain” or the “second brain.” Recent neural tracing studies, however, challenge this canon of ENS autonomy.

The view that the ENS is independent can be traced to several types of observation. Early research on the relationship of the ENS to the central nervous system (CNS) noted that the ENS could support motor activity and a number of local reflexes when the viscera were decentralised or isolated. Langley (1922),1 in his seminal analyses, reinforced the idea of a largely autonomous ENS by noting that the preganglionic axons projecting to the ENS were far outnumbered by the multitude of enteric neurones in the viscera. In the case of the vagus nerve in particular, Langley envisioned a few specialised “mother cells” or “vagal cells” in the ENS that were contacted by preganglionic inputs and that then distributed the relatively small number of inputs to a bigger group of effector neurones within the enteric plexuses. In a related but more modern view, Wood (1987)2 suggested that autonomic preganglionics might project to “command neurones” within the ENS.3 On the sensory side, early observations also noted that individuals with intact peripheral nerves connecting the ENS and CNS had little awareness of visceral afferent feedback.

Although the view of an all-but-autonomous ENS still tends to dominate thinking and teaching about the innervation of the gastrointestinal tract, it should be stressed that the …