Chapter 21 - Cellular Neurophysiology of Enteric Neurons

Abstract

Neurons of the enteric division of the autonomic nervous system function like an independent integrative nervous system intrinsic to the gastrointestinal tract. Cell bodies of neurons of the enteric nervous system are in ganglia, positioned inside the walls of the specialized organs that make up the gastrointestinal tract. The structure, function, and neurochemistry of enteric ganglia differ significantly from that of other autonomic ganglia (e.g., ganglia of the sympathetic division of the autonomic nervous system). Whereas sympathetic ganglia function mainly as relay distribution centers for signals transmitted from the central nervous system, the ganglia of the enteric nervous system form a system that integrates and processes information like the brain and spinal cord. The enteric nervous system is sometimes called the little brain in the gut on this basis. Like the brain and spinal cord, the enteric nervous system has three functional categories of neurons: sensory neurons, interneurons, and motor neurons. The enteric nervous system is found in the walls of the esophagus, stomach, small and large intestine, and gallbladder. It consists of ganglia, primary interganglionic fiber tracts, and secondary and tertiary fiber projections to the effector systems it controls. Enteric neurons are organized into two major plexuses. Cell bodies of the neurons are clustered in ganglia. Unmyelinated C fibers projecting from the cell bodies to other ganglia make up the interganglionic fiber tracts. The enteric ganglia together with the interganglionic connectives form two ganglionated plexuses. One of these is the myenteric plexus, also called Auerbach’s plexus, and the other is the submucosal plexus, also called Meissner’s plexus. The ganglionated plexuses are continuous around the circumference of the gastrointestinal tract and along its length. Enteric ganglia have Dogiel Types I, II, and III neurons that can be defined based on morphology and electrophysiological and synaptic behavior. Different types of enteric neurons have specific neurotransmitters and synaptic connections, which use 20 or more different neurotransmitters for chemical signaling and transfer of information as the enteric nervous system carries on its normal activities. Enteric neurons, like neurons elsewhere, use propagated action potentials as a mechanism for transmitting coded signals from one part of the neuron to the other. Transmission of signals from neuron to neuron in the neural circuits is achieved by mechanisms of synaptic transmission similar to those in circuits of the brain and spinal cord. Enteric neurons are classified as S-type or AH-type 2 based on their electrophysiological and synaptic behavior. Chemically mediated neurotransmission consists of excitatory postsynaptic potentials, inhibitory postsynaptic potentials, presynaptic inhibition, and presynaptic facilitation. Paracrine signaling from non-neuronal cells (e.g., enteric mast cells) is an important aspect of homeostatic function of the enteric nervous system.