The gastrointestinal (GI) tract is in a continuous contractile, absorptive, and secretory state. The control of this state is complex, with contributions by the muscle and epithelium, local nerves of the enteric nervous system (ENS), the autonomic nervous system (ANS), and circulating hormones. Of these, perhaps the most important regulator of physiological gut function is the ENS (Figure 46–1).
The neuronal network that initiates and generates the peristaltic response. Mucosal stimulation leads to release of serotonin by enterochromaffin cells (8), which excites the intrinsic primary afferent neurons (1), which then communicate with ascending (2) and descending (3) interneurons in the local reflex pathways. The reflex results in contraction at the oral end via the excitatory motor neuron (6) and aboral relaxation via the inhibitory motor neuron (5). The migratory myoelectric complex (see text) is shown here as being conducted by a different chain of interneurons (4). Another intrinsic primary afferent neuron with its cell body in the submucosa also is shown (7). MP, myenteric plexus; CM, circular muscle; LM, longitudinal muscle; SM, submucosa; Muc, mucosa. (Adapted with permission of Annual Reviews, from Kunze WA, Furness JB. The enteric nervous system and regulation of intestinal motility. Annu Rev Physiol, 1999;61:117–142. Permission conveyed through Copyright Clearance Center, Inc.)
The ENS is an extensive collection of nerves that constitutes the third division of the ANS. It is the only part of the ANS truly capable of autonomous function if separated from the central nervous system (CNS). The ENS lies within the wall of the GI tract organized into 2 connected networks of neurons and nerve fibers: the myenteric (Auerbach) plexus, found between the circular and longitudinal muscle layers, and the submucosal (Meissner) plexus, located in the submucosa. The former is largely responsible for motor control, whereas the latter regulates secretion, fluid transport, and blood flow. The ENS and the ANS are also involved in host defense and innervate organs and cells of the immune system.
GENERATION AND REGULATION OF GI ACTIVITY
The ENS is responsible for the largely autonomous nature of most GI activity. This activity is organized into relatively distinct programs that respond to input from the local environment of the gut, as well as the ANS-CNS. Each program consists of a series of complex, but coordinated, patterns of secretion and movement that show regional and temporal variation. The fasting program of the gut is called the MMC (migrating myoelectric complex when referring to electrical activity and migrating motor complex when referring to the accompanying contractions) and consists of a series of 4 phasic activities. The most characteristic, phase III, consists of clusters of rhythmic contractions that occupy short segments of the intestine for a period of 6-10 min before proceeding caudally (toward the anus). Phase II of the MMC is associated with the release of ...