The various components of the gastrointestinal (GI) tract serve several functions, including
digestive, excretory, endocrine, and exocrine. Control of these functions
requires neuronal activity from both local and higher centers.
The GI system has a complex collection of highly organized neurons
called the enteric nervous system(ENS) located in the intestinal walls
(Figure 36–1). The ENS may be considered a third division
of the autonomic nervous system, and includes the myenteric plexus
and the submucosal plexus. These neuronal networks receive preganglionic
fibers from the parasympathetic system as well as postganglionic
sympathetic axons. They also receive sensory input from within the
wall of the gut. Fibers from the cell bodies in these plexuses travel
to the smooth muscle of the gut to control motility. Other motor
fibers go to the secretory cells. Sensory fibers transmit information
from the mucosa and from stretch receptors to motor neurons in the
plexuses and to postganglionic neurons in the sympathetic ganglia.
The parasympathetic and sympathetic fibers that synapse on enteric plexus
neurons appear to play a modulatory role.
A highly simplified diagram of the intestinal wall and
some of the circuitry of the enteric nervous system (ENS). The ENS
receives input from both the sympathetic and the parasympathetic
systems and sends afferent impulses to sympathetic ganglia and to
the central nervous system. Many transmitter or neuromodulator substances
have been identified in the ENS. These agents include acetylcholine
(ACh), norepinephrine (NE), nitric oxide (NO), neuropeptides (NP),
substance P (SP), and serotonin (5-HT). Additional abbreviations:
longitudinal muscle layer (LM), myenteric plexus (MP), circular
muscle layer (CM), and submucosal plexus (SMP).
Multiple neurotransmitters, neuromodulators, and autocrine factors
are present in the GI system. Autacoids are endogenous molecules
that have powerful physiologic and pharmacologic effects but do
not fall into traditional autonomic or hormonal groups. Two important
amineautacoids, histamine (H) and serotonin (5-hydroxytryptamine,
5-HT), and numerous peptide autacoids are present. Both of these
amine-autacoids are discussed in previous chapters; histamine in
the respiratory system (Chapter 35) and 5-HT in the central nervous
system (Chapter 19). Other well-documented autacoids are cytokines
(Chapter 32) and prostaglandins (Chapter 34).
As previously discussed (Chapter 35), stimulation of histamine
type 1 receptors results in mucous secretion in the respiratory
system and contraction of several types of smooth muscle. In contrast, stimulation
of histamine type 2 (H2)
receptors results in gastric acid secretion in the stomach. Serotonin
is produced from the amino acid tryptophan and stored in vesicles
in the enterochromaffin cells of the gut as well as neurons in the
ENS. In addition to its activity as a central nervous system neurotransmitter
(Chapters 12 and 19), 5-HT has a physiologic role as a neurotransmitter
in the ENS and perhaps a role as a local hormone that modulates ...