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U N I T 8
Gastrointestinal and Hepatobiliary Function
Gastrointestinal Innervation
and Motility
The motility of the GI tract propels food and fluids
along its length, from mouth to anus, in a manner that
facilitates digestion and absorption. The movements of
the GI tract can be either rhythmic or tonic. The
rhyth-
mic movements
move food forward and keep the GI
contents mixed. Rhythmic movements are found in the
esophagus, antrum of the stomach, and small intestine.
The
tonic movements
consist of a constant level of con-
traction or tone without regular periods of relaxation.
They are found in the lower esophagus, the upper region
of the stomach, the ileocecal valve, and the internal anal
sphincter.
All of the contractile tissue in the GI tract is smooth
muscle, except for that in the mouth and pharynx, the
upper third of the esophagus, and the external anal
sphincter. Although the smooth muscle found in each
region of the GI tract exhibits structural and functional
differences, certain basic properties are common to all
of the muscle cells. For example, all of the smooth mus-
cle of the GI tract is unitary smooth muscle, in which the
cells are electrically coupled by low-resistance pathways
so that electrical signals initiating muscle contractions
can move rapidly from one fiber to the next.
Like the self-excitable cardiac muscle cells in the
heart, some smooth muscle cells throughout the GI tract
function as pacemaker cells. These cells display rhyth-
mic, spontaneous oscillations in membrane potentials,
called
slow waves
, ranging in frequency from about
3 per minute in the stomach to 12 per minute in the
duodenum. Slow waves are generated by a thin layer
of interstitial cells located between the longitudinal and
circular muscle layers.
Neural Control of Gastrointestinal
Motility
The motility of the GI tract can be modulated by input
from two sets of nerves, the intrinsic and extrinsic ner-
vous systems. The intrinsic nervous system, called the
enteric nervous system
, has cell bodies that are con-
tained within the wall of the GI tract. The extrinsic ner-
vous system, which has nerves with cell bodies that are
located outside the digestive tract, is part of the auto-
nomic nervous system (ANS). In addition, a number of
peptides, including neurotransmitters and GI hormones,
assist in regulating GI motility.
Enteric Nervous System Innervation
The GI tract has a nervous system all its own called the
enteric nervous system which lies entirely within the wall
of the GI tract, beginning in the esophagus and continu-
ing all the way to the anus. The enteric nervous system is
composed of two plexuses: an outer myenteric (Auerbach)
plexus and an inner submucosal (Meissner) plexus. These
two plexuses are networks of nerve fibers and ganglion cell
bodies. Interneurons in the plexuses connect afferent sen-
sory fibers, efferent motor neurons, and secretory cells to
form reflex circuits that are located within the GI tract wall.
The
myenteric plexus
consists mainly of a linear chain of
interconnecting neurons that is located between the circu-
lar and longitudinal muscle layers of the muscular externa.
Because it lies between the two muscle layers and extends
all the way down the GI tract, it is concerned mainly
with motility along the length of the gut. The
submucosal
plexus
, which lies between the submucosal and mucosal
layers of the wall, is mainly concerned with controlling the
function of each segment of the GI tract. It integrates sig-
nals received from the mucosal layer into local control of
motility, intestinal secretions, and absorption of nutrients.
The activity of the neurons in the myenteric and
submucosal plexuses is regulated by local influ-
ences, input from the ANS, and interconnecting fibers
that transmit information between the two plexuses.
Mechanoreceptors monitor the stretch and distention
of the GI tract wall, and chemoreceptors monitor the
chemical composition (i.e., osmolarity, pH, and digestive
products of protein and fat metabolism) of its contents.
These receptors can communicate directly with gangli-
onic cells in the intramural plexuses or with visceral
afferent fibers that influence ANS control of GI function.
Autonomic Innervation
The autonomic innervation of the GI system is mediated
by both the sympathetic and parasympathetic nervous
systems (see Chapter 34, Fig. 34-23). In general, stimu-
lation of the parasympathetic nervous system causes a
general increase in activity of the entire enteric nervous
system, whereas sympathetic stimulation inhibits activ-
ity causing many effects.
Parasympathetic innervation to the stomach, small
intestine, cecum, ascending colon, and transverse colon
occurs through the vagus nerve. The remainder of the
colon is innervated by parasympathetic fibers that exit
the sacral segments of the spinal cord by way of the
of the small intestine—is where most of the
digestive and absorptive processes occur.The
lower segment—the cecum, colon, and rectum of
the large intestine—serves as a storage channel
for the efficient elimination of waste.
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Throughout its length, except for the mouth,
pharynx, and upper esophagus, the wall of the
digestive tract is composed of four layers: an inner
mucosal layer that produces mucus, secretes
digestive enzymes, and absorbs the breakdown
products of digestion; an underlying submucosal
layer that contains blood vessels, lymph vessels,
and nerves that control the secretory activity
of the mucosal glands; a layer of circular and
longitudinal smooth muscle fibers that mix and
propel the gut contents along its length; and an
outer serosal or adventitial layer that forms the
peritoneum (serosa) or attaches structures of the
GI tract to the body wall (adventitia).
SUMMARY CONCEPTS
(continued)
