The gastrointestinal (GI)
tract consists of the esopha-gus, stomach, small intestine, and colon. It
processes ingested boluses of food and drink and expels waste material.
Intervention by disease or pharmacological therapy may alter function of the GI
From the mid esophagus to the
anus, smooth muscle surrounds the alimentary canal and is responsible for
active movement and segmentation of intestinal con-tents. This smooth muscle,
which lies in the muscularis propria, consists of a circular and a longitudinal
layer of muscle.
From the gastric body to the
colon, repetitive spon-taneous depolarizations originate in the interstitial
cells of Cajal, from which they spread to the circular muscle layer and then to
the longitudinal muscle layer. The rate of slow-wave contraction varies in
different regions of the gastrointestinal tract, occurring approximately 3 per
minute in the stomach, 12 per minute in the proximal in-testine, and 8 per
minute in the distal intestine. The in-creased frequency of contraction in the
proximal intes-tine forms a gradient of contraction, and intestinal contents
are therefore propelled distally. Though the stomach has fewer spontaneous
contractions than does the small intestine, there is normally no retrograde
spread of a depolarization wave from duodenum to stomach.
The underlying intrinsic
smooth muscle motility is modulated by neurohormonal influences. Afferent
sen-sory neurons, extrinsic motor neurons, and intramural neurons innervate the
gut. It also has mucosal sensory receptors for monitoring chemical, osmotic, or
painful stimuli and muscle receptors to monitor degrees of stretch.
Both the parasympathetic and
sympathetic nervous systems provide extrinsic gastrointestinal innervation.
Parasympathetic stimulation increases muscle con-traction of the gut, while
sympathetic stimulation in-hibits contractions. Stimulation of either α- or β- adrenoceptors will result
in inhibition of contractions. The intramural nervous system consists of a
myenteric (Auerbach’s) plexus between the circular and longitudi-nal muscle
areas and a submucosal (Meissner’s) plexus between the muscularis mucosa and the
circular muscle layers. These two
plexuses contain stimulatory choliner-gic neurons.
Ingested liquids are rapidly
emptied from the stom-ach into the intestine, while digestible solids are first
mechanically broken down in the stomach by peristaltic contractions.
Stimulation of osmotic, carbohydrate, and fat receptors in the small bowel
inhibits gastric peri-staltic contractions and retards gastric emptying.
The small intestinal motility
in the fed state consists of random slow-wave contractions that result in slow
transit and long contact of food with enzymes and ab-sorptive surfaces. With
fasting, an organized peristaltic wave, termed the interdigestive migrating motor com-plex, begins to
cycle every 84 to 112 minutes. During the
migrating motor complex, a peristaltic contraction ring travels from the
stomach to the cecum at 6 to 8 cm per minute. In the stomach the contractions
sweep against a widely patent pylorus, permitting the passage of undigestible
solids. In the small intestine this is to clear the intestine of undigested
material: it functions as an intestinal housekeeper. The migrating motor
complex appears to correlate with motilin
hormonal levels and is modulated by vagal innervation. Motilin is a 22–amino
acid polypeptide released from the duode-nal mucosa as a regulator of normal GI
motor activity. Exogenous motilin is a potent inducer of gastric motor
Colonic motor function also
has cyclic slow waves in the proximal colon. These contractions are primarily
retrograde in the proximal colon, allowing segmenta-tion and liquid
reabsorption. In the distal colon a propulsive mass movement occurs
intermittently. This may be stimulated by food ingestion and is termed the gastrocolonic reflex.
Approximately 1 to 1.5 L of
fluid is ingested per day, and coupled with secretions from the stomach,
pan-creas, and proximal duodenum, approximately 8 L of chyme enters the jejunum
per day. Reabsorption of 6 to 7 L occurs within the small bowel, leaving a
residual of 1.5 L fluid, 90% of which is reabsorbed in the colon. This pattern
of liquid reabsorption permits the elimination of fecal waste containing an
average of 0.1 to 0.2 L fluid per day. Diarrhea
occurs if there is an altered rate of in-testinal motility, if mucosal function
or permeability is altered, or if the fluid load entering the colon over-whelms
colonic reabsorption. Constipation
may occur if intestinal movement is inhibited or if there is a fixed