Chapter 23
The Digestive System
873
23
the stomach musculature is thicker, they become much more
powerful. Consequently, the contents of the fundus and body
(food storage area) remain relatively undisturbed, while food-
stuffs in and around the pyloric antrum receive a truly lively
pummeling and mixing.
Te pyloric part of the stomach, which holds about 30 ml of
chyme, acts as a dynamic filter that allows only liquids and small
particles to pass through the barely open pyloric valve. Nor-
mally, each peristaltic wave reaching the pyloric muscle squirts
3 ml or less of chyme into the small intestine. Because the con-
traction also
closes
the valve, which is normally partially relaxed,
the rest (about 27 ml) is propelled backward into the stomach,
where it is mixed further
(Figure 23.19)
. Tis back-and-forth
pumping action (retropulsion) effectively breaks up solids in the
gastric contents.
Although the intensity of the stomach’s peristaltic waves can
be modified, their rate is constant—always around three per
minute. Tis contractile rhythm is set by
enteric pacemaker cells
,
muscle-like noncontractile cells formerly called
interstitial cells
of Cajal
(kă-hal
9
). Located in the longitudinal smooth muscle
layer, the pacemaker cells depolarize and repolarize spontane-
ously three times each minute, establishing the so-called
cyclic
slow waves
of the stomach, or its
basic electrical rhythm (BER)
.
Since gap junctions couple the pacemakers electrically to the
rest of the smooth muscle sheet, their “beat” is transmitted ef-
ficiently and quickly to the entire muscularis.
Te pacemakers set the maximum frequency of contrac-
tion, but they do not initiate the contractions or regulate their
force. Instead, they generate subthreshold depolarization waves,
which are then “ignited” (enhanced by further depolarization
and brought to threshold) by neural and hormonal factors.
Te same factors that increase the strength of stomach con-
tractions also enhance gastric secretions. Distension of the
stomach wall by food activates stretch receptors and gastrin-
secreting cells, both of which ultimately stimulate gastric
Regulation of Gastric Motility
and Emptying
Stomach contractions not only accommodate its filling and
cause its emptying, but they also compress, knead, and mix
the food with gastric juice to produce chyme. Te processes of
mechanical breakdown and propulsion are inseparable in the
stomach due to a unique type of peristalsis.
Response of the Stomach to Filling
Te stomach stretches to accommodate incoming food, but
internal stomach pressure remains constant until about 1.5 L
of food have been ingested. TereaFer, the pressure rises. Te
relatively unchanging pressure in a filling stomach is due to two
factors:
Receptive relaxation
of smooth muscle in the stomach fun-
dus and body which occurs both in anticipation of and in
response to food moving through the esophagus and into
the stomach. Te swallowing center of the brain stem coor-
dinates this process, which is mediated by the vagus nerves
acting on serotonin- and NO-releasing enteric neurons.
Gastric accommodation
, which is an example of smooth
muscle
plasticity
. It is the intrinsic ability of visceral smooth
muscle to exhibit the
stress-relaxation response
, in other
words, it can stretch without greatly increasing its tension
and contracting expulsively. As we described in Chapter 9,
this capability is very important in hollow organs, like the
stomach, that must serve as temporary reservoirs.
Gastric Contractile Activity
As in the esophagus, the stomach exhibits peristalsis. AFer a
meal, peristalsis begins near the gastroesophageal sphincter,
where it produces gentle rippling movements of the thin stom-
ach wall. But as the contractions approach the pylorus, where
1
Propulsion:
Peristaltic waves move
from the fundus toward the pylorus.
2
3
Grinding:
The most vigorous
peristalsis and mixing action
occur close to the pylorus.
Retropulsion:
The pyloric end of the
stomach acts as a pump that delivers small
amounts of chyme into the duodenum,
simultaneously forcing most of its contained
material backward into the stomach.
Pyloric
valve
closed
Pyloric
valve
closed
Pyloric
valve
slightly
opened
Figure 23.19
Peristaltic waves in the stomach.
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