Page 253 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Stomach Structure and Motility (0.5–4 mV) of the waves increases as they
spread to the pylorus. Whether and how often
Structure. The cardia connects the esophagus contraction follows such an excitatory wave
to the upper stomach (fundus), which merges depends on the sum of all neuronal and hor-
with the body (corpus) followed by the antrum monal influences. Gastrin increases the re-
of the stomach. The lower outlet of the stom- sponse frequency and the pacemaker rate.
ach (pylorus) merges with the duodenum Other hormones like GIP inhibit this motility
(! A). Stomach size is dependent on the directly, whereas somatostatin (SIH) does so
degree of gastric filling, but this distension is indirectly by inhibiting the release of GRP
mainly limited to the proximal stomach (! A, (! D1 and p. 234).
B). The stomach wall has an outer layer of Gastric emptying. Solid food remains in the
longitudinal muscle fibers (only at curvatures; stomach until it has been broken down into
Nutrition and Digestion lique muscle fibers. The mucosa of the tubular the duodenum. The time required for 50% of
regulates stomach length), a layer of powerful
small particles (diameter of ! 1 mm) and sus-
pended in chyme. The chyme then passes to
circular muscle fibers, and an inner layer of ob-
the ingested volume to leave the stomach var-
glands of the fundus and corpus contain chief
ies, e.g., 10—20 min for water and 1–4 hours for
cells (CC) and parietal cells (PC) (! A) that pro-
solids (carbohydrates ! proteins ! fats). Emp-
duce the constituents of gastric juice (! p.
tying is mainly dependent on the tone of the
242). The gastric mucosa also contains en-
docrine cells (that produce gastrin in the an-
lates emptying of the stomach (tone of proxi-
trum, etc.) and mucous neck cells (MNC).
mal stomach rises, pylorus dilates), whereas
Functional anatomy. The stomach can be
10 divided into a proximal and a distal segment proximal stomach and pylorus. Motilin stimu-
decreases in the pH or osmolality of chyme or
(! A). A vagovagal reflex triggered by swal- increases in the amount of long-chain free
lowing a bolus of food causes the lower fatty acids or (aromatic) amino acids inhibit
esophageal sphincter to open (! p. 238) and gastric emptying. Chemosensitive enterocytes
the proximal stomach to dilate for a short pe- and brush cells of the small intestinal mucosa,
riod (receptive relaxation). This continues enterogastric reflexes and certain hormones
when the food has entered the stomach (CCK, GIP, secretin and gastrin; ! p. 234) me-
(vagovagal accommodation reflex). As a result, diate these regulatory activities (! D2). The
the internal pressure hardly rises in spite of the pylorus is usually slightly open during the
increased filling. Tonic contraction of the prox- process (free flow of “finished” chyme). It con-
imal stomach, which mainly serves as a reser- tracts only 1) at the end of “antral systole” (see
voir, slowly propel the gastric contents to the above) in order to retain solid food and 2)
distal stomach. Near its upper border (middle when the duodenum contracts in order to pre-
third of the corpus) is a pacemaker zone (see vent the reflux of harmful bile salts. If such re-
below) from which peristaltic waves of con- flex does occur, refluxed free amino acids not
traction arise due mainly to local stimulation normally present in the stomach elicit reflex
of the stomach wall (in response to reflex closure of the pylorus (! D2).
stimulation and gastrin; ! D1). The peristaltic Indigestible substances (bone, fiber, foreign
waves are strongest in the antrum and spread bodies) do not leave the stomach during the
to the pylorus. The chyme is thereby driven digestive phase. Special contraction waves
towards the pylorus (! C5, 6, 1), then com- called migrating motor complexes (MMC) pass
pressed (! C2, 3) and propelled back again through the stomach and small intestine
after the pylorus closes (! C3, 4). Thereby, the roughly every 1.5 hours during the ensuing in-
food is processed, i.e., ground, mixed with gas- terdigestive phase, as determined by an intrin-
tric juices and digested, and fat is emulsified. sic “biological clock.” These peristaltic waves
The distal stomach contains pacemaker transport indigestible substances from the
cells (interstitial Cajal cells), the membrane stomach and bacteria from the small intestine
potential of which oscillates roughly every to the large intestine. This “clearing phase” is
240 20 s, producing characteristic slow waves (! p. controlled by motilin.
244). The velocity (0.5–4 cm/s) and amplitude
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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