points to links to figures)Integrated Response to a Meal
The response to a meal can be divided into 4 Phases: Cephalic, Gastric, Intestinal and Interdigestive.
| Cephalic
Phase |
The cephalic phase begins with presentation and
ingestion of a meal. The (1) sight,
smell and taste of food as well as mechanical stimulation of the oral cavity and
swallowing initiate a number of "long" reflexes (2-6)
which alter GI activities . During this phase (2)
salivation is stimulated
by cholinergic reflexes mediated by parasympathetic nerves. Return.
Reflexes initiated during swallowing (3)
stimulate primary peristalsis in the esophagus and cause relaxation of the upper
and lower esophageal sphincters. Vagal reflexes (4)
inhibit contractile activity in the proximal stomach [receptive relaxation].
Vagal reflexes initiated during the cephalic phase also
(5) stimulate
acid secretion by parietal cells in the stomach. These "long"
reflexes, acting through cholinergic neurons of the enteric nerve plexus,
stimulate secretion of HCl by parietal cells and secretion of histamine by
enterochromaffin-like (ECL) cells. Histamine in turn stimulates parietal
cell secretion of acid. In addition, vagal reflexes to the antrum
stimulate secretion of the hormone gastrin from G cells. Elevated levels
of circulating gastrin increase acid secretion by direct stimulation of parietal
cells and by stimulating histamine secretion from ECL cells. Return.
Vagal reflexes also induce a relatively small (6)
stimulation of enzyme secretion by the exocrine pancreas.
During the cephalic phase the process of digestion of
complex carbohydrates is begun due to the activity of ptyalin secreted in
saliva. Little absorption occurs during this phase.
| Gastric
Phase |
The gastric phase begins when food enters the
stomach. Distention (
)
of the stomach activates stretch receptors initiating a number of reflexes (1-7)
which alter gastric, intestinal, colonic and pancreatic activities.
Gastric acid secretion is stimulated by (1)
"long" vago-vagal reflexes and (2)
"short" reflexes mediated by the enteric nerve plexus . These
reflexes enhance
acid secretion by: 1) cholinergic stimulation of parietal cells, 2) stimulation
of histamine secretion by enterochromaffin-like (ECL) cells and 3) stimulation
of gastrin secretion by G cells. The hormone gastrin stimulates acid
secretion by direct stimulation of parietal cells and by stimulating histamine
secretion by ECL cells. If the meal contains protein, protein
digestion products (eg peptides) produced by the activity of acid and pepsin
will stimulate G cell secretion of gastrin further enhancing acid
secretion. As acid is secreted, the pH of the lumenal contents
decreases. Reduction of the pH will stimulate secretion of somatostatin
from D cells in the stomach. Somatostatin acts to prevent excessive
production of acid by inhibiting secretion of gastrin from G cells and histamine
from ECL cells. Return.
In addition to stimulating gastric acid secretion, long
and short reflexes (1, 2) and
elevated circulating gastrin stimulate gastric motility. Long and short (3,
4) cholinergic reflexes stimulate secretion of pepsinogen from chief
cells. Gastrin released during the gastric phase causes an (5)
increase in the contractile activity of the ileum and relaxation of the
ileocecal sphincter [gastroileal reflex] while long reflexes may induce (6)
mass movement in the distal colon [gastrocolic reflex]. Vagal reflexes
also cause a relatively small (7)
stimulation of enzyme secretion by the exocrine pancreas.
In the stomach some proteins are digested by acid and
pepsin. Little absorption occurs during this phase.
| Intestinal
Phase |
The intestinal phase begins when chyme enters the
small intestine. Chyme entering the small intestine initiates reflexes and
hormonal mechanisms (1-5) which alter the activities of the
small intestine, pancreas, gallbladder and stomach.
As chyme empties from the stomach into the small
intestine, the resulting distention (1)
stimulates intestinal contractile activity. Intestinal chyme also (2)
stimulates exocrine pancreatic secretion.
Acidic chyme entering the small intestine will stimulate S cells in the mucosa
of the duodenum to secrete the hormone secretin. Elevated circulating
secretin stimulates volume and bicarbonate secretion by pancreatic ductal
cells. Fat and protein digestion products in intestinal chyme stimulate
secretion of cholecystokinin (CCK) from I cells in the duodenum and upper
jejunum. The hormone CCK stimulates secretion of enzymes by pancreatic
acinar cells. Return.
In addition to stimulating pancreatic enzyme secretion,
CCK (3) induces contraction of the
gallbladder and relaxation of the sphincter of Oddi.
Acidic chyme, chyme that differs significantly from
isotonicity and chyme that contains fat will activate duodenal receptors which
act to (4) reduce the rate of gastric
emptying. While the pathways responsible for this inhibition of emptying
have not as yet been defined, it is likely that CCK is involved. The
presence of acidic or hyperosmotic chyme or chyme containing fat in the small
intestine (5) causes a reduction
in gastric secretion of acid. Part of this inhibition is due to neural
reflexes and part is the result of secretion of as yet unidentified hormones
collectively known as enterogastrones. Return.
Most digestion and absorption occurs while chyme is in
the small intestine. Organic nutrients such as carbohydrates and proteins
are first digested by pancreatic enzymes and then by enzymes in the brush border
membrane of enterocytes. Fats are digested by pancreatic enzymes with
digestion and absorption being facilitated by bile salts (acids) secreted by the
liver. Some organic nutrients are absorbed by secondary active transport
(e.g. glucose, amino acids, dipeptides), some by facilitated diffusion (e.g.
fructose) and others by simple diffusion (e.g. monoglycerides, fatty acids,
cholesterol, fat soluble vitamins).
In addition to organic nutrients, most electrolytes
(e.g. sodium, potassium, chloride, bicarbonate) are absorbed/reabsorbed
from the small intestine. Absorption of organic nutrients and electrolytes
(particularly NaCl) sets up an osmotic gradient resulting in absorption of
fluid. Most fluid (>90%) is absorbed/reabsorbed out of the small
intestine.
| Interdigestive
Phase |
The interdigestive phase is the period of fasting
after most of the components of the previous meal have been digested and
absorbed and the stomach and small intestine are relatively empty. This
phase is characterized by brief periods of intense peristaltic contractions
appearing first in the (1) stomach and then (2)
in the upper, middle and finally the distal small intestine. These periods
of intense contractile activity last for only 5-10 min and are followed by
longer relatively quiescent periods. This pattern repeats at intervals of
approximately 90 min and has been called the migrating motility complex (MMC).
The MMC appears to be initiated in the stomach by cyclical changes in the level
of circulating motilin and is abolished by eating.
In the colon, some components of the chyme are degraded
by colonic bacteria while residual electrolytes and fluid are absorbed.
Because chyme remains in the colon for a protracted period of time, colonic
processing usually occurs during all four phases of the digestive process.
N.B. Division of the digestive process into these four phases is somewhat arbitrary since there is considerable temporal overlap among the phases. Thus, control mechanisms from different phases may be operating concurrently.
Return to: #Cephalic, #Gastric, #Intestinal, #Interdigestive
Please send comments and suggestions to:
Robert-Beesley@ouhsc.edu
