NORMAL GASTRIC NEUROMUSCULAR PHYSIOLOGY
Normal gastric physiology is best described in terms of its fasting patterns and the responses of the stomach in response to food intake (the fed response).
Fasting gastric contractile
patterns are characterized by a cyclic motor phenomenon called the migrating
motor complex. In healthy people in the fasting state, it occurs
approximately once every 90 minutes, most prominently at night. The fasting
state generally starts approximately 4 hours after meal ingestion, when the
stomach has completely emptied a meal. The fasting contractile patterns consist
of a period of quiescence (phase I), a
period of intermittent pressure activity (phase II), and an activity
front, during which the stomach and small intestine contract at their highest
frequency (phase III). During phase III of the migrating motor complex,
contraction frequencies reach 3 a minute in the stomach and 11 or 12 a minute
in the proximal small intestine. This interdigestive contraction wave
(intestinal housekeeper) progresses down the stomach and small intestine
and serves to help empty the stomach of indigestible solids and transport them
down the small intestine into the colon. These contractile pressures,
especially in the fasting period, are generally recorded with antroduodenal
manometry, a catheter recording pressures from the stomach (primarily antral
region) and the small intestine (primarily duodenum).
GASTRIC RESPONSES TO MEAL INGESTION
Normally with solid food ingestion,
the upper stomach relaxes (gastric accommodation), allowing the proximal
stomach to accommodate the ingested meal. This is followed by a progressively
tonic contraction of the fundus to deliver food into the distal stomach. Within
the antrum, regular peristaltic contractions grind down solid food so that it
can be passed down the stomach toward the pylorus. The pyloric sphincter opens
in a coordinated fashion with antral contractions, allowing the smaller
particle size (food chyme) to empty from the stomach into the small intestine.
Larger food particles are retropulsed back toward the body of the stomach,
where they are ground down further; this process is repeated until the
particles are small enough to empty through the pylorus.
Gastric accommodation is a
postprandial, vagally mediated reflex resulting in reduced gastric tone,
primarily in the proximal stomach,
that occurs with eating a meal. Gastric accommodation provides a reservoir for
ingested foods without a significant increase in intra-gastric pressure.
The accommodation reflex has two
principal components. Receptive relaxation occurs within seconds of
eating and is triggered by both oropharyngeal and gastric stimulation. This
response involves relaxation of both the lower esophageal sphincter and
proximal stomach. Adaptive relaxation is a slower process triggered by
gastric or duodenal distention and is perhaps also modified by specific
nutrients. The accommodation reflex is vagally mediated and represents the balance
between the cholinergic excitatory drive and nonadrenergic noncholinergic
inhibitory input. The afferent signal is generated by activation of
stretch-sensitive mechanoreceptors in the stomach wall, osmo- receptors, and
chemoreceptors in the stomach and duodenum. The efferent nonadrenergic
noncholinergic signal involves nitric oxide as the principal neurotransmitter. A
role for vasoactive intestinal polypeptide has also been suggested.
GASTRIC EMPTYING OF A MEAL
Normal gastric emptying reflects a
coordinated effort between the fundus, antrum, pyloric sphincter, and duodenum.
Coordination of these fundic-antral- pyloric-duodenal motor events is carefully
regulated and governed by (1) gastrointestinal electrical activity through the
interstitial cells of Cajal, (2) neural connectivity through enteric nerves in
the enteric nervous system, and (3) vagal efferent nerves from the central
nervous system (CNS). Feedback from nutrients and the volume in the stomach and
small bowel affects the process of gastric emptying and is conveyed through nlocal enteric sensory nerves, vagal afferent nerves, and hormones.
Fundic and antral smooth muscle
contractions are primarily cholinergically mediated. Rhythmic antral
contractions, generally at 3 cycles per minute, triturate large food particles
into an appropriate size for intestinal digestion. The rate of these
contractions is governed by the electrical pacemaker of the stomach and the
interstitial cells of Cajal.
Pyloric sphincter relaxation, often
synchronized with antral contractions, allows smaller food particles and chyme
to pass out of the stomach into the duodenum. Pyloric relaxation is mediated
through release of inhibitory nerves, especially nitric oxide and, possibly,
vasoactive intestinal polypeptide.
Solid foods and liquids leave the
stomach at different rates. Liquids empty at an exponential rate because their
emptying depends primarily on the gastroduodenal pressure gradient and less on
pyloric opening. Solids are initially retained selectively within the stomach
until particles have been triturated to a size smaller than 2 mm, at which
point they can be emptied at a linear rate from the stomach.
