Gastric Motility: Filling and EmptyingContractions of gastric smooth muscle serves two basic functions. First, it allows the stomach to grind, crush and mix ingested food, liquefying it to form what is called "chyme." Second, it forces the chyme through the pyloric canal, into the small intestine, a process called gastric emptying. The stomach can be divided into two regions on the basis of motility pattern: an accordion-like reservoir that applies constant pressure on the lumen and a highly contractile grinder.
The proximal stomach, composed of the fundus and upper body, shows low frequency, sustained contractions that are responsible for generating a basal pressure within the stomach. Importantly, these tonic contractions also generate a pressure gradient from the stomach to small intestine and are thus responsible for gastric emptying. Interestingly, swallowing of food and consequent gastric distention inhibits contraction of this region of the stomach, allowing it to balloon out and form a large reservoir without a significant increase in pressure - this phenomenon is called "adaptive relaxation."
The distal stomach, composed of the lower body and antrum, develops strong peristaltic waves of contraction that increase in amplitude as they propagate toward the pylorus. These powerful contractions constitute a very effective gastric grinder; they occur about 3 times per minute in people and 5 to 6 times per minute in dogs. There is a pacemaker in the smooth muscle of the greater curvature that generates rhythmic slow waves from which action potentials and hence peristaltic contractions propagate. As you might expect and at times hope, gastric distention strongly stimulates this type of contraction, accelerating liquefaction and hence, gastric emptying. The pylorus is functionally part of this region of the stomach - when the peristaltic contraction reaches the pylorus, its lumen is effectively obliterated - chyme is thus delivered to the small intestine in spurts.
Motility in both the proximal and distal regions of the stomach is controlled by a very complex set of neural and hormonal signals. Nervous control originates from the enteric nervous system as well as parasympathetic (predominantly vagus nerve) and sympathetic systems. A large battery of hormones have been shown to influence gastric motility - for example, both gastrin and cholecystokinin act to relax the proximal stomach and enhance contractions in the distal stomach. The bottom line is that the patterns of gastric motility likely are a result from smooth muscle cells integrating a large number of inhibitory and stimulatory signals.
Liquids readily pass through the pylorus in spurts, but solids must be reduced to a diameter of less than 1-2 mm before passing the pyloric gatekeeper. Larger solids are propelled by peristalsis toward the pylorus, but then refluxed backwards when they fail to pass through the pylorus - this continues until they are reduced in size sufficiently to flow thought the pylorus.
At this point, you may be asking "What happens to solids that are indigestible - for example, a rock or a penny? Will it remain forever in the stomach?" If the indigestible solids are large enough, they indeed cannot pass into the small intestine, and will either remain in the stomach for long periods, induce a gastric obstruction or, as every cat owner knows, be evacuated by vomition. However, many of the indigestible solids that fail to pass through the pylorus shortly after a meal do pass into the small intestine during periods between meals. This is due to a different pattern of motor activity called the migrating motor complex, a pattern of smooth muscle contractions that originates in the stomach, propagates through the intestines and serves a housekeeping function to periodically sweep out the gastrointestinal tract.