Gastric Motility
| Overview |
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- Physical motion within the stomach first facilitates entry of food into the esophagus via "Receptive Relaxation", subsequently mixes food with gastric secretions in a process known as "Gastric Mixing", and finally allows for regulated "Gastric Emptying" of mixed food into the small intestine for further digestion and absorption of nutrients.
| Receptive Relaxation |
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- Overview
- Receptive Relaxation refers to the muscular relaxation of the orad stomach in response to entry of food from the esophagus. The orad stomach then acts as a storage depot for ingested food within the stomach prior to its mixing.
- Regulation
- Receptive relaxation is completely coordinated by the brainstem via the autonomic nervous system. Afferent sensory information about the entry of food is transmitted to the brain via the vagus nerve, and efferent motor information initiating relaxation is also conducted by the vagus. Thus receptive relaxation is termed a 'Vagovagal Reflex' since both afferent and efferent arms of the reflex travel through the vagus nerve.
| Gastric Mixing |
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- Gastric mixing is mediated by peristaltic waves that begin in the mid-stomach and progress through the caudad stomach. During mixing the pylorus muscle is contracted, thus preventing any stomach contents from emptying into the small intestine. The waves become stronger as they approach the closed pylorus, thus helping mix and break up the food into a semi-liquid slurry known as 'Chyme'.
| Gastric Emptying |
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- Overview
- Gastric emptying occurs when the pressures created by the gastric peristaltic waves exceed the closing pressure of the pylorus muscle, causing 'Chyme' to be forced through the pyloric sphincter. This occurs either by increasing the peristaltic pressures or relaxing the pylorus muscle. In general, gastric emptying is delayed if there is excessive acid or undigested fats within the small intestine.
- Nervous Regulation
- A number of nerve reflex pathways exist which sense the situation in the duodenum and then relay that information either directly to the stomach or via external pathways going through the spinal cord or brainstem. These sensory pathways sense a number of factors in the duodenum but are especially sensitive to duodenal pH. When duodenal pH drops (i.e. H+ concentration increases) excessively, these reflex pathways reduce the strength of gastric peristaltic contractions and increase the tone of the pylorus muscle.
- Hormonal Regulation
- A number of hormones are also released by the small intestine epithelium which regulate gastric emptying, the most important of which is cholecystokinin. Cholecystokinin is released by epithelial cells of the duodenum and jejunum in response to the presence of fats in the small intestinal lumen and reduces the strength of gastric peristaltic contractions.