Gastric Motility and Emptying The Role of the Stomach as a Reservoir and a Churn

Motility of the stomach allows it to serve as a reservoir, act as a churn to fragment food and mix it with gastric secretions to aid digestion, and to empty gastric contents into the duodenum at a controlled rate. Gastric motility and storage are complex and subject to multiple regulatory mechanisms. The first aspect of gastric motility is gastric filling. Accommodation of large changes in volume when a meal is eaten is achieved by the plasticity of the stomach smooth musculature and by receptive relaxation. Plasticity means that smooth muscle can maintain constant tension over a wide range of lengths without changing tension. Mechanoreceptors in the proximal stomach signal the degree of distension and, beyond a certain level, a stretch-activated contraction is initiated and pacesetter cells are depolarized. These properties are augmented by receptive relaxation of the deep folds of the stomach (known as rugae), which is mediated by the vagus nerve and associated with eating, possibly via stimulation of the taste buds.

A group of pacesetter cells located high on the greater curvature of the stomach generates slow wave potentials, which sweep down the length of the stomach 3 times per minute. These spontaneous depolarizations are known as the basic electrical rhythm (BER) of the stomach. The stomach's circular smooth muscle layer may be stimulated to contract in peristaltic waves synchronized with the BER. The contractions in the thinly muscled fundus and body are weak but become stronger in the thickly muscled antrum. Food emptied into the stomach from the esophagus is therefore stored in the body of the stomach and gradually fed into the antrum where mixing takes place. The antrum can contain 30 ml of chyme but only a few milliliters of chyme are forced through the pyloric sphincter into the duodenum with each peristaltic wave. Each wave causes the sphincter to contract more forcefully, blocking passage into the duodenum. This process is called retropulsion and achieves thorough mixing of chyme in the antrum. These events are known as the gastric phase of digestion, which is initiated as soon as food enters the stomach. The dominant hormone of the gastric phase is gastrin (Table 4).

Table 4 Gastrointestinal peptide hormones affecting gastric motility and emptying

Hormone Amino acid no. Cell source Stimuli Effects on stomach

(chromosomal location)

Gastrin

Gastric inhibitory peptidea

Motilin

Secretin

Cholecystokinin

34(17q21)

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