Gastric motility and secretion are, to some extent, automatic. Waves of contraction that serve to push chyme through the py-loric sphincter, for example, are initiated spontaneously by pacesetter cells in the greater curvature of the stomach. Likewise, the secretion of HCl from parietal cells and pepsinogen from chief cells can be stimulated in the absence of neural and hormonal influences by the presence of cooked or partially digested protein in the stomach. This action involves other cells in the gastric mucosa, including the G cells (that secrete the hormone gastrin); the enterochromaffin-like (ECL) cells, which secrete histamine; and the D cells, which secrete somatostatin.
The effects of autonomic nerves and hormones are superimposed on this automatic activity. This extrinsic control of gastric function is conveniently divided into three phases: (1) the cephalic phase; (2) the gastric phase; and (3) the intestinal phase. These are summarized in table 18.6.
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