Vomiting

Abstract

Vomiting is a complex coordinated defense mechanism which is primarily a function of the respiratory and not of the gastrointestinal system. It is controlled by a neural mechanism consisting of a superficially placed chemoceptive trigger zone in the area postrema in the 4th ventricle and a co-ordinating center in the lateral reticular formation of the medulla oblongata. The chemoceptive trigger zone is composed of specialized cells which are sensitive to circulating chemical agents, and when properly stimulated, activate the more deeply placed reticular vomiting center. The latter may also be activated directly by visceral afferents from the gut. Chlorpromazine, a clinically effective antiemetic, was found to act on both areas. In small doses it is believed to compete with circulating emetic agents for receptors in the chemoceptive trigger zone, thus acting as a competitive inhibitor. In larger doses it has a depressant effect on the reticular vomiting center itself.