Some Physiology

Useful vomiting occurs as a protective mechanism for eliminating irritant or harmful substances from the upper gastrointestinal tract. The act of emesis is controlled by the vomiting centre in the medulla. Close to it lie other visceral centres, including those for respiration, salivation and vascular control, which give rise to the prodromal sensations of vomiting. These centres are not anatomically discrete but comprise interconnected networks within the nucleus of the tractus solitarius). The vomiting centre does not initiate, but rather it coordinates the act of emesis on receiving stimuli from various sources, namely,

• The chemoreceptor trigger zone (CTZ), a nearby area that is extremely sensitive to the action of drugs and other chemicals

• The vestibular system

• The periphery, e.g. distension or irritation of the gut, myocardial infarction, biliary or renal stone

• Cortical centres.

The vomiting centre and the nucleus of the tractus solitarius contain many muscarinic cholinergic and histamine H1 receptors, and the CTZ is rich in dopamine D2 receptors; drugs that block these receptors are effective antiemetics. The precise role and location of 5-HT3 receptors (see ondansetron, below) in relation to emesis remains to be defined but both central and peripheral mechanisms may be involved.

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