Emesis

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Chapter: Essential pharmacology : Drugs For Emesis, Reflux And Digestive Disorders

Vomiting occurs due to stimulation of the emetic (vomiting) centre situated in the medulla oblongata. Multiple pathways can elicit vomiting.


EMESIS

 

Vomiting occurs due to stimulation of the emetic (vomiting) centre situated in the medulla oblongata. Multiple pathways can elicit vomiting (Fig. 47.1). The chemoreceptor trigger zone (CTZ) located in the area postrema and the nucleus tractus solitarius (NTS) are the most important relay areas for afferent impulses arising in the g.i.t, throat and other viscera. The CTZ is also accessible to bloodborne drugs, mediators, hormones, toxins, etc. because it is unprotected by the blood-brain barrier. Cytotoxic drugs, radiation and other g.i. irritants release 5HT from enterochromaffin cells acts on 5HT3 receptors present on extrinsic primary afferent neurones (PAN) of the enteric nervous system (ENS), which connect with vagal and spinal visceral afferents to send impulses to NTS and CTZ. Released in large quantity, 5HT may also spill into circulation and reach CTZ. It may as well be released from platelets by inflammatory mediators. However, 5HT is not the only mediator of such signals: many peptides and other messengers are also involved.

 


 

The CTZ and NTS express a variety of receptors, e.g. histamine H1, dopamine D2, serotonin 5HT3, cholinergic M and opioid μ through which the emetic signals are relayed and which could be targets of antiemetic drug action.

 

The vestibular apparatus generates impulses when body is rotated or equilibrium is disturbed or when ototoxic drugs act. These impulses reach the vomiting centre mainly relayed from the cerebellum and utilize muscarinic as well as H1 receptors. Various unpleasant sensory stimuli such as bad odour, ghastly sight, severe pain as well as fear, recall of an obnoxious event, anticipation of an emetic stimulus (repeat dose of cisplatin) cause nausea and vomiting through higher centres.

 

Nausea is accompanied by reduced gastric tone and peristalsis. In the emetic response fundus and body of stomach, esophageal sphincter and esophagus relax, while duodenum and pyloric stomach contract in a retrograde manner. Rhythmic contractions of diaphragm and abdominal muscles then compress the stomach and evacuate its contents via the mouth. Conditions that inhibit gastric emptying predispose to vomiting.

 

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