MECHANISMS Vomiting is coordinated by the brainstem and is effected by responses in the gut, pharynx, and somatic musculature. Mechanisms underlying nausea are poorly understood but likely involve the cerebral cortex, as nausea requires cognitive and emotional input and is associated with autonomic responses including diaphoresis, pallor, and altered heart rate. Functional brain imaging studies support this idea showing activation of cerebral regions including the insula, anterior cingulate cortex, and amygdala during nausea. Coordination of Emesis Brainstem nuclei—including the nucleus tractus solitarius; dorsal vagal and phrenic nuclei; medullary nuclei regulating respiration; and nuclei that control pharyngeal, facial, and tongue movements—coordinate initiation of emesis involving neurokinin NK1 , serotonin 5-HT3 , endocannabinoid, and vasopressin pathways. Somatic and visceral muscles respond stereotypically during emesis. Inspiratory thoracic and abdominal wall muscles contract, increasing intrathoracic and intraabdominal pressures to evacuate the stomach. Under normal conditions, distally migrating gut contractions are coordinated by an electrical phenomenon, the slow wave, which cycles at 3 cycles/min in the stomach and 11 cycles/min in the duodenum. During emesis, slow waves are abolished and replaced by orally propagating spikes that evoke retrograde contractions to facilitate expulsion of gut contents. Activators of Emesis Emetic stimuli act at several sites. Emesis evoked by unpleasant thoughts or smells originates in the brain. Motion sickness and inner ear disorders act on labyrinthine pathways. Gastric irritants and cytotoxic agents like cisplatin stimulate gastroduodenal vagal afferent nerves. Nongastric afferents are activated by bowel obstruction and mesenteric ischemia. The area postrema, in the medulla, responds to bloodborne stimuli (emetogenic drugs, bacterial toxins, uremia, hypoxia, ketoacidosis) and is termed the chemoreceptor trigger zone. Neurotransmitters mediating vomiting are selective for different sites. Labyrinthine disorders stimulate vestibular muscarinic M 1 and histaminergic H 1 receptors. Vagal afferent stimuli activate 5-HT 3 receptors. The area postrema is served by nerves acting on 5-HT 3 , M1 , H 1 , and dopamine D 2 subtypes. NK 1 receptors in the central nervous system (CNS) mediate both nausea and vomiting. Cannabinoid CB 1 pathways may participate in the cerebral cortex and brainstem. Therapies for vomiting act on these receptor-mediated pathways.