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You are in Home >> Resources >> Clinical anaesthesia >> Nausea and vomiting

Physiology of PONV

Created: 13/7/2004

Vomiting centre: located in dorso-lateral reticular formation of the medulla.

Chemoreceptor trigger zone (CTZ): located in the area postrema of the medulla, on the lateral walls of the 4th ventricle (outside the blood-brain barrier).

Nausea and vomiting can be induced by many physiological and pathological factors, as well as drugs or ingested toxins. Nausea and vomiting is primarily controlled by the vomiting centre. This is an area in the brainstem that integrates responses. Efferent impulses from these medullary centres influence related brainstem nuclei to initiate the vomiting reflex.

Afferent stimuli arrive from chemoreceptors and pressure receptors in the gut and CNS, as well as peripheral pain receptors. Other sites of input in the CNS include the cerebral cortex (pain, fear and anxiety), vestibular and cerebellar nuclei and the CTZ.

The CTZ comprises a group of cells close to the area postrema on the floor of the fourth ventricle. Raised intracerebral pressure is thought to cause vomiting via increased pressure on the floor of the fourth ventricle. The CTZ is outside the blood-brain- and cerebrospinal fluid-brain barriers and is extremely sensitive to emetic stimuli. Dopamine and 5-HT play an important role in the activity of the CTZ. 5-HT may play a large role in drug-induced emesis. 5-HT3 receptors appear to play an important part in the mediation of nausea and vomiting induced by high doses of cytotoxic agents. These receptors are located in the small intestine, the CTZ and the area postrema.

The neurotransmitters implicated in the control of nausea and vomiting include acetylcholine, dopamine and 5-HT. Hence anti-emetics act as antagonists to one or more of these neurotransmitters.

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