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Monday, October 3, 2011

Vomiting - How does it occur, the receptors and areas invloved in vomiting

How does vomiting occur / Mechanism of vomiting?

In the brain(within the lateral medullary reticular formation of the brain stem), there is an area called Vomiting Center (nucleus of tractus solitarius), which controls vomiting. This vomiting center gets stimulated by different stimulus, eg gastric irritation, drugs (see causes of vomiting). The vomiting center then sends signals to the gastrointestinal tract and other areas which ultimately leads to vomiting.



As you can see in the figure above, there are four main sites from where the different stimuli come from:

1. Chemoreceptor trigger zone / Area postrema: stimulated by emetogenic stimuli (eg toxins) in blood or cerebrospinal fluid (CSF).
2. Vestibular system: stimulated by disturbances in the balancing system of the body eg motion sickness
3. Gastrointestinal tract and heart: Stimuled by substances that cause gastric irritation (drugs, acute infections, toxins, radiation, distension), myocardial infarction.
4. Central nervous system: stimulated by psychiatric disoreders, stress, anticipatory vomiting prior to cancer treatment or surgery.

The different emetogenic stimuli acts through different receptors. These receptors vary in different sites. The activation of different receptors depends on the cause or causes of vomiting and nausea. Based on these receptors the different classed of anti-emetic drugs are prescribed.

Receptors associated with vomiting:

Zone
Vomiting center
Chemoreceptor trigger zone / Area Postrema
Vestibular system
Gastrointestinal tract and heart
Central nervous system
Receptors(R)
· Histamine 1 (H1)
· Muscarinic 1 (M1)
· Neurokinin 1 (NK1)
· 5-HT3
· Dopamine D2
· Opioid
· Seretonin 5-HT3
· Neurokinin 1 (NK1)
· Muscarinic M1
· Histamine H1
· 5 HT3
· Mechano-R
· Chemo-R
No receptors. But stimulates through areas like cortex, thalamus, hypothalamus, meninges

The diagram below describes the whole process of vomiting in brief:

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