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Vomiting is a forceful expulsion of the contents of the stomach and sometimes the gut. The whole mechanism is guided and controlled by the brain and its vomiting centre.
The fourth ventricle of the brain hosts the vomiting centre. The floor of the fourth ventricle contains an area called the chemoreceptor trigger zone (CTZ). It is also called the area postrema. When the CTZ is stimulated, vomiting may occur.
The CTZ contains receptors for dopamine, serotonin, opioids, acetylcholine and the neurotransmitter substance P. When stimulated, each of these receptors gives rise to pathways leading to vomiting and nausea. Present in high concentrations in the vomiting centre, Substance P seems to be involved in the final common pathways that give rise to vomiting.
The CTZ lies outside the blood-brain barrier (BBB). Normally, the BBB controls substances that affect the brain. Medications and chemicals in the blood have selective access to the brain when protected by the BBB. As the CTZ, however, lies outside the BBB, drugs and medications are capable of stimulating this area to trigger vomiting. Medications that control or are used to treat vomiting may inhibit this area to prevent vomiting.
The CTZ in the brain is stimulated by various inputs from different parts of the body and this leads to vomiting. Some of the inputs to the CTZ include:
The process of vomiting involves several phases and steps. These include: