The area postrema is a medullary structure in the brain located at the base of the cavity called fourth ventricle. It is bathed by cerebrospinal fluid, which in turn carries substances that come from the blood; therefore, it is also called a circumventricular organ. Along with several cranial nerves, parts of the gastrointestinal tract, thorax, and abdomen, the area postrema induces vomiting when a person is exposed to emetic stimuli such as toxins, drugs, and also when metabolic wastes are present in the blood.
Unpleasant thoughts or smells can stimulate vomiting. The signals for vomiting can therefore come from the nervous system, particularly the cortex of the brain. Alternatively, cranial nerves that supply the tongue and the pharynx can be activated. When the posterior area of the oral cavity is accidentally stimulated, such as when a person is brushing his teeth and tongue, the gag reflex is activated and vomiting can occur. Vomiting can also be triggered by motion sickness and other disorders that affect the ear.
The area postrema is called the chemoreceptor trigger zone for vomiting because it is sensitive to bloodborne substances that serve as emetic stimuli. These bloodborne substances include drugs such as cisplatin, bacterial toxins, and metabolic wastes that become elevated as a result of diseases. Examples of bacteria that produce toxins that induce vomiting are Staphylococcus aureus, Bacillus cereus, and Vibrio cholerae. Uremia or elevation of nitrogenous wastes like urea can lead to vomiting.
The area postrema is also an important component of the autonomic nervous system. This means that it is involved in the involuntary control of the cardiovascular and respiratory systems. It is an entry and integration point for sensory information from the heart, stomach, intestines, kidneys, liver, and other organs. Based on studies, the area postrema also appears to play an important role in the regulation of blood pressure.
When this region of the brain is invaded by tumors or removed surgically, a person's ability to detect toxins in the blood is greatly diminished. Additionally, the brain is not able to respond to certain physiological stimuli. Experiments on rats have shown when their area postrema is damaged they were not able to detect lithium chloride, a substance that is toxic when consumed at high concentrations. Normally, rats are able to detect this substance and avoid it.
Medications can stimulate the area postrema, and this stimulation can lead to nausea and vomiting. Dopamine agonists, which are drugs used for treating Parkinson’s disease, is one of the drug types that causes this stimulation. These drugs aim to normalize the movements of a person with Parkinson's Disease by increasing the concentration of dopamine, and brain neurotransmitter. Dopamine, however, is a strong stimulant of the area postrema.
