postrema

The postrema, or area postrema, is a structurally and functionally distinct region located at the caudal medulla oblongata on the dorsal surface, near the floor of the fourth ventricle close to the obex. It is one of the circumventricular organs, characterized by a relatively permeable capillary network that allows circulating signals to influence nearby neural circuits.

Unlike most of the brain, the area postrema lacks a typical blood–brain barrier. Its fenestrated capillaries and specialized surrounding glial and ependymal elements enable it to detect circulating toxins, hormones, and other chemical stimuli that enter the bloodstream. This sensory role positions the area postrema as a chemoreceptor trigger zone for emesis.

Functionally, neurons in the area postrema participate in initiating vomiting by projecting to autonomic and visceral motor centers, especially the nucleus tractus solitarius and the dorsal motor nucleus of the vagus. It responds to a range of emetic signals, including serotonin (5-HT3), dopamine (D2), and neurokinin-1 (NK1) receptor activation, as well as other chemical and osmotic cues. Through these pathways, the area postrema helps coordinate the complex motor and autonomic responses involved in vomiting.

Clinical relevance centers on its role in nausea and vomiting related to toxins and chemotherapy. Anti-emetic drugs—such as 5-HT3 antagonists, NK1 receptor antagonists, and dopamine antagonists—often exert part of their effects by modulating signals involving the area postrema.