Area postrema

The area postrema is a highly vascular structure located in the caudal part of the floor of the fourth ventricle of the brainstem, positioned just rostral to the obex and separated from the vagal triangle by a semitransparent ridge called the funiculus separans.

Structurally, area postrema consists of vascular and cellular components. The vascular portion contains sinusoidal fenestrated capillaries, forming a portal system, while the cellular part includes flattened ependymal cells, glial cells, and small neurons . Specialized cells known as tanycytes, derived from thickened ependymal cells, feature microvilli that extend into the cerebrospinal fluid (CSF).

The area postrema is one of the brain's circumventricular organs, lacking a blood-brain barrier. This allows it to detect circulating substances in the bloodstream, as its fenestrated capillaries enable molecules and chemical signals to readily reach its neurons . It plays an important role in monitoring blood-borne signals, such as baroreceptor information (pressure-sensing signals) from the carotid sinus and aorta, which helps brain regulate cardiovascular responses, such as adjusting heart rate and vascular constriction. It also detects osmoreceptor signals, which detect the concentration of solutes (like sodium) in the blood, from the liver and other organs. These inputs play a role in managing fluid balance and metabolic processes. Additionally, the area postrema detects the hormone angiotensin II, which triggers pathways to increase arterial blood pressure without significantly impacting the heart rate.

Notably, the area postrema is also part of the dorsal vagal complex and is well-known as the brain's "vomiting center," making it critical for detecting and responding to emetic stimuli in the blood and CSF.