Area postrema

The area postrema is a structure in the medulla oblongata in the brainstem that controls vomiting. Its location in the brain also allows it to play a vital role in the control of autonomic functions by the central nervous system. It is one of the circumventricular organs, enabling the dual role of being a sensor for circulating chemical messengers in the blood, as well as integrating neural inputs in the brainstem. The area postrema is a structure in the medulla oblongata in the brainstem that controls vomiting. Its location in the brain also allows it to play a vital role in the control of autonomic functions by the central nervous system. It is one of the circumventricular organs, enabling the dual role of being a sensor for circulating chemical messengers in the blood, as well as integrating neural inputs in the brainstem. The area postrema is a small protuberance found at the inferoposterior limit of the fourth ventricle. Specialized ependymal cells are found within the area postrema. These cells differ slightly from the majority of ependymal cells (ependymocytes), forming a unicellular epithelial lining of the ventricles and central canal. The area postrema is separated from the vagal triangle by the funiculus separans, a thin semitransparent ridge. The vagal triangle overlies the dorsal vagal nucleus and is situated on the caudal end of the rhomboid fossa or 'floor' of the fourth ventricle. The area postrema is situated just before the obex, the inferior apex of the caudal ventricular floor. Both the funiculus separans and area postrema have a similar thick ependyma-containing tanycyte covering. Ependyma and tanycytes can participate in the transport of neurochemicals into and out of the cerebrospinal fluid from its cells or adjacent neurons, glia or vessels. Ependyma and tanycytes may also participate in chemoreception. The area postrema is considered a circumventricular organ because of its proximity to the ventricular system. In a morphological study, area postrema capillaries in the ventral subregion of area postrema were shown to be relatively impermeable like those of the brain, whereas medial and dorsal area postrema capillaries had microscopic characteristics of high permeability, a characteristic called sinusoidal. Subregional capillary density of the area postrema was highest near the ventricular interface, and was nearly twice as dense as the capillary densities of the adjacent solitary nucleus (SN), and dorsal motor nucleus of the vagus nerve. A tanycyte barrier partially compensates for high capillary permeability in the area postrema. Physiological subregional studies of the area postrema indicated that its blood volume is relatively large, and blood flow and transit time for blood markers relatively slow, thereby amplifying the sensing capability for circulating compounds, such as hormones or transmitters. The area postrema connects to the solitary nucleus, or nucleus tractus solitarii (NTS), and other autonomic control centers in the brainstem. It is excited by visceral afferent impulses (sympathetic and vagal) arising from the gastrointestinal tract and other peripheral trigger zones, and by humoral factors. The area postrema makes up part of the dorsal vagal complex, which is the critical termination site of vagal afferent nerve fibers, along with the dorsal motor nucleus of the vagus and the NTS. Nausea is most likely induced via stimulation of the area postrema via its connection to the NTS, which may serve as the beginning of the pathway triggering vomiting in response to various emetic inputs. However, this structure plays no key role for nausea induced by the activation of vagal nerve fibers or by motion, and its function in radiation-induced vomiting remains unclear. Because the area postrema and a specialized region of NTS have permeable capillaries, peptides and other hormonal signals in the blood have direct access to neurons of brain areas with vital roles in the autonomic control of the body. As a result, the area postrema is considered a site of integration for various physiological signals in the blood as they enter the central nervous system. The area postrema, one of the circumventricular organs, detects toxins in the blood and acts as a vomit-inducing center. The area postrema is a critical homeostatic integration center for humoral and neural signals by means of its function as a chemoreceptor trigger zone for vomiting in response to emetic drugs. It is a densely vascularized structure with subregional capillary specializations for high permeability for circulating blood signals, allowing it to detect various chemical messengers in the blood and cerebrospinal fluid. Capillary blood flow appears to be uniquely slow in the area postrema, prolonging the contact time for blood-borne hormones to interact with neuronal receptors involved in regulation of blood pressure, body fluids, and emetic responses. The fenestrated sinusoidal capillaries of the area postrema and a specialized region of NTS make this particular region of the medulla critical in the autonomic control of various physiological systems, including the cardiovascular system and the systems controlling feeding and metabolism. Angiotensin II causes a dose-dependent increase in arterial blood pressure without producing considerable changes in the heart rate, an effect mediated by the area postrema.