The area postrema is located at the rhomboid fossa in the brainstem and is part of the vomiting center. This functional unit of the nervous system elicits vomiting when appropriately stimulated, thereby performing a protective role. Antiemetics inhibit this response as part of the treatment of traumatic brain injury and other neurologic conditions.
What is the area postrema?
Medicine counts the area postrema as one of the circumventricular organs. These are characterized by the fact that they are located in the brain ventricles. The cerebral ventricles represent cavities in the brain that contain water (cerebrospinal fluid), which appears dark or white on MRI images, depending on the weighting. In addition to their location, the circumventricular organs are characterized by a special type of tissue: ependyma. Its relatively large surface area creates space for numerous receptors and allows the area postrema to be sensitive to potential contaminants. This circumstance is highly significant because the brain region is part of the vomiting center. Together with other structures, it controls involuntary vomiting in response to toxins, drugs, signals from the digestive tract, and other stimuli.
Anatomy and structure
As a circumventricular organ, the area postrema has the ependyma, a special type of tissue composed of glial cells found in few other structures. For a long time, scientists assumed that glial cells performed mainly static functions and were stabilizing filler; the name “glia” means “glue” and is a reminder of this erroneous conclusion. Today, however, it is known that they are extremely important for the nervous system to function properly. They insulate the axons of nerve fibers, contribute to optimal nutrient supply to neurons, and exert hygienic functions at the micro level. Most other circumventricular organs are also circumventricular organs located in the ventricles of the brain. The organs in the cerebral ventricles use the ependyma to demarcate themselves from the fluid. However, they do not have a blood-brain barrier, which is the barrier between the bloodstream and brain tissue elsewhere in the brain and is designed to keep pathogens, toxins, and other substances from entering the brain. In the immediate vicinity of the area postrema lies the nucleus solitarius or nucleus tractus solitarii. It is considered the gustatory nucleus and also belongs to the vomiting center. There are close connections between the two brain structures, which allow them to work together on a functional level.
Function and tasks
The area postrema represents an important part of the vomiting center. The role of the associated structures is to protect the body: specialized receptors sense stimuli that indicate toxins in the food or environment, pose a threat through mechanical pressure, or suggest other dangers. The area postrema includes the chemoreceptor trigger zone. Because the circumventricular organ itself lacks a blood-brain barrier, it performs an additional guardian function for the barrier behind it. The receptors of the area postrema respond to various chemicals indicative of toxins or putrefaction; the fenestrated structure of the ependyma surrounding the area postrema ensures particularly reliable screening of blood seeking to enter brain tissue. Serotonin and dopamine are the main neurotransmitters acting in the vomiting center. The vomiting center communicates with other parts of the brain through numerous connections. Nerves leading to the cerebral cortex link the area postrema and the rest of the vomiting center to both the olfactory center and higher cognitive functions. The vestibular organ, gastrointestinal nerves, certain areas of the medulla oblongata, and various muscle groups also connect to the vomiting center. Pathways that lead directly or via other switch points to muscles serve the motor execution of the vomiting act. This process is automatic.
Diseases
In the context of disease, the area postrema occupies a dual position; on the one hand, as part of the vomiting center, it plays an important role in protecting against toxins (for example, in food); on the other hand, erroneous or persistent irritation of this brain region may be a sign of disease and may itself contribute to physical distress. To reduce the constant nausea and stop vomiting, doctors therefore use antiemetics.The drugs do not form a homogeneous group, but represent different active substances, each of which acts specifically on the area postrema. During normal stimulus initiation, the substances dock onto receptors and open ion channels in the cell membrane, through which electrically charged particles can flow in. This response of the receptor changes the electrical voltage of the cell: the neuron depolarizes. Antiemetics, which act at the area postrema, prevent this response by inhibiting the receptors. The drug metoclopramide inhibits the function of dopamine and serotonin receptors in the area postrema, while scopolamine inhibits the muscarinic acetylcholine receptors and 5-HT3 antagonists affect only a very specific serotonin receptor. More rarely, physicians also turn to apomorphine for treatment. As part of its normal functioning, the area postrema responds to potential pollutants and mechanical agents such as pressure. An increase in incranial pressure, or intracranial pressure, can result from a variety of causes. Possible triggers include cerebral edema, stroke, circulatory disturbances, traumatic effects such as in traumatic brain injury, or drainage disturbances. The area postrema is located in the cerebral ventricle; the organism uses this region as a buffer zone to keep damage from the tissues when intracranial pressure increases. The vomiting center is therefore very sensitive to changes in incranial pressure thanks to this spatial position. In addition, stimuli from the digestive tract, olfactory center, and vestibular organ can activate the vomiting center.
