Chemoreceptor Trigger Zone - Chemoreception

Chemoreception

Since the CTZ is located in the medulla oblongata, it therefore does not have a specific blood-brain barrier. This means that large polar molecules, such as emetic toxins, can diffuse through to and reach the CTZ quite easily. This is because the medula oblangata is located in the area of the brain, the most inferior portion, which does not have a robust and highly developed blood-brain barrier. Without this barrier, emetic drugs and toxins are free to interact with a receptor (biochemistry), or multiple receptors located in the CTZ. These receptors in the CTZ are called chemoreceptors because they interact with different types of chemicals which are usually referred to as neurotransmitters. These neurotransmitters implement their effects on the CTZ receptors by binding to them which sets off a chain of events which produces an action potential. Studies have shown that neurons in the CTZ increase their rate of firing when exposed to emetic substances. The CTZ has many different types of receptors, which are specific to different types of toxins or drugs that might be present in the bloodstream and thus that can affect the CTZ. Types of CTZ receptors include dopamine, serotonin, histamine, substance P, opioid, and acetylcholine receptors. It has been discovered that the cholinergic neurons are actually nicotinic. These receptors are meant to monitor the amount of associated neurotransmitter of these receptors in the blood. For instance, the CTZ has opioid receptors that monitor the level of opioids in the blood, and when the amount of opioids in the blood reach a certain level, the opioid receptors in the CTZ will signal to the vomiting center to initiate vomiting. This is because the CTZ sends the "vomit" command through action potentials, and these specific action potentials that trigger emesis are only produced when a certain amount of opioids bind to a certain amount of opioid receptors in the CTZ. Neurons in the CTZ, and area postrea in general, actually have two types of receptors: those at the surface of the neuron and those that are located deeper down in the dendrites. The receptors on the surface of the neuron are chemoreceptors that are activated from direct contact of emetic substances in the blood, whereas the receptors that are deeper down on the dendrites are receptors that are activated in response to the activated chemoreceptors on the surface.