Chemoreceptors

A chemoreceptor, also known as chemosensor, is a specialized cell which acts as a sensory receptor which transduces (responds to) a chemical substance and generates a biological signal. This signal may be in the form of an action potential if the chemoreceptor is a neuron (nerve cell), or in the form of a neurotransmitter that can activate a nearby nerve fiber if the chemosensor is a specialized sensory cell, such as a sensory cell in a taste bud or in an internal chemoreceptor such as the carotid body. In more general terms, a chemosensor detects chemicals in the internal or external environment and transmits that information to the nervous system.

There are two main classes of the chemosensor: direct and distance.

When inputs from the environment are significant to the survival of the organism, the input must be detected. As all life processes are ultimately based on chemistry it is natural that detection and passing on of the external input will involve chemical events. The chemistry of the environment is, of course, relevant to survival, and detection of chemical input from the outside may well articulate directly with cell chemicals.

Chemoreception is important for the detection of food, habitat, conspecifics including mates, and predators. For example, the emissions of a predator's food source, such as odors or pheromones, may be in the air or on a surface where the food source has been. Cells in the head, usually the air passages or mouth, have chemical receptors on their surface that change when in contact with the emissions. It passes in either chemical or electrochemical form to the central processor, the brain or spinal cord. The resulting output from the CNS (central nervous system) makes body actions that will engage the food and enhance survival.

Particular chemoreceptors, called ASICs, detect the levels of carbon dioxide in the blood. To do this, they monitor the concentration of hydrogen ions in the blood, which decrease the pH of the blood. This can be a direct consequence of an increase in carbon dioxide concentration, because aqueous carbon dioxide in the presence of carbonic anhydrase reacts to form a proton and a bicarbonate ion.

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