A Chemosensor, also known as chemoreceptor, is a cell or group of cells that transduce a chemical signal into an action potential. There are two main classes of the chemosensor: direct and distance. Two examples of distance chemoreceptors are olfactory receptor neurons in the olfactory system, and those in the vomeronasal organ that detect pheromones. Examples of direct chemoreceptors include taste buds in the gustatory system and carotid bodies that detect changes in pH inside the body.
Chemoreceptors and breathing rate
Chemoreceptors in the medulla, carotid arteries and aorta, detect the levels of carbon dioxide in the blood. To do this, they monitor the concentration of hydrogen ions in the blood, which increases the pH of the blood, as a direct consequence of the raised carbon dioxide concentration.
The response is that the inspiratory centre (in the medulla), sends nervous impulses to the external intercostal muscles and the diaphragm, via the phrenic nerve to increase breathing rate and the volume of the lungs during inhalation.
Chemoreceptors and heart rate
Chemoreceptors in the medulla, carotid arteries and aorta, detect the levels of carbon dioxide in the blood, in the same way as applicable in the Breathing Rate section.
In response to this high concentration, a nervous impulse is sent to the cardiovascular centre in the medulla, which will then feedback to the accelerator nerve, increasing nervous impulses here, and prompting the sinoatrial node to stimulate more contractions of the myogenic cardiac muscle (increase heart rate).
Chemoreceptors and sense organs
In taste sensation, the tongue is composed of 5 different taste buds: salty, sour, sweet, bitter, and unami. The salty and sour tastes work directly through the ion channels, the sweet and bitter taste work through G protein-coupled receptors, and the unami sensation is activated by glutamate.
Noses in vertebrates and antennae in many invertebrates act as distance chemoreceptors. Molecules diffused through the air and bind to specific receptors on olfactory sensory neurons, activating an opening ion channel via G-proteins.
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.
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 souce 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. The change does not stop there. 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.
See also: Sensory receptor