Chemoreceptor

A chemoreceptor, also known as chemosensor, is a sensory receptor that transduces a chemical signal into an action potential. In more general terms, a chemosensor detects certain chemical stimuli in the environment.

Classes
There are two main classes of the chemosensor: direct and distance.
 * Examples of distance chemoreceptors are:
 * olfactory receptor neurons in the olfactory system
 * neurons in the vomeronasal organ that detect pheromones
 * Examples of direct chemoreceptors include
 * Taste buds in the gustatory system
 * Carotid bodies and aortic bodies detect changes primarily in oxygen. They also sense increases in CO2 partial pressure and decreases in arterial pH, but to a lesser degree than for O2.
 * The chemoreceptor trigger zone is an area of the medulla in the brain that receives inputs from blood-borne drugs or hormones, and communicates with the vomiting center to induce vomiting.

Cellular antennae
Within the biological and medical disciplines, recent discoveries have noted that primary cilia in many types of cells within eukaryotes serve as cellular antennae. These cilia play important roles in chemosensation. The current scientific understanding of primary cilia organelles views them as "sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."

Breathing rate
Chemoreceptors 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 is a direct consequence of an increase in carbon dioxide concentration, because carbon dioxide becomes carbonic acid in an aqueous environment.

The response is that the respiratory centre (in the medulla), sends nervous impulses to the external intercostal muscles and the diaphragm, via the intercostal nerve and the phrenic nerve, respectively, to increase breathing rate and the volume of the lungs during inhalation.

Chemoreceptors that affect breathing rate are broken down into two categories.


 * central chemoreceptors are located on the ventrolateral surface of medulla oblongata and detect changes in pH of cerebrospinal fluid. They do not respond to a drop in oxygen, and eventually desensitize.


 * peripheral chemoreceptors: Aortic body detects changes in blood oxygen and carbon dioxide, but not pH, while carotid body detects all three. They do not desensitize. Their effect on breathing rate is less than that of the central chemoreceptors.

Heart rate
The response to stimulation of chemoreceptors on the heart rate is complicated. Stimulation of peripheral chemoreceptors directly activates the medullary vagal center and slows the heart rate. However, a number of other factors are usually at play in this situation which obscure this response. These factors include activation of stretch receptors due to increased ventilation and the release of circulating catecholamines. Hence, although the stimulation of peripheral chemoreceptors causes bradycardia, this may not be the net result.

Sensory organs
In taste sensation, the tongue is composed of 5 different taste buds: salty, sour, sweet, bitter, and savory. The salty and sour tastes work directly through the ion channels, the sweet and bitter taste work through G protein-coupled receptors, and the savory sensation is activated by glutamate.

Noses in vertebrates and antennae in many invertebrates act as distance chemoreceptors. Molecules are 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 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. 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.