Chemosensor
Encyclopedia
A chemoreceptor, also known as chemosensor, is a sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

 that transduces
Transduction (physiology)
In physiology, transduction is the conversion of a stimulus from one form to another.Transduction in the nervous system typically refers to stimulus alerting events wherein a mechanical/physical/etc stimulus is converted into an action potential which is transmitted along axons towards the central...

 a chemical signal into an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

. 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 neuron
      Olfactory receptor neuron
      An olfactory receptor neuron , also called an olfactory sensory neuron , is a transduction cell within the olfactory system. J. Rospars, Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor...

      s in the olfactory system
      Olfactory system
      The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...

    • neurons in the vomeronasal organ
      Vomeronasal organ
      The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....

       that detect pheromones
  • Examples of direct chemoreceptors include
    • Taste buds in the gustatory system
      Gustatory system
      The gustatory system is the sensory system for the sense of taste.- Importance :The gustatory system allows humans to distinguish between safe and harmful food. Bitter and sour foods we find unpleasant, while salty, sweet, and meaty tasting foods generally provide a pleasurable sensation...

    • Carotid bodies
      Carotid body
      The carotid body is a small cluster of chemoreceptors and supporting cells located near the fork of the carotid artery ....

       and aortic bodies
      Aortic body
      In the human heart, the aortic body is one of several small clusters of chemoreceptors, baroreceptors, and supporting cells located along the aortic arch.-Function:...

       detect changes primarily in oxygen
      Oxygen
      Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

      . They also sense increases in CO2 partial pressure and decreases in arterial pH
      PH
      In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

      , but to a lesser degree than for O2.
  • The chemoreceptor trigger zone
    Chemoreceptor trigger zone
    The chemoreceptor trigger zone , is an area of the medulla that receives inputs from blood-borne drugs or hormones, and communicates with the vomiting center, to initiate vomiting. The CTZ is close to the area postrema on the floor of the fourth ventricle and is outside of the blood-brain barrier...

     is an area of the medulla
    Medulla oblongata
    The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

     in the brain that receives inputs from blood
    Blood
    Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

    -borne drug
    Drug
    A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage.In pharmacology, a...

    s or hormone
    Hormone
    A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

    s, and communicates with the vomiting center to induce vomiting
    Vomiting
    Vomiting is the forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose...

    .

Cellular antennae

Within the biological
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 and medical
Medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 disciplines, recent discoveries have noted that primary cilia in many types of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 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
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 in the blood. To do this, they monitor the concentration of hydrogen ion
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....

s in the blood, which decrease the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 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
Phrenic nerve
The phrenic nerve originates mainly from the 4th cervical nerve, but also receives contributions from the 5th and 3rd cervical nerves in humans....

, 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
    Central chemoreceptors
    Central chemoreceptors of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the pH of their environment....

     are located on the ventrolateral surface of medulla oblongata
    Medulla oblongata
    The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...

     and detect changes in pH of cerebrospinal fluid. They have also been shown experimentally to respond to hypercapnic hypoxia (elevated CO2, decreased O2), and eventually desensitize.

  • peripheral chemoreceptors
    Peripheral chemoreceptors
    Peripheral chemoreceptors act principally to detect variation of the oxygen concentration in the arterial blood, whilst also monitoring arterial carbon dioxide and pH. They are located in the aortic body and carotid body, on the arch of the aorta and on the common carotid artery, respectively. The...

    : Aortic body
    Aortic body
    In the human heart, the aortic body is one of several small clusters of chemoreceptors, baroreceptors, and supporting cells located along the aortic arch.-Function:...

     detects changes in blood oxygen and carbon dioxide, but not pH, while carotid body
    Carotid body
    The carotid body is a small cluster of chemoreceptors and supporting cells located near the fork of the carotid artery ....

     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
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

 sensation, the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

 is composed of 5 different taste buds: salty, sour, sweet, bitter, and savory
Umami
Umami , popularly referred to as savoriness, is one of the five basic tastes together with sweet, sour, bitter, and salty.-Etymology:Umami is a loanword from the Japanese meaning "pleasant savory taste". This particular writing was chosen by Professor Kikunae Ikeda from umai "delicious" and mi ...

. The salty and sour tastes work directly through the ion channels, the sweet and bitter taste work through G protein-coupled receptor
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...

s, and the savory sensation is activated by glutamate.

Nose
Nose
Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes through the pharynx, shared with the...

s in vertebrates and antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 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
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

 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 odor
Odor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...

s or pheromone
Pheromone
A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

s, 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
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 or spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...

. The resulting output from the CNS (central nervous system
Central nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

) makes body actions that will engage the food and enhance survival.

See also

  • Diffuse chemosensory system
    Diffuse chemosensory system
    The diffuse chemosensory system is an anatomical structure composed of solitary chemosensory cells and chemosensory clusters. The concept of DCS has been advanced in 2005, after the discovery that cells similar to gustatory elements are present in several organs of the respiratory and digestive...

  • Chemosensory clusters
    Chemosensory clusters
    Chemosensory clusters are aggregates formed by a small number of chemosensory cells with characteristics similar to those found in the taste cells of the oro-pharyngeal cavity. The chemosensory clusters are similar to the taste buds but are smaller, resembling the developing taste buds...

  • Chemoreceptor trigger zone
    Chemoreceptor trigger zone
    The chemoreceptor trigger zone , is an area of the medulla that receives inputs from blood-borne drugs or hormones, and communicates with the vomiting center, to initiate vomiting. The CTZ is close to the area postrema on the floor of the fourth ventricle and is outside of the blood-brain barrier...

  • Molecular sensor
    Molecular sensor
    A molecular sensor or chemosensor is a molecule that interacts with an analyte to produce a detectable change. Molecular sensors combine molecular recognition with some form of reporter so the presence of the guest can be observed...

  • Solitary chemosensory cells
    Solitary chemosensory cells
    Solitary chemosensory cells are isolated elements located in epithelia of the apparatuses of endodermic origin . In the aquatic vertebrates, SCCs are also present in the skin. In oral cavity, SCCs precedes the development of taste buds...

  • Sensory receptor
    Sensory receptor
    In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

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