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: Olfaction involves the ability to detect chemicals in the gaseous state. In vertebrates, the olfactory system detects odorants and pheromones in the nasal cavity. Within the olfactory system there are two anatomically distinct organs: the main olfactory epithelium (MOE) and the vomeronasal organ(VNO). It was initially thought that the MOE is responsible for the detection of odorants, while the VNO detects pheromones. The current view, however, is that both systems can detect odorants and pheromones.^[1] Olfaction in invertebrates differs from olfaction in vertebrates. For example, in insects, olfactory sensilla are present on their antennae.^[2]
• Examples of direct chemoreceptors include:
  • Taste buds in the gustatory system: The primary use of gustation as a type of chemoreception is for the detection of tasteants. Aqueous chemical compounds come into contact with chemoreceptors in the mouth, such as taste buds on the tongue, and trigger responses. These chemical compounds can either trigger an appetitive response for nutrients, or a defensive response against toxins depending on which receptors fire. Fish and crustaceans, who are constantly in an aqueous environment, use their gustatory system to identify certain chemicals in the mixture for the purpose of localization and ingestion of food.
  • Insects use contact chemoreception to recognize certain chemicals such as cuticular hydrocarbons and chemicals specific to host plants. Contact chemoreception is more commonly seen in insects but is also involved in the mating behavior of some vertebrates. The contact chemoreceptor is specific to one type of chemical.^[2]

Sensory Organs

• Olfaction: In vertebrates, olfaction occurs in the Nose. Volatile chemical stimuli enter the nose and eventually reach the olfactory cleft (where the main olfactory epithelium is located). Embedded in the olfactory epithelium are three types of cells: supporting, basal, and olfactory sensory neurons (OSN's). While all three types of cells are integral to the olfactory pathway, only OSN cells make contact with the stimulus. After the stimulus is received by the cilia protruding from the OSNs, the signal activates an action potential which then travels through the bony cribiform plate to the glomeruli within the olfactory bulb. While the proximity of the olfactory cleft to the brain is close, the signals are often slow to illicit response and often require numerous molecules to trigger the corresponding action potential. In insects, antennae act as distance chemoreceptors. For example, antennae on moths are made up of long feathery hairs that increase sensory surface area. Each long hair from the main antenna also has smaller sensilla that are used for volatile olfaction.^[3] Since moths are mainly nocturnal animals, the development of greater olfaction aids them in navigating the night.
• Gustation: In many vertebrates, the tongue serves as the primary gustatory sensory organ. As a muscle located in the mouth, it acts to manipulate and discern the composition of food in the initial stages of digestion. The tongue is rich in vasculature, allowing the chemoreceptors located on the top surface of the organ to transmit sensory information to the brain. Salivary glands in the mouth allow for molecules to reach chemoreceptors in an aqueous solution. The chemoreceptors of the tongue fall into two distinct superfamilies of G protein-coupled receptors. GPCR’s are intramembrane proteins than bind to an extracellular ligand- in this case chemicals from food- and begin a diverse array of signaling cascades that can result in an action potential registering as input in an organism’s brain. Large quantities of chemoreceptors with discrete ligand-binding domains provide for the five basic tastes: sour, salty, bitter, sweet, 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.
• Contact Chemoreception: Contact chemoreception is dependent on the physical contact of the receptor with the stimulus. The receptors are short hairs or cones that have a single pore at, or close to the tip of the projection. They are known as uniporous receptors. Some receptors are flexible, while others are rigid and do not bend with contact. They are mostly found in the mouthparts, but can also occur on the antennae or legs of some insects. There is a collection of dendrites located near the pores of the receptors, yet the distribution of these dendrites changes depending on the organism being examined. The method of transduction of the signal from the dendrites differs depending on the organism and the chemical it is responding to.
• 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."^[4]

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.

In Physiology

• 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.

Control of Breathing

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.

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 regulate the depth and rhythm of breathing 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 have also been shown experimentally to respond to hypercapnic hypoxia (elevated CO₂, decreased O2), and eventually desensitize . These are sensitive to pH and CO₂.
• peripheral chemoreceptors: consists of aortic and carotid bodies. 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.^[5]

See also

• Cell surface receptor
• Chemosensory clusters
• Chemoreceptor trigger zone
• Diffuse chemosensory system
• Haptic technology
• Molecular sensor
• Odor
• Olfaction
• Solitary chemosensory cells
• Sensory receptor
• Taste
• Vomeronasal organ

References

1. Shi, P.; Zhang, J. (2009). "Extraordinary Diversity of Chemosensory Receptor Gene Repertoires Among Vertebrates". Chemosensory Systems in Mammals, Fishes, and Insects. Results and Problems in Cell Differentiation 47. pp. 57–75. doi:10.1007/400_2008_4. ISBN 978-3-540-69918-7.
2. Chapman RF (1998) "Chemoreception" in The Insects: structure and function 4th edition. Cambridge University Press, Cambridge. 639.
3. http://www.ncbi.nlm.nih.gov/books/NBK55976/[]
4. Satir, Peter; Christensen, Søren T. (2008). "Structure and function of mammalian cilia". Histochemistry and Cell Biology 129 (6): 687–93. doi:10.1007/s00418-008-0416-9. PMC 2386530. PMID 18365235.
5. Levy, MN; AJ Pappano (2007). Cardiovascular Physiology 9ed. Philadelphia USA: Elsevier. pp. 89–91.

External links

• Chemoreceptors at the US National Library of Medicine Medical Subject Headings (MeSH)