The term chemoreception refers to an organism’s ability to perceive and react to different chemicals. It is comprised of a sensory system that is activated by exposure to chemicals. Special sensory receptors called chemoreceptors are stimulated by chemicals and relay signals to the nervous system. The nervous system will then respond accordingly. This mechanism is important for survival because these receptors enhance the organism’s perceptions of its surrounding.
Olfaction and gustation are the systems most often discussed when referring to chemoreception. Both sensory systems utilize chemoreceptors. Olfaction refers to the ability to smell, and gustation refers to the ability to taste. Both systems utilize chemical interaction to create signals for the organism to process. Chemoreceptors are made up of specialized cells called neurons, which enable them to convert chemical signals into action potential. These signals are then relayed to the central nervous system.
Anatomy and Physiology
Different organisms have varying chemoreception systems. The scientific community believes that all organisms have some level of chemoreception, and many vertebrates have different classes of chemoreception (Starenchak & Bissonnette, 2014). Chemoreception is controlled by the autonomic nervous system. In vertebrates, chemoreceptors function as pH balance sensors to maintain homeostasis (respiratory chemoreceptors), detect vapors (olfactory chemoreceptors), and taste potential food (gustatory chemoreceptors). The respiratory chemoreceptors are made up of the central and peripheral chemoreceptors. In vertebrates, the central chemoreceptors are located on the medulla oblongata of the brain. Periphery chemoreceptors, also known as arterial receptors, are located on the carotid and aortic bodies. The carotid body can be found on the bifurcation of the carotid artery of the heart and the aortic body by the aortic arch. Olfactory chemoreceptors are found at the top of the nasal passages. The synapses of these nerves are ultimately connected to the olfactory cortex of the brain. Gustatory chemoreceptors are found in the mouth and are commonly called taste buds. However, these chemoreceptors are found on taste cells that are located within taste buds.
Classes of Chemoreceptors
Chemoreceptors are divided into two classes: indirect chemoreceptors and direct chemoreceptors (Starenchak & Bissonnette, 2014). Indirect chemoreceptors work from a distance. Only a low concentration of chemical is necessary for these chemoreceptors to convert the stimulus into an action potential. The olfactory system is an example of a sensory system that utilizes indirect chemoreceptors. The olfaction system can detect odors and associate them to a source that may be located quite far away. Direct chemoreceptors involve many of the chemical compounds making actual contact with the neurons. There needs to be a high enough concentration of the chemical to trigger a cascade of action potentials all the way to the central nervous system. More stimuli will result in more frequent and rapid action potentials. An example of a direct chemoreceptor is the gustation system. Chemicals must physically touch the taste cells within the taste buds in order for the brain to interpret taste. The more flavor something has, the stronger the taste. Respiratory chemoreceptors also fall under the class of direct chemoreceptors because these receptors maintain homeostatic pH levels by directly detecting hydrogen ions in the blood.
Diseases and Defects
A defect in chemoreceptors can impair an organism’s ability to sense its environment. This may leave the organism vulnerable to predators, especially those who rely heavily on their olfactory senses. Impaired olfactory chemoreceptors are also known to affect appetite. Defects in gustatory chemoreceptors are presented in many forms including reduced ability to taste (hypogeusia), phantom taste perception (parageusia), and even complete loss of tasting ability (ageusia). The respiratory chemoreceptors are arguably the most vital for survival. Diseases such as coronary heart disease and chronic obstructive pulmonary disorder (COPD) have been shown to affect these receptors (van Gestel & Steier, 2010). COPD may develop following chronic inhalation of toxic smoke, which impairs the body’s ability to expel carbon dioxide. Aortic bodies are thought to provide a compensatory mechanism in these patients, but research in this field is currently in its infancy.
See also: Ageusia; Hypogeusia; Taste Bud; Taste System
Starenchak, Holly, & Nicole Bissonnette. (2014). Developmental and comparative aspects of chemoreception in the vertebrates. Anatomy, Phylogeny, and Ontogeny of Chemoreceptors. Retrieved from https://sites.google.com/a/ncsu.edu/anatomy-phylogeny-and-ontogeny-of-chemoreceptors/
Sundin, Lena, Mark L. Burleson, Adriana P. Sanchez, Jalile Amin-Naves, Richard Kinkead, Luciane H. Gargaglioni, … Mogens L. Glass. (2007). Respiratory chemoreceptor function in vertebrates: Comparative and evolutionary aspects. Integrative and Comparative Biology, 47(4), 592—600. Retrieved from http://icb.oxfordjournals.org/content/47/4/592.full
van Gestel, Arnoldus J. R., & Joerg Steier. (2010). Autonomic dysfunction in patients with chronic obstructive pulmonary disease (COPD). Journal of Thoracic Disease, 2(4), 215—222. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3256465/