Chemoreception is a perceptual quality of the sense of smell and taste and registers chemical substances in the air via chemoreceptors. For example, the chemoreceptors measure the partial pressure of oxygen and initiate respiration to prevent hypoxia. In patients with MCS (a minimally conscious state), chemoreception is impaired.
What is chemoreception?
Chemoreception is a perceptual quality of the sense of smell and taste, registering chemical substances in the air via chemoreceptors. Together with interoception, exteroception makes up the entirety of the human perceptual system. In medicine, exteroception is the perception of external stimuli. Perception systems for this type of perception are the sense of sight, the sense of hearing, the sense of touch and the sense of smell and taste. The senses of smell and taste are closely interconnected and partially overlap in processing. Various perceptual qualities play a role for the senses of smell and taste. One of the most important perceptual qualities in this area is chemoreception. This is a physiological process that allows chemical signals from the environment to bind to chemoreceptors of the sense of smell and taste. The presence of a particular molecule, for example, can initiate the binding. The chemoreceptors convert the stimuli into an action potential and make them accessible to the central nervous system. Chemoreceptors are located in the mucous membranes of the nose as well as in the oral mucosa. They are specialized in the perception of airborne or liquid-dissolved chemical substances and, last but not least, regulate respiration. Humans possess about 320 different chemoreceptors. Animals with a keen sense of smell have chemoreceptors for more than 1000 different chemical molecules.
Function and task
Chemoreceptors protect humans from chemicals in the air and in liquids. They are also involved in regulating respiration, adjusting vascular tone, and regulating acid-base balance. Medicine distinguishes chemoreception into perception by central chemoreceptors and sensory input from peripheral chemoreceptors. Central chemoreceptors are located in the circulatory center of the brainstem (formatio reticularis) and measure the pH and the CO2 partial pressure of the cerebrospinal fluid. Peripheral chemoreceptors are located in the glomera carotica and glomera aortica. Their area of responsibility is sensitivity to protons of pH, to potassium, to partial pressure of O2, and to partial pressure of CO2. All peripheral chemoreceptors have a markedly high sensitivity to oxygen. When O2 partial pressure falls below the so-called O2 threshold of 110 mm Hg, they excite the afferent nerves of the respiratory center and regulate vital respiration to prevent hypoxia. Peripheral chemoreceptors are among the glomus cells and represent vascular nodules that receive blood through the side branches of adjacent arteries. This blood supply makes them some of the best perfused organs. In the context of peripheral chemoreceptors, type I and type II glomus cells are distinguished. These cells are located bilaterally at the dividing sequence of the common carotid artery and in the aorta of the glomus aorticum. From this area, they extend to the subclavian artery. Information about hypoxia travels as impulses to the vagus nerve and reaches the respiratory center via the glossopharyngeal nerve. In the respiratory center, breathing is initiated based on this information. In addition to the trigger zone of chemoreceptors at the base of the fourth ventricle within the area postrema, chemosensors are located in the mucosa of the gastrointestinal tract. These sensors play an important role in reflex vomiting. The sensors have sensitivity to substances such as bacterial toxins, emetine, higher concentrated saline, and copper sulfate. Thus, although chemosensors are primarily responsible for interoception in the sense of chemical sensing of liquids and gases in one’s own body, they also protect humans from consumption of certain substances from outside in the sense of exteroception.
Diseases and ailments
A special position among chemoreception-associated diseases is occupied by multiple chemical intolerance.This is a syndrome with strong intolerance to volatile chemicals such as fragrances, cigarette smoke, solvents or exhaust fumes. For a long time, it was debated whether the disease should be classified as psychosomatic or toxicological. According to recent studies, it is a multifactorial disorder with aspects of both fields. MCS sufferers are mainly affected by fatigue, exhaustion, concentration disorders, headaches and burning eyes. In addition, they are often plagued by a loss of memory, shortness of breath, dizziness or musculoskeletal complaints. Gastrointestinal complaints and dermatological problems may also occur. Several theories have been advanced regarding the suspected causes of the disease. One of these theories is that the intolerance is due to occupational or environmental contexts and may be subject to genetic involvement. Poisoning, faulty nervous and hormonal functions, or respiratory problems and lowering of the nervous trigger threshold are thought to play a causative role. Chemical triggers include solvents, pesticides, metals, and combustion products. Other theories assume an initial exposure to neurotoxic pollutants, which is accompanied by the non-specific symptoms of neurotoxic poisoning. After this initial exposure, the effects are reversible, but in the context of additional stressors or in susceptible individuals, the initial exposure may progress to a chronic form. A third theory assesses intolerance as a purely psychiatric disorder and relates it to depression, neurosis, or chemophobia. In addition to this disorder, discomfort or even a failure of chemoreception predominantly related to the conducting nerve pathways and the processing brain areas play a role. In the case of lesions in the involved areas of the central nervous system, severe dysregulation can occur, affecting, among other things, respiration and acid-base balance. As part of the autonomic nervous system, the respiratory center is less vulnerable to neurologic diseases, such as multiple sclerosis, than to stroke, ischemia, or related phenomena.
