Receptors receive stimuli and signals from the environment and transmit them for processing. In biochemistry, certain biomolecules and in physiology, sensory cells act as receptors.
What are receptors?
In the broadest sense, a receptor is a signaling device that responds to specific influences. Thus, both biochemistry and physiology refer to receptors. In biochemistry, they are proteins or protein complexes that can bind signaling molecules. Each biochemical receptor can bind only one molecule according to the lock-and-key principle. It has exactly the functional group that is the right fit for the receiving molecule. The receptors therefore already exist for a large number of possible signals. Whether they react now depends on the presence of the appropriate signal molecule. In physiology, sensory cells are considered receptors. In the meantime, however, the concept of receptors is undergoing change. Today, sensory receptors are also referred to as sensors. These are in turn divided into primary and secondary sensory cells. While primary sensory cells form action potentials, secondary sensory cells only receive the signals. In sensors, signal reception is also triggered by biochemical receptors.
Anatomy and structure
Biochemical receptors are located either on the surface of biomembranes or in the cytoplasm or nucleus, respectively. The membrane receptors are proteins that are chemically modified and can bind signaling molecules. Each receptor can bind only one specific signal molecule. When this binding occurs, electrical or chemical processes are triggered, causing a response from the cell, tissue, or the entire body. Membrane receptors are divided into ionotropic and metabotropic receptors according to their mode of action. Ionotropic receptors represent ion channels that open upon binding to ligands, resulting in the alteration of membrane electrical conductivity. Metabotropic receptors cause concentration changes of secondary messengers. Intracellular nuclear receptors bind in the cytoplasm or in the nucleus as signal molecules, for example steroid hormones, and in this way control the expression of genes in the cell nucleus. In this way, they mediate certain hormone responses. In physiology, as already mentioned, sensory cells are called receptors. There are different types of receptors such as baroreceptors (for pressure stimulus), chemoreceptors, photoreceptors, thermoreceptors, pain receptors, or proprioceptors.
Function and tasks
In general, receptors have the function of receiving and transmitting signals or stimuli. Receptor molecules operate on the lock-and-key principle, with a separate receptor for each signal molecule. Ligand binding either generates and transmits electrical signals or induces intracellular signaling cascades through changes in the concentration of messenger molecules. Nuclear receptors mediate hormonal responses through gene activation, for example. Sensory cells also receive physical or chemical signals via biochemical receptors. Nevertheless, they are also called receptors or sensors in parallel. In this context, the different types of sensory cells perform different tasks. For example, the chemoreceptors are responsible for the perception of taste and odor impressions. Furthermore, they regulate respiration by measuring the concentrations of oxygen, carbon dioxide and hydrogen ions. Baroreceptors constantly register arterial and venous blood pressure and transmit the values to the brain. Thus, they are responsible for the proper functioning of the cardiovascular system. Photoreceptors receive light stimuli and play a significant role in the visual process. Thermoreceptors serve to perceive temperature and temperature change. Thus, there are special receptors for heat or for cold. Some thermoreceptors also regulate the homeostasis of body temperature. Special receptors, such as proprioceptors (muscle spindles), sense the length of skeletal muscles, for example.
Diseases
Several diseases are directly caused by malfunctioning receptors. For example, when there is dysfunction in the mechanoreceptors of the cervical spine, dizziness and nausea result. Diseases of the cervical spine are not so rare.In addition to dizziness, such symptoms as hearing loss, tinnitus, visual disturbances, concentration disorders and other sensory disturbances also occur. Other diseases such as cardiac arrhythmias, angina pectoris, gastrointestinal disorders, bladder disorders or bronchial asthma can also develop on the basis of receptor disorders. Type II diabetes develops as part of the metabolic syndrome. Insulin resistance can develop as a result of certain metabolic processes. In insulin resistance, sufficient insulin is still produced, but the insulin receptor no longer responds properly. The effectiveness of the insulin decreases. Therefore, the pancreas is animated to produce even more insulin. This can lead to its complete exhaustion. The diabetes becomes manifest. Many mental illnesses are caused by disturbances in the transmission of stimuli. Here, so-called neurotransmitters act as biochemical messengers. These neurotransmitters pass on their information by binding to receptors. If the receptors are blocked by other substances or if they do not function properly for other reasons, significant mental disorders can result. Some psychotropic drugs target the receptors directly in their mode of action. Some mimic the function of the neurotransmitter and bind to the appropriate receptor. Other psychotropic drugs are used to block receptors for physiological neurotransmitters in the presence of increased psychological irritability. Therefore, there are always side effects when taking these drugs, which lead to the limitation of performance. Furthermore, there are also some genetic diseases related to receptors. Thus, more and more receptor mutations are discovered, which can lead to their ineffectiveness. On the other hand, autoimmune diseases are also known to target receptors. A well-known example is the autoimmune disorder myasthenia gravis, where signal transmission between nerve and muscle is disturbed.
