Receptor Definition – Receptors are known to be biological transducers that convert energy from both the outside and inside environments into electrical impulses. They can congregate to form a sense organ, such as the eye or ear, or they can be dispersed, as in the skin and viscera.
Afferent nerve fibres transmit signals from receptors to the central nervous system. The receptive field is a region or area in the peripheral nervous system from which a neuron in the central nervous system receives input. Receptive fields are not static entities, but rather fluid entities.
Types of Receptors
Receptors come in a variety of shapes as well as sizes, and they are classified in a variety of ways. For example, steady-state receptors generate impulses as long as a specific state, such as temperature, which generally remains constant. On the other hand, changing-state receptors, respond to changes in a stimulus’s intensity or position. Exteroceptive (which means reporting the external environment), interoceptive (which means sampling the environment of the body itself), and proprioceptive receptors are also classified (sensing the posture and movements of the body). Exteroceptors transmit information from the senses of sight, hearing, smell, taste, and touch. Interoceptors transmit information about the bladder, the alimentary canal, blood pressure, and the osmotic pressure of blood plasma. Proprioceptors report the position and movement of body parts as well as the body’s position in space.
Nerve ending receptors can also be classified based on the types of stimuli to which they respond. Chemoreceptors are sensitive to substances taken into the mouth (generally which are taste or gustatory receptors), inhaled through the nose (smell or olfactory receptors), or found in the body itself (detectors of glucose or of acid-base balance in the blood). The skin’s receptors are classified as thermoreceptors, mechanoreceptors, and nociceptors, with the latter being sensitive to noxious or potentially damaging stimulation to the body’s tissues.
There are basically two types of thermoreceptors: warm and cold. Warmth fibres are stimulated by rising temperatures and inhibited by falling temperatures, whereas cold fibres respond in the opposite direction.
Structure of Receptors
A portion of the receptor protrudes from the cell membrane. The same is true for cell organelle membranes. The primary function of a receptor is to recognise and respond to a specific ligand, such as a neurotransmitter or hormone. Some receptors are activated by changes in the ‘transmembrane potential’ (the difference in electric potential between the inside and the outside of a cell).
The middle section, located within the membrane itself, is a protein-lined pore through the membrane known as an ‘ion channel.’ When the ligand binds to the surface, the pore opens up and allows ions to pass through.
In other cases, when there are differences in electric potential, the receptor changes shape, causing changes inside the cell.
The receptor’s interior (or cytoplasmic) part interacts with the interior of the cell or organelle. There are several types of receptors, each of which acts differently.
Enzyme-Linked Receptors
Enzyme-linked receptors are known as cell-surface receptors with intracellular domains that are associated with an enzyme. In various cases, the intracellular domain of the receptor itself is an enzyme or the enzyme-linked receptor has an intracellular domain that interacts directly with an enzyme. The enzyme-linked receptors normally have large extracellular as well as intracellular domains, but the membrane-spanning region consists of a single alpha-helical region of the peptide strand.
When a ligand binds to the extracellular domain, a signal is sent across the membrane, activating the enzyme and initiating a chain of events within the cell that eventually results in a response. An example of an enzyme-linked receptor type is the tyrosine kinase receptor. Phosphate groups are transferred to tyrosine molecules by the tyrosine kinase receptor. Signal molecules bind to the extracellular domains of two nearby tyrosine kinase receptors, causing them to dimerize. Phosphates are then added to tyrosine residues on the intracellular domain of the receptors, allowing the signal to be transmitted to the next messenger in the cytoplasm.
Internal Neural Receptors
Internal neural receptors, also known as intracellular or cytoplasmic receptors, are found in the cell’s cytoplasm and respond to hydrophobic ligand molecules that can cross the plasma membrane. Many of these molecules bind to proteins that generally act as mRNA synthesis regulators once inside the cell. Remember that mRNA transports genetic information from the nucleus of a cell’s DNA to the ribosome, where the protein is assembled.
When the ligand in an internal neural receptor binds to the internal receptor, a shape change occurs, exposing a DNA-binding site on the receptor protein. The ligand-receptor complex enters the nucleus, binds to specific regions of DNA, and promotes mRNA production from specific genes. Internal receptors have the ability to directly influence gene expression (how much of a specific protein is produced from a gene) without requiring the signal to be passed on to other receptors or messengers.
Ion Channel-Linked Receptors
Ion channel-linked receptors bind to a ligand as well open a channel through the membrane, allowing specific ions to pass through. The ion channel-linked type of cell-surface receptor has a large membrane-spanning region that allows it to form a channel. When a ligand binds to the extracellular region of the channel, the protein’s structure changes, allowing ions such as sodium, calcium, magnesium, and hydrogen to pass through.
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