Receptors

Receptors are biological transducers that convert energy from both the outside and inside environments into electrical impulses. They can be massed together 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 connect receptors to the central nervous system. The receptive field is the region or area in the periphery from which a neuron in the central nervous system receives input. 

Types of Receptors

Receptors come in a variety of shapes and 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, remains constant. Changing-state receptors, on the other hand, respond to changes in a stimulus's intensity or position.

Exteroceptive (reporting the external environment), interoceptive (sampling the environment of the body itself), and proprioceptive receptors are also classified (sensing the posture and movements of the body).

Proprioceptors report the position and motion of body parts as well as the body's position in space.

Receptors are also classified based on the types of stimuli to which they respond. 

Chemoreceptors are sensitive to substances taken into the mouth (taste or gustatory receptors), inhaled through the nose (smell or olfactory receptors), or found in the body itself (detectors of glucose or 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.

Internal Receptors

Internal 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 act as mRNA synthesis regulators to mediate gene expression once inside the cell. 

The cellular process of converting the information in a cell's DNA into a sequence of amino acids that eventually forms a protein is known as gene expression. When the ligand binds to the internal receptor, a conformational change on the protein exposes a DNA-binding site.

The ligand-receptor complex enters the nucleus, binds to specific regulatory regions of chromosomal DNA, and stimulates transcription. Internal receptors have the ability to directly influence gene expression without requiring the signal to be passed on to other receptors or messengers.

Cell-Surface Receptors

Cell-surface receptors, also known as transmembrane receptors, are proteins that bind to external ligand molecules on the cell surface, membrane, or within the cell. This type of receptor spans the plasma membrane and converts an extracellular signal into an intracellular signal. Ligands that interact with cell-surface receptors do not need to enter the cell. Because they are specific to individual cell types, cell-surface receptors are also known as cell-specific proteins or markers.

Each Cell-Surface Receptor is Made up of Three Major Parts: 

• an external ligand-binding domain (extracellular domain), 

• a hydrophobic membrane-spanning region, and 

• an intracellular domain within the cell. 

Depending on the type of receptor, the size and extent of each of these domains vary greatly. The majority of signaling in multicellular organisms is mediated by cell-surface receptors. 

Cell-Surface Receptors are Classified Into Three Types: 

1. Ion Channel-Linked Receptors: 

Ion channel-linked receptors bind to a ligand and open a channel through the membrane, allowing specific ions to pass through. This type of cell-surface receptor has a large membrane-spanning region that allows it to form a channel. Many of the amino acids in the membrane-spanning region are hydrophobic in order to interact with the phospholipid fatty acid tails that form the centre of the plasma membrane. The amino acids that line the inside of the channel, on the other hand, are hydrophilic, allowing water or ions to pass through.

2. G-Protein-Linked Receptors:

G-protein-linked receptors bind a ligand and activate a G-protein, which is a membrane protein. The activated G-protein then interacts with a membrane ion channel or enzyme. Although all G-protein-linked receptors have seven transmembrane domains, each receptor has a unique extracellular domain and G-protein-binding site.

Cell signalling via G-protein-linked receptors is a cyclical process. Before the ligand binds, the inactive G-protein can bind to a newly discovered binding site on the receptor. When the G-protein binds to the receptor, the resulting shape change activates the G-protein, causing GDP to be released and GTP to be picked up. The G-protein subunits were then divided into the subunit and the subunit. As a result, one or both of these G-protein fragments may be able to activate other proteins. Later, the GTP on the active G-protein subunit is hydrolyzed to GDP, and the subunit is deactivated. The subunits recombine to form the inactive G-protein, and the cycle begins again.

3. Enzyme-Linked Receptors:

Enzyme-linked receptors are cell-surface receptors that have intracellular domains linked to an enzyme. In some cases, the receptor's intracellular domain is an enzyme, or the enzyme-linked receptor has an intracellular domain that directly interacts with an enzyme. 

Usually, enzyme-linked receptors have large intracellular and extracellular domains, but the membrane-spanning region is made up of a single alpha-helical peptide strand. 

When a ligand binds to the extracellular domain, a signal is transmitted across the membrane and activates the enzyme, triggering a series of events within the cell that eventually results in a response.

Receptors in the Immune System

T Cell Receptors:

T cell receptor (TCR) is a protein complex found on the surface of T cells, also known as T lymphocytes, that recognises antigen fragments as peptides bound to major histocompatibility complex (MHC) molecules. 

TCR-antigen peptide-binding has a low affinity and is degenerate, which means that many TCRs recognise the same antigen peptide and many antigen peptides are recognised by the same TCR.

The TCR is made up of two distinct protein chains (that is, it is a heterodimer). In humans, 95% of T cells have an alpha (α) chain and a beta (β) chain (encoded by TRA and TRB, respectively), while 5% of T cells have gamma and delta (δ) chains (encoded by TRG and TRD, respectively). This ratio shifts during development and in diseased states (such as leukaemia). It also varies by species. 

Orthologues of the four loci have been found in a variety of species. Each locus can generate a wide range of polypeptides with both constant and variable regions.

When the TCR interacts with an antigenic peptide and MHC (peptide/MHC), the T lymphocyte is activated via signal transduction, a series of biochemical events mediated by associated enzymes, co-receptors, specialised adaptor molecules, and activated or released transcription factors. 

The TCR belongs to the Non-catalytic tyrosine-phosphorylated receptors family based on the initial receptor triggering mechanism (NTRs).

B Cell Receptors:

A transmembrane protein on the surface of a B cell is known as the B cell receptor (BCR). B cell receptors are made up of immunoglobulin molecules that combine to form a type 1 transmembrane receptor protein and are typically found on the surface of lymphocyte cells. 

The BCR regulates B cell activation via biochemical signaling and physically acquiring antigens from immune synapses. B cells can gather and grab antigens by utilising biochemical modules for receptor clustering, cell spreading, the generation of pulling forces, and receptor transport, which ultimately leads to endocytosis and antigen presentation.

The mechanical activity of B cells follows a pattern of negative and positive feedback that regulates the amount of removed antigen by directly manipulating the dynamic of BCR–antigen bonds. Grouping and spreading, in particular, increase the antigen-BCR relationship, resulting in sensitivity and amplification. Pulling forces, on the other hand, delink the antigen from the BCR, thereby testing the quality of antigen-binding.

The binding moiety of the receptor is made up of a membrane-bound antibody that, like all antibodies, has two identical paratopes that are unique and determined at random. The BCR for an antigen is a vital sensor required for B cell activation, survival, and development.

When a B cell encounters an antigen that binds to its receptor (its "cognate antigen"), it proliferates and differentiates to produce a population of antibody-secreting plasma B cells and memory B cells. When the B cell receptor (BCR) interacts with the antigen, it performs two critical functions. 

Signal transduction is one function that involves changes in receptor oligomerization.