Physiology, Cellular Receptors

The ability of cells to communicate is crucial for maintaining cell function and homeostasis. Cells communicate through gap junctions, juxtacrine signaling, or secreted chemical messengers. Gap junctions enable the direct transfer of signaling molecules between the cytoplasm of 2 connected cells. Juxtacrine signaling involves a membrane-bound chemical messenger that directly interacts with a receptor on the plasma membrane of an adjacent cell.

Secreted chemical messengers are released from the originating cell and are classified into the following 4 types:

1. Paracrine signaling molecules: target neighboring cells close to the release site.

2. Autocrine signaling molecules: target the same cell that secreted them.

3. Hormones: are released by endocrine cells and reach target cells in one or more distant body locations via blood circulation.

4. Neurotransmitters: are released by neurons and affect other neurons or effector cells near the release site.

Signal transduction begins when a chemical messenger, acting as a ligand, binds to a specific cellular receptor on the target cell. This binding induces a conformational change in the receptor, leading to its activation. Depending on the receptor type, a sequence of signaling events occurs, leading to specific responses such as altered gene expression, changes in cell morphology, modulation of proliferation or growth rate, and adjustments in metabolism.

Although cellular receptors primarily bind to endogenous ligands, 2 notable exceptions exist—pathogenic viruses, which can bind to host cellular receptors to infect a cell, and bacterial components, which can bind to receptors on immune cells to initiate an immune response.

Cellular receptors can be either intracellular or cell surface proteins. Intracellular receptors that located in the cytoplasm or nucleus of target cells bind lipid-soluble chemical messengers. Cell surface receptors are transmembrane proteins that bind water-soluble chemical messengers.