Retseptorile

Retseptorile (receptors) are cellular proteins that bind ligands such as hormones, neurotransmitters, drugs, or sensory stimuli, allowing cells to perceive and respond to external and internal signals. They are located on the cell surface or inside the cell, and they translate chemical binding into a cellular response through signaling pathways or changes in gene expression.

There are several major classes: membrane receptors, including G protein–coupled receptors (GPCRs), receptor tyrosine kinases, and ligand-gated ion channels; and intracellular receptors, such as nuclear receptors that respond to lipophilic signals by modulating transcription. Sensory receptors are specialized types found in senses, including photoreceptors in the retina, olfactory receptors in the nose, and various mechanoreceptors.

Binding of a ligand to a receptor is often highly specific, and the strength of binding (affinity) and the number of receptors (density) influence cellular responsiveness. Receptors can be activated by agonists, which mimic the natural ligand, or blocked by antagonists. Some ligands act as inverse agonists or allosteric modulators that alter receptor activity in other ways. Receptor signaling can proceed through second messengers such as cAMP, IP3, or calcium ions, or through changes in kinase activity and gene transcription.

Receptor regulation includes desensitization, internalization, and up- or down-regulation in response to prolonged exposure to ligands. Understanding retseptorile is central to physiology, pharmacology, and medicine, as many drugs exert their effects by targeting specific receptors.