Receptors are specialized protein molecules that help cells communicate and respond to signals in their environment, playing a critical role in how drugs, hormones, and neurotransmitters work in the body. These proteins are the primary target for many medications and are essential for various physiological functions.
What Are Receptors?
Receptors are cellular macromolecules, usually proteins, that bind to specific molecules called ligands (such as drugs, hormones, or neurotransmitters), leading to changes in cell function. When a ligand attaches to a receptor, the receptor activates or inhibits a biological response, acting like a lock that only fits a specific key (the ligand).
Basic Functions of Receptors
Recognition: Receptors recognize and bind only to specific ligands.
Transduction: Once the ligand binds, the receptor changes shape or activity, starting a chain reaction in the cell.
Response: The result can be fast (like muscle contraction) or slow (like changes in gene expression).
Classification of Receptors
Receptors can be classified based on their location, structure, or the type of signal they transmit. The most widely-used classification relates to how they work and their position in or on the cell.
1. Cell Surface (Membrane) Receptors
These receptors are embedded in the cell membrane and interact with ligands that are too large or polar to enter the cell.
G Protein-Coupled Receptors (GPCRs): The largest family, these receptors indirectly activate enzymes or channels inside the cell through G proteins. Examples: Beta-adrenergic receptors, dopamine receptors.
Ligand-Gated Ion Channels: These form pores in the cell membrane, which open or close when a ligand binds, allowing ions like sodium or calcium to enter or exit. Example: Nicotinic acetylcholine receptor.
Enzyme-Linked Receptors: These are linked to enzymes such as kinases. When activated, they initiate cascades that affect growth, metabolism, or cell function. Example: Insulin receptor.
2. Intracellular (Nuclear) Receptors
These receptors are found inside the cytoplasm or nucleus. Only ligands that can cross the cell membrane (like steroid hormones) can activate them.
Type I Nuclear Receptors: Located in the cytoplasm, move to the nucleus after ligand binding (e.g., glucocorticoid receptor).
Type II Nuclear Receptors: Always in the nucleus, activate gene transcription directly (e.g., thyroid hormone receptor).
3. Sensory Receptors
These are specialized cells or proteins that respond to environmental stimuli:
Mechanoreceptors: Respond to mechanical pressure or stretch (e.g., skin touch receptors).
Thermoreceptors: Detect temperature changes.
Chemoreceptors: Respond to chemical stimuli (e.g., taste buds, olfactory cells).
Photoreceptors: Respond to light (e.g., rods and cones in the eye).
Nociceptors: Sense pain.
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