Essential for body function, environmental sensing, and response.
Understanding it is crucial because:
Disorders can lead to diseases (e.g., nervous and endocrine system).
Most drugs act by modifying cell-to-cell communication.
Local Signaling:
Paracrine: signaling molecule acts on nearby cells (e.g., immune responses).
Synaptic: communication between neurons (e.g., thoughts and memories).
Long Distance Signaling:
Endocrine: hormone released into bloodstream, travels to target cell (e.g., insulin).
Reception: signaling molecule binds to receptor.
Transduction: signaling pathway initiated.
Response: cellular response activated (e.g., enzyme activation).
Proteins that bind to signaling molecules (ligands/agonists) to elicit a response.
Hydrophilic molecule receptors are on the cell surface.
Hydrophobic molecule receptors are intracellular.
Transduction pathway and response depend on receptor type and cell type.
Ligand-gated ion channels
G protein-coupled receptors
Tyrosine kinase receptors
Steroid receptors
Ligand-gated ion channels:
Fast, direct neurotransmission.
Ions flow into/out of cell, changing membrane voltage (IPSPs and EPSPs).
G protein-coupled receptors:
Slower, indirect neurotransmission.
Activation modifies depolarization or hyperpolarization.
Large family; many drug targets.
Activation causes a cascade of effects.
Phosphorylation (addition of phosphate groups by kinases) controls protein activity.
Dephosphorylation (removal of phosphate groups by phosphatases) also controls protein activity.
Amplification enables pronounced cellular response from low neurotransmitter concentrations.
Effect depends on receptors expressed by the postsynaptic cell.
Membrane receptors responding to hormones.
Important for metabolism and growth.
Receptor dimerization leads to autophosphorylation and activation of other proteins.
Often involves phosphorylation cascades leading to gene transcription.
Intracellular receptors for lipophilic hormones.
Translocation to nucleus, triggers gene transcription (slow process).
Cell Communication & Receptor Families