An overview of cellular signaling, including the types of signaling molecules involved, the importance of cell communication, and the general mechanisms of signal transduction within cells.
Introduction to Cellular Signaling
Fundamentals, components, and major pathways
| Term | Definition |
|---|---|
| Ligand | Extracellular molecule (hormone, growth factor, neurotransmitter, cytokine, etc.) that initiates a signal. |
| Receptor | Protein (membrane‑bound or intracellular) that specifically binds a ligand and converts the binding event into a cellular response. |
| Signal Transduction | Sequence of molecular events that convey the signal from the receptor to downstream effectors. |
| Second Messenger | Small intracellular molecules (cAMP, Ca²⁺, IP₃, DAG) that amplify and propagate the signal. |
| Effector | Protein or enzyme that executes the final cellular response (e.g., kinase, transcription factor). |
| Feedback | Positive or negative regulation that modulates signal strength/duration. |
| Cross‑talk | Interaction between distinct signaling pathways, allowing integration of multiple cues. |
| Category | Location | Typical Ligand | Signal‑Transmission Mechanism |
|---|---|---|---|
| Ion‑channel (Ligand‑gated) | Plasma membrane | Neurotransmitters (e.g., GABA, acetylcholine) | Ligand binding opens channel → ion flux (Na⁺, Cl⁻, Ca²⁺) → rapid depolarization/hyperpolarization |
| G‑protein‑coupled receptors (GPCRs) | Plasma membrane | Hormones, photons, odorants | Ligand → conformational change → G‑protein activation → downstream effectors (adenylyl cyclase, PLC, ion channels) |
| Receptor tyrosine kinases (RTKs) | Plasma membrane | Growth factors (EGF, insulin) | Dimerization → autophosphorylation → recruitment of adaptor proteins → MAPK, PI3K pathways |
| Serine/Threonine kinase receptors | Plasma membrane | TGF‑β family | Ligand‑induced heteromeric complex → serine/threonine phosphorylation → SMAD signaling |
| Intracellular (Nuclear) receptors | Cytoplasm or nucleus | Steroid hormones, thyroid hormone, retinoic acid | Ligand diffuses in → receptor‑ligand complex binds DNA → transcriptional regulation |
| Cytokine receptors (non‑kinase) | Plasma membrane | Interleukins, interferons |