Second messenger system

An intracellular signaling pathway in which small molecules or ions relay and amplify signals from activated cell-surface receptors to cellular targets.

G protein-coupled receptor: A cell-surface receptor that activates intracellular heterotrimeric G proteins in response to an external signal. Many second-messenger pathways begin when these receptors activate G proteins.
Cell signaling: The processes by which cells detect signals and produce responses. Second-messenger systems are intracellular routes within the broader process of cell signaling.
Hormone: A signaling molecule produced by an organism that regulates activity in target cells. Many hormones trigger second-messenger pathways after binding cell-surface receptors.
Receptor tyrosine kinase: A cell-surface receptor that activates its own tyrosine kinase activity after ligand binding. These receptors often signal through protein interactions and phosphorylation rather than classic diffusible messengers.
Calcium signaling: The use of changes in intracellular calcium concentration to regulate cellular processes. Calcium acts in diverse pathways, raising the question of how cells encode distinct responses.
Adenylyl cyclase: An enzyme that converts ATP into cyclic AMP. Activated G proteins regulate this enzyme to control cyclic AMP production.
Cell-surface receptor: A membrane protein that detects extracellular molecules and initiates intracellular signaling. These pathways commonly relay signals that begin at receptors embedded in the plasma membrane.
Epinephrine: A hormone and neurotransmitter that helps mobilize rapid physiological responses to stress. Its receptors activate cyclic AMP signaling in tissues such as liver and heart.
Notch signaling pathway: A juxtacrine pathway in which ligand binding triggers cleavage of Notch and release of its intracellular domain. Notch signaling transfers a receptor fragment into the nucleus instead of relying on a diffusible second messenger.
Calcium wave: A propagating increase in intracellular calcium concentration across a cell or tissue. Waves show how the timing and spatial spread of a messenger can carry information.

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