Second Messengers

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Second messengers are small molecules and ions that relay signals received by cell-surface receptors to effector proteins.
Second messengers are produced in response to extracellular stimuli, and also in response to stimuli from within the cell.
They are typically present at low concentrations in resting cells and can be rapidly produced or released when cells are stimulated.
The first second messenger, which was discovered in 1957, was cyclic adenosine monophosphate, or cAMP.
Second messengers fall into four classes: cyclic nucleotides, lipids, ions, and gases and free radicals.
They have diverse properties that allow them to signal within membranes, within the cytosol or between the two...