Cell Signaling Cell signaling requires both extracellular signaling molecules and a complementary set of receptor proteins in each cell. Signals such as neurotransmitters secreted by nerve cells act locally on the postsynaptic cells that their axons contact. Exerts centralized control.
Some signal molecules can cross the plasmamembrane.
They are small and hydrophobic. Cell surface receptors Fig. Three main classes of cell-surface receptors. 15-23 cAMP activates protein kinase A. Fig. Calcium is an important second messenger, which regulates a number of signaling cascades.