Alternative definition : KEGG: Ca2 that enters the cell from the outside is a principal source of signal Ca2
. Entry of Ca2 is driven by the presence of a large electrochemical gradient across
the plasma membrane. Cells use this external source of signal Ca2 by activating various
entry channels with widely different properties. The voltage-operated channels (VOCs)
are found in excitable cells and generate the rapid Ca2 fluxes that control fast
cellular processes. There are many other Ca2 -entry channels, such as the receptor-operated
channels (ROCs), for example the NMDA (N-methyl-D-aspartate) receptors (NMDARs) that
respond to glutamate. There also are second-messenger-operated channels (SMOCs) and
store-operated channels (SOCs).;