Why are calcium channels anchored within nanometres of the vesicles they trigger? · Grey Matter

Calcium entering through an open channel is buffered and pumped away so fast that its concentration is high only in a tiny cloud around the channel mouth, tens of nanometres across, for well under a millisecond.

Why are calcium channels anchored within nanometres of the vesicles they trigger?

Calcium entering through an open channel is buffered and pumped away so fast that its concentration is high only in a tiny cloud around the channel mouth, tens of nanometres across, for well under a millisecond. A vesicle docked inside that cloud sees enough calcium to bind its sensor and fuse almost at once; one docked farther away sees too little, too late. Tethering proteins at the active zone therefore hold the channels next to the docked vesicles, which is what lets fusion follow calcium entry by about 60 microseconds at body temperature. The arrangement also keeps release tied to the spike, since the cloud vanishes as soon as the channels close.