Depolarization-evoked opening of Ca V 2.1 (P/Q-type) Ca 2+ -channels triggers neurotransmitter release, while voltage-dependent inactivation (VDI) limits channel availability to open, contributing to synaptic plasticity. The mechanism of Ca V 2.1 response to voltage is unclear. Using voltage-clamp fluorometry and kinetic modeling, we optically tracked and physically characterized the structural dynamics of the four Ca V 2.1 voltage-sensor domains (VSDs). VSD-I seems to directly drive opening and convert between two modes of function, associated with VDI. VSD-II is apparently voltage-insensitive. VSD-III and VSD-IV sense more negative voltages and undergo voltage-dependent conversion uncorrelated with VDI. Auxiliary β-subunits regulate VSD-I-to-pore coupling and VSD conversion kinetics. Ca V 2.1 VSDs are differentially sensitive to voltage changes brief and long-lived. Specifically the voltage-dependent conformational changes of VSD-I are linked to synaptic release and plasticity.