Membrane capacitance ($C_m$)

Membrane capacitance is the capacitance associated with separated electric charge across cell membrane. A local ideal capacitor has charging electric current $I_C=C_{m}dV/dt$. If a known actual membrane voltage change is $\Delta V$, integrating the charging transient after subtracting sustained ionic and leak components gives $C_m=\Delta V^{-1}\int I_{C}dt$. Pipette capacitance, finite patch-clamp series resistance and imperfect transient subtraction can bias the estimate. A capacitive transient is not by itself an ion channel opening.

When a single ion species supplies the net charging of a cell membrane, its bulk concentration increment is $\Delta c=C_{m}A\Delta V/(zF\mathcal{V})$. Opposing ionic Electric currents can make gross entry exceed this minimum net electric charge budget. A large membrane potential change can coexist with approximate bulk electroneutrality.