Capacitive currents
Movement of charges onto (and away from) capacitor plates such as the inside and outside of the membrane is referred to as a current flow "through" the capacitor. In electrophysiology it is important to be aware that such currents flow ONLY when the voltage across a capacitor is changing with respect to time (the capacitor is being "charged").
In voltage clamp experiments, for example, no capacitive current flows when the voltage level is steady. If the rate of change of the voltage is zero, then the capacitive current, Icap, is zero and only ionic current flows. The voltage step charges the membrane during the transition and then the steady new level holds the charge on the capacitor (the membrane). This is one of the principle reasons Hodgkin and Huxley developed the voltage clamp: to eliminate capacitative currents, and allow the measurement of the ionic currents which underlie both the resting potential and the action potential.
Capacitance (C) = charge (Q) / volts (V).
The size of a capacitor (C) is specified in terms of the ratio of the charge
it holds (Q) to the voltage across it (V). The unit of capacitance (C) is the
farad (F).
Units?
Charge (Q) is measured in coulombs (a current of 1 amp will move one coulomb
of charge per second). A farad (F) is 1 coulomb per volt. That is, if the capacitor
had a capacitance of 1 farad, each coulomb of charge applied to the membrane
would put one volt across it. With respect to the membrane, these numbers are
ridiculously large (see below). And what is a volt? A volt is a unit of electromotive
force, and is equal to one ampere times one ohm; in other words, a battery which
produced one ampere of current across a one ohm resistor would be 1 volt.
In a membrane, capacitance is expressed in units of µF / cm^2.
The larger the area of membrane, the more charge it can hold, and thus the greater
its total capacitance. Most biological membranes have a capacitance of about
1 µF/cm^2.
Capacitive current (Icap) = C * dV/dt.
The current flow onto a capacitor equals the product of the capacitance and
the rate of change of the voltage. (For those not inclined to take our word
for it, the simple derivation of this equation is
provided). If a constant current is injected across a lipid bilayer, the steady
current flow (Icap) will produce a constant rate of change (dV/dt) of
the voltage across the capacitance (C) of the membrane, as you just saw
in your simulation. This equation also shows again that whenever the voltage
is constant (dV/dT=0), there is no capacitance current.