As the membrane capacitance becomes more and more charged, an increasing fraction of the injected current flows across the membrane resistance.

Thus, the amplitude of the initial fast current is entirely determined by the size of the voltage step and the electrode resistance (which is defined as the sum of the resistance of the electrode and the resistance of the electrode’s connection to the cell). How do the properties of the electrode and cell membrane influence the shape of the current curve (see Figure 1B)? Initially, the entire current charges the membrane capacitance with no current flowing across the membrane resistance. Controlling the membrane voltage and measuring the resulting current in this way constitutes a basic voltage clamp experiment. Contrary, when the voltage step is reversed, we observe a large instantaneous current of the opposite direction that decreases exponentially until it reaches steady state again. This current then decreases exponentially (falling flank) to reach a steady state. Initially, a square shaped voltage step leads to an instantaneous jump in current (the initial peak). To understand how the RC filter properties of the membrane determine a cell’s voltage response, consider how a voltage step applied to the inside of a cell alters the current injected through an electrode (see Figure 1B).

Solve elec setup