Do you see the white triangular thing to the right of the figure? That is a micropipette that records the electrical activity from inside the cell. I do that in the laboratory to measure the chloride response of a cell aka poke and prod the cell until it dies. Also, I get to play God! I hold the cell at any voltage I like and see how it behaves at that voltage.
Okay now don’t think I am a sadist or a narcissist … I just described an important electrophysiological technique – The whole cell patch clamping!