Neurons and muscle cells have electric potentials across the cell membrane that are held by ion gradients. A depolarization can carry information along the cell membrane in the form of an "action potential", e.g. to excite the contraction of a muscle cell. (Place mouse over Figure 1 for a demonstration.)
To understand the function of exciteable cells, we measure voltage and electric currents on a variety of scales from the entire cell down to the activity of single ion channels. An interesting field of research is to study the influence of various drugs on these electrical properties.