FIGURE 4-5 Oscilloscope Recording (A) Basic wave shapes are displayed on a digital oscilloscope, a versatile electronic instrument used to visualize and measure electrical signals as they change. (B) On the graph of a trace produced by an oscilloscope, S stands for stimulation. The horizontal axis measures time, and the vertical axis measures voltage. By convention, the axon voltage is represented as negative, in millivolts (mV). On the right, one trace of two action potentials from an individual neuron as displayed on a digital oscilloscope screen.