You can measure the voltage of a neuron by using electrodes. First, place the tip of one electrode inside the neuron. Then, measure the voltage with respect to a reference electrode outside the neuron. This voltage can be displayed on a graph of millivolts (y-axis) versus time (x-axis).

Whenever the voltage is not changing, it will appear as a flat line in the display. Before you place an electrode inside a neuron, the display will read 0 mV (because both electrodes are outside the neuron). Once you place an electrode inside a neuron, the display will switch to a steady negative voltage. This voltage is the neuron's resting potential.

When a neuron generates an action potential, it will appear as a spike on the display. Action potentials like these are extremely fast events. In this lab, the width of the entire display represents only about half a second!