Measuring the Resting Membrane Potential

A difference in electrical charge between the inner and outer surface of a plasma membrane is called an electrical potential. All living cells have an electrical potential. The electrical potential of a resting cell is the resting membrane potential. The membrane potential of a cell can be measured in millivolts (mV) with the use of a microelectrode and an oscilloscope.
(a) Both recording (needle) and reference (block) electrodes are outside of the cell, and no electrical potential (0 mV) is recorded. (b) The recording electrode is inside the cell, the reference electrode is outside, and an electrical potential difference of about 85 mV is recorded, with the inside of the plasma membrane negative with respect to the outside of the plasma membrane.