Understanding membrane potential

Membrane potential is a fundamental property of all cells.

- Tapan Kumar Maitra

It results from an excess of negative charge on one side of the plasma membrane and an excess of positive charge on the other side. Cells at rest normally have an excess of negative charge inside and an excess of positive charge outside the cell; the resulting electrical potential is called the resting membrane potential, denoted Vm. The membrane potential can be measured by placing one tiny electrode inside the cell and another outside the cell; its value is generally given in millivolts (mV). (Do not confuse the potential Vm and the unit mV.) The electrodes compare the ratio of negative to positive charge inside the cell and outside the cell. Because the inside of a cell typically has an excess of negative charge, that cell has a negative resting membrane potential. For example, the resting membrane potential is approximately -60 mV for the squid giant axon.

Nerve, muscle, and certain other cell types such as the islet cells of the pancreas of vertebrates exhibit a special property called electrical excitability. In electrically excitable cells, certain types of stimuli trigger a rapid sequence of changes in membrane potential known as an action potential. During an action potential, the membrane potential changes from negative to positive values and then back to negative values again, all in a little over a millisecond. In nerve cells, the action potential has the specific function of transmitting an electrical signal along the axon. To understand how nerve cells use action potentials to transmit signals, we must first examine how cells generate a resting membrane potential and how the membrane potential changes during an action potential.