The potential on the cell membrane is measured by comparing the potentials inside and outside the cell because there is a difference in the concentration or concentration gradient of charged particles called ions inside the cell. These concentration gradients in turn lead to electronic and chemical imbalances that drive ion balance imbalances, and more different imbalances provide greater power or drive to compensate for imbalances. For this reason, ions generally move from the high concentration side of the membrane to the low concentration side. The two ions of interest at the action potential are potassium cation (K + ) and sodium cation (Na + ), which can be found both inside and outside the cell. The concentration of K + in the cells is higher relative to the outside. The Na + concentration outside the cell is high relative to the internal concentration, which is about 10 times. When there is no action potential in progress (ie, the cell is “stationary”), the potential of the neuron is at a resting membrane potential, which is typically measured to be about -70 mV. This means that the potential inside the battery is 70 mV lower than the outside. It should be noted that this refers to an equilibrium state – the ions are still in and out of the cell, but in a way that maintains the resting membrane potential at a fairly constant value. The resting membrane potential can be maintained because the cell membrane contains proteins that form ion channels – pores that allow ions to flow into and out of the cell – and a sodium/potassium pump that can pump ions into and out of the cell. Ion channels are not always on; some types of channels are only open under certain conditions. These channels are therefore referred to as “gated” channels. The leaking channels are randomly turned on and off to help maintain the resting membrane potential of the cells. The sodium leak channel allows Na+ to slowly enter the cell (since the concentration of Na+ is higher externally relative to the interior), while the potassium channel allows K+ to move out of the cell (because the concentration of K+ is higher internally) relative to the outside). However, potassium leaks more channels than sodium, so potassium moves out of the cell at a faster rate than sodium enters the cell. Therefore, there is more positive charge outside the cell, resulting in a negative resting membrane potential. The sodium/potassium pump maintains the resting membrane potential by moving sodium from the cells or potassium back into the cells. However, the pump brings two K + ions to every three Na + ions, maintaining a negative potential. Voltage-gated ion channels are important for action potentials. Most of these channels remain closed as the cell membrane approaches its resting membrane potential. However, when the potential of the cells becomes more positive (less negative), these ion channels will open.