In cells of all types, there is an electrical potential difference between the inside of the cell and the surrounding extracellular fluid. This is termed the membrane potential of the cell. While this phenomenon is present in all cells, it is especially important in nerve and muscles cells, because changes in their membrane potentials are used to code and transmit information.
First, what is an electrical potential difference? An electrical potential difference exists between two locations when there is a net separation of charge between the two locations. This is illustrated in the figure on the right. Electrical potentials are measured in units of volts. (A volt is defined in terms of energy per unit charge; that is, one volt is equal to one joule/coloumb.)
When a nerve or muscle cell is at "rest", its membrane potential is called the resting membrane potential. In a typical neuron, this is about –70 millivolts (mV). The minus sign indicates that the inside of the cell is negative with respect to the surrounding extracellular fluid.
It is essential to realize that only a very small number of negative and positive ions need to be separated by the membrane to create the resting membrane potential. For example, for each pair of negative and positive ions separated by the membrane, there are roughly 1000 pairs of positive and negative ions within the cytosol of the neuron.
Thus, two energetic factors influence the movement of an ion across a membrane.
The concentration gradient, of course, applies to uncharged molecules too. But with ions, we must always consider the electrical potential difference as well. Thus, the total energy change for the movement of an ion across the membrane is the the sum of the energy change due to the concentration gradient and the energy change due to electrical potential difference. These two factors may act in the same direction or in opposite directions.
If some event, such as the opening of a gated ion channel, causes the membrane potential to become less negative, this is termed depolarization. Conversely, if some factor causes the membrane potential to become more negative, this is termed hyperpolarization.