Having read the resting potential explanation, you now know how a neuron can have a potential difference or a voltage at rest. But remember that the development of the voltage required differential permeability of the membrane to ions (in this case, sodium was permeable while chloride was not).

What happens if the permeability of the membrane changes with time? Let's consider a situation where there is a 10:1 ratio of KCl again but neither potassium or chloride ions are permeable. In this case, there is no charge separation, so the potential difference between compartments is 0 V. Now let's make the membrane suddenly permeable to potassium. The potassium ions would rush out of the left compartment and create a "resting potential" of -58 mV. You've generated an electrical signal! The method a neuron uses to control permeability of ions through the membrane is by means of a protein that penetrates the membrane and has a pore that can be opened or closed. These proteins are known as transmembrane ion channels.

In reality, the situation in the neuron is a lot more complicated than in our little tank. There are two concentration gradients that are important in a real cell. First, there is much more potassium ions inside the cell than outside. At rest, the membrane is moderately permeable to potassium ions, so there is a resting potential. (typically this is on the order of -40 mV to -80 mV) Second, there is much more sodium ions outside the cell than inside. Normally, the membrane is not permeable to sodium, but if it were, movement of sodium ions would tend to make the inside positive as the sodium ions rush in. Together, these ions can either move the inside less positive (potassium) or more positive (sodium).