What causes the resting potential in a neurone?

A neurone which is not conducting a nerve impulse has a negative charge of -70mV compared to the outside environment; there is a potential difference across the membrane. This is partly caused by movement of sodium ions (Na+) and potassium ions (K+), which have a positive charge. Sodium-potassium pumps in the plasma membrane actively transport potassium ions into the cell and sodium ions out. Three sodium ions are actively transported out for every two potassium ions actively transported in, which causes the inside of the neurone to become more negative than the outside environment. 

There are also organic anions within the neurone, which carry a negative charge. As the neurone's membrane is impermeable to the organic anions, these add to the potential difference across the membrane. Finally, the membrane of the neurone is slightly permeable to potassium ions, which means that there is also slight loss of the potassium ions from the neurone, after they are actively transported in. This also helps to generate the negative charge across the membrane.