The lipid bilayer is a environment. This means that it is to most molecules including (they are usually ). This means that intracellular and extracellular compartments are established with very different compositions. Ion are used to the gradient.

The inside of a cell contains a slight excess of which sets up a voltage inside the cell. This voltage is known as the (Em). In neurons this is usually about mV. Em is determined by the of charges.

Ion movement across the membrane is . Ion channels are therefore required. There are a few ions (ions that can pass through the channels): Na+, K+, , Cl-. There are three different types of ion channels that are available:
Non-gated (): these set the of the resting membrane
: generate the action potential
(chemical): generate the Em changes at the

The of the cell is about -65mV. There are several factors that cause this:
1) the gradient - this is set up due to the .
2) the electrical force - the results in the inside being more than the outside. This causes influx. At there is no net movement of .
3) the of the membrane to each ion - how easy it is for ions to move across the membrane through ion .
The following assumptions are made:
- the membrane is only permeable to
- the Em is initially