Potassium ion channels then open, allowing potassium ions to move out of the neuron (Bond and McConkey, 2001). .
The channels consist of proteins that traverse the plasma neural membrane (Gleitman, 1999 & Kandel et al, 1995) (see figure 1). The membrane itself is almost completely impermeable to ions and therefore the channels are necessary to allow the ions to move and out of the neuron (Kendel et al, 1995). Most of the ion channels involved in neural signalling are gated, meaning that they can be opened and closed when required. When they are opened ions are able to move in and out of the neuron. This occurs at an extremely rapid rate, up to 100 million ions may pass through a single ion channel per second (Kandel at al, 1995). As well as gated ion channels the neuron also contains resting channels that are usually open, allowing for the resting potential to be maintained when the neuron is not being stimulated (Kendel et al, 1995). .
Ion channels are highly selective, only allowing the passing of certain ions in and out of the neuron (Kendel et al, 1995). Many models have been proposed to explain this selectivity, but the most highly supported theory is that of a selectivity filter (Kendel et al, 1995) (see figure 2). As the selected ion moves through the filter it binds to an active site. Because it is hydrophilic, the ion has a cloud of water molecules surrounding it. When it binds to an active site, the ion sheds most of its waters and in their place forms weak chemical bonds with charged amino acid residues that line the walls of the channel. An ion will move along the channel only if the energy of interaction with the selectivity filter compensates for the loss of waters. Since different ions have different sized clouds of water molecules surrounding them, the filter ensures that only certain types of ions pass through the channel (Kendel et al, 1995). Ultimately, the filter is discriminating between the ions on the basis of their ionic charge (Kendel at al, 1995).
