Neurons process information in the form of electrical signals (nerve impulses or action potentials) that travel along their axons (long extensions of cell membrane). Electrical charges move across the membrane as charged ions, but the cell membranes of most cells, including neurons, are relatively impermeable to charged ions. However, proteins that act as ion channels and ion pumps are embedded in the cell membrane and make it possible for ions to move, or to be moved, selectively across the membrane.
In this animation, we review how ion channels are responsible for a voltage difference (called the resting potential) across the cell membrane of a neuron.
In a typical mammalian neuron, there is a large difference in the concentration of ions, such as sodium and potassium, between the intracellular and the extracellular environments. In addition, the interior of the neuron has a high concentration of organic anions, including proteins and nucleic acids.
The cell membrane is composed of a lipid bilayer. Its hydrophobic nature prevents the diffusion of ions across the membrane.
The only way ions can diffuse across the lipid bilayer is by passing through specialized channels. These channels are transmembrane pores that permit the movement of particular ions while excluding others. Such channels can be in an open or closed state.
When a neuron is at rest, most ion channels are closed. However, some potassium channels are open, permitting potassium ions to diffuse out of the cell down their concentration gradient. Note that sodium channels are normally closed, and thus sodium ions cannot cross the membrane when the neuron is at rest.
In a typical neuron, the internal concentration of potassium is higher than the external concentration. Potassium ions are pulled by two opposing forces. First, a diffusion force drives potassium ions down their concentration gradient towards the exterior of the cell.
The movement of potassium ions out of the cell increases the internal negative charge. The positively charged potassium ions are attracted to the internal negative charge, and this electrical force pulls potassium ions back into the cell.
The diffusion and electrical forces eventually come into balance, and an electrical potential, or voltage, is reached at which the electrical force exactly balances the diffusion force. At this point, there is no net movement of potassium ions into or out of the cell.
The electrical potential across the membrane can be measured by inserting an electrode into the cell. A neuron at rest has a voltage difference of about –60 millivolts across the membrane. This value is the neuron's resting membrane potential and is governed by the relative concentrations of ion species between the intracellular and extracellular space.
When a neuron is at rest, the cell membrane is far more permeable to potassium (K+) ions than to other ions present, such as sodium (Na+) and chloride (Cl-). The electrochemical equilibrium that results from the distribution of these ion species across the membrane, together with the relative permeabilities of each ion, is responsible for the –60mV charge that can be measured across the membrane. This charge is called the resting membrane potential.