Ion channels and action potentials

The action potential results from the changes in the permeability of cell membranes to ions. At rest, the membrane permeability of a nerve fibre is thought to depend on ion channelsthrough which specific ions can move. An ion channel consists of a protein molecule spanning the membrane, with a pore through the centre. Sodium ions move through one type of channel and potassium ions through another. There are many more of these ion channels for potassium than for sodium, therefore at rest the membrane permeability to potassium ions is much greater than that to sodium ions.

During an action potential, special ion channels control ion movements across the membrane. These channels are believed to have voltage-sensitive gates that open and close in response to voltage changes, and are therefore called voltage-gated ion channels.

During the resting potential, the voltage-gated sodium and potassium ion channels are closed. When a stimulus is applied, sodium ion channels open rapidly, sodium ions move in, and the inside becomes more positive. If the stimulus reaches the threshold level, an action potential occurs. When the action potential reaches its peak, the sodium ion channels close slowly and potassium ion channels open slowly. Sodium ions stop moving into the cell but potassium ions diffuse more rapidly out. These changes cause the potential difference to drop. When the membrane returns to its resting potential, potassium ion channels close, but because they do this slowly, the potential dips below the resting level. Finally, when the potassium ion channels are closed, the membrane returns to its resting condition.

So far, we have examined how an action potential is generated at the point of stimulation. However, this is only the first step in the propagation of a nerve impulse along a neurone. These localised action potentials are converted into nerve impulses which transmit information from one part of a neurone to another neurone or to an effector such as a muscle or a gland.

Action potentials obey the all-or-none law.This means that no matter how strong the stimulus, the size of an action potential is always the same. Therefore, information about the strength of a stimulus is carried along a nerve fibre not as variations in the size of nerve impulses, but by changes in their frequency. The next spread discusses these points more fully.