In order for an action potential to occur enough sodium ions must enter the cell so that the membrane potential reaches a specific threshold. This then causes the sodium gates to open. However, if not enough sodium enters, the depolarisation is not large enough and no action potential occurs. In other words, there’s no half way point in which some ion channels open and other don’t. This is the all or nothing law.

An action potential is always the same size: it always reaches +40mV. It never decreases, no matter how long the axon of a neurone is. It’s the frequency of an impulse which dictates how strong a stimulus is: the higher the frequency, the stronger the stimulus.

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The Refractory Period

Ion channels assist the effectiveness of a nerve impulse in two ways:

• the all or nothing law (the action potential must reach the threshold value to start)
• the refractory period

Once an ion channel has been opened it requires a ‘rest period’ before it reopens. This is known as the refractory period. It only last about 2 milliseconds but means that the action potential only moves in one direction along the axon because the upstream channel aren’t open.

It’s also needed in order for the proteins of the ion channels to revert back to their original polarity.

During the absolute refractory period the sodium channels are either open or recovering. It’s not possible for a second stimulus to start a new action potential. This means that the impulses are discrete or separate from each other.

However, if the stimulus is much greater it is possible for one to be produced. This is called the relative refractory period. At this point the potassium channels are open and the membrane is hyperpolarized (at -80mV). Relative to the resting potential (-70mV) action potentials are harder to start.

The refractory period is able to limit the number of action potentials. On average, it allows approximately 100 per second.