International A Level Biology / The nervous system and neurones
The action potential
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.4. Free lesson.
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An action potential is a rapid change in the electrical potential across an axon membrane, caused by Na⁺ and K⁺ ions moving through channels. Its phases, threshold and refractory period explain how nerve impulses are produced and controlled.
How to read an action potential graph
The graph records the membrane potential at one point on an axon. It shows the charge inside compared with outside, in millivolts, over a few milliseconds. At rest, the potential is −70 mV. If a stimulus brings it to the threshold of about −55 mV, an action potential fires. The graph rises to about +40 mV, falls, dips below resting potential briefly, then returns to rest. The rise is depolarisation: the inside becomes positive. The fall is repolarisation: the inside becomes negative again. The brief dip below −70 mV is hyperpolarisation.
How Na⁺ and K⁺ ions produce each phase
Ions cross the membrane through channel proteins. Voltage-gated channels open or close when the membrane potential changes. At first, a stimulus opens a few Na⁺ channels. Na⁺ ions diffuse into the axon down their electrochemical gradient, because both their concentration and the electrical charge pull them in. At threshold, more voltage-gated Na⁺ channels open. More Na⁺ enters and opens still more channels. This is positive feedback and causes rapid depolarisation, up to about +40 mV. At about +40 mV, Na⁺ channels close and voltage-gated K⁺ channels open. K⁺ ions diffuse out, making…
How permeability changes during the action potential
Permeability means how easily an ion crosses the membrane. The changes in permeability explain the shape of the action potential graph. Na⁺ permeability rises first as Na⁺ channels open, then falls quickly when they close. K⁺ permeability rises later and stays high for longer, helping repolarisation and causing the brief hyperpolarisation. The key pattern is: Na⁺ permeability rises first and falls fast; K⁺ permeability rises later and stays high for longer.
Why action potentials are all or nothing
An action potential either fires at its usual size or does not happen. If threshold is reached, the spike reaches about +40 mV. If threshold is not reached, there is no action potential. A stronger stimulus does not make a bigger spike. Instead, it makes action potentials fire more often, increasing their frequency.
How the refractory period affects impulses
The refractory period is a short time after an action potential when that part of the axon membrane cannot fire again. The voltage-gated Na⁺ channels are closed and cannot reopen yet, however strong the stimulus. This keeps action potentials separate, limits their frequency and makes impulses travel in one direction. If tetrodotoxin (TTX) blocks voltage-gated Na⁺ channels, Na⁺ cannot diffuse into the axon. Depolarisation cannot occur, so threshold is not reached and no action potentials are produced. They are not just smaller.