After an action potential there is a short time during which the axon membrane cannot be stimulated to produce another action potential, or can only be stimulated by a much stronger stimulus. This is the refractory period. It lasts a few milliseconds and has two parts.
Absolute refractory period (about 1 ms): the voltage-gated sodium ion channels are inactivated after depolarisation. They cannot open again until the membrane has repolarised, so no stimulus, however strong, can produce a new action potential.Relative refractory period: the Na+ channels have recovered, but the membrane is still hyperpolarised because some K+ channels are still open. A new action potential can be produced, but only by a stimulus stronger than normal.The refractory period has three important consequences:• Impulses travel in one direction only. The region of membrane behind an action potential is refractory, so local currents can only depolarise the membrane ahead of it.
• Action potentials are separate (discrete) events. A new action potential cannot overlap the previous one.
• It limits the frequency of impulses. The absolute refractory period sets a maximum number of action potentials per second.Worked example: a neurone has an absolute refractory period of 2 ms.maximum frequency = 1 ÷ 0.002 s = 500 impulses per second (500 Hz) Because stimulus strength is coded by the frequency of impulses, the refractory period sets an upper limit on the stimulus strength a single neurone can signal.
Absolute refractory period (about 1 ms): the voltage-gated sodium ion channels are inactivated after depolarisation. They cannot open again until the membrane has repolarised, so no stimulus, however strong, can produce a new action potential.Relative refractory period: the Na+ channels have recovered, but the membrane is still hyperpolarised because some K+ channels are still open. A new action potential can be produced, but only by a stimulus stronger than normal.The refractory period has three important consequences:• Impulses travel in one direction only. The region of membrane behind an action potential is refractory, so local currents can only depolarise the membrane ahead of it.• Action potentials are separate (discrete) events. A new action potential cannot overlap the previous one.
• It limits the frequency of impulses. The absolute refractory period sets a maximum number of action potentials per second.Worked example: a neurone has an absolute refractory period of 2 ms.