Hyperpolarisation is the brief period after repolarisation when the membrane potential becomes more negative than the resting potential, falling to about −80 mV instead of −70 mV.
It happens because the voltage-gated potassium ion channels are slow to close. During repolarisation K+ diffuses out of the axon. When the potential gets back to −70 mV, many K+ channels are still open, so K+ keeps leaving and the inside becomes more negative than at rest. This is sometimes called the undershoot.The resting potential is then restored in two ways:• the voltage-gated K+ channels close, so the extra loss of K+ stops
• the sodium-potassium pump continues to move 3 Na+ out and 2 K+ in, and with the K+ leak channels this returns the membrane to about −70 mVHyperpolarisation matters because the membrane is further from threshold. A larger stimulus than normal is needed to reach threshold and trigger another action potential. This forms part of the relative refractory period, which helps to keep one action potential separate from the next.Hyperpolarisation is also used elsewhere in the nervous system. At an inhibitory synapse, the neurotransmitter causes chloride ions (Cl−) to enter, or K+ to leave, the postsynaptic neurone, hyperpolarising it and making an action potential less likely. In the retina, rod cells hyperpolarise when light falls on them.On a graph, hyperpolarisation is the dip below the resting potential line that follows the downward slope of repolarisation.
It happens because the voltage-gated potassium ion channels are slow to close. During repolarisation K+ diffuses out of the axon. When the potential gets back to −70 mV, many K+ channels are still open, so K+ keeps leaving and the inside becomes more negative than at rest. This is sometimes called the undershoot.The resting potential is then restored in two ways:• the voltage-gated K+ channels close, so the extra loss of K+ stops• the sodium-potassium pump continues to move 3 Na+ out and 2 K+ in, and with the K+ leak channels this returns the membrane to about −70 mVHyperpolarisation matters because the membrane is further from threshold. A larger stimulus than normal is needed to reach threshold and trigger another action potential. This forms part of the relative refractory period, which helps to keep one action potential separate from the next.Hyperpolarisation is also used elsewhere in the nervous system. At an inhibitory synapse, the neurotransmitter causes chloride ions (Cl−) to enter, or K+ to leave, the postsynaptic neurone, hyperpolarising it and making an action potential less likely. In the retina, rod cells hyperpolarise when light falls on them.On a graph, hyperpolarisation is the dip below the resting potential line that follows the downward slope of repolarisation.