What is the resting potential of a neurone?
About –70 mV; inside is more negative than outside due to Na+/K+ pump and leaky K+ channels.
What is the role of the sodium–potassium pump at rest?
It pumps 3 Na+ out and 2 K+ in, maintaining a negative inside and the resting potential.
What is meant by the threshold potential?
The membrane potential at which enough Na+ channels open to trigger a full action potential, usually around –55 mV.
What happens at the start of depolarisation?
A stimulus causes some voltage-gated Na+ channels to open and Na+ begins to enter the neurone.
What happens during full depolarisation?
Many voltage-gated Na+ channels open, Na+ floods in and the membrane potential becomes positive (around +30 mV).
What happens to sodium channels at the peak of the action potential?
Voltage-gated Na+ channels close and become inactivated so no more Na+ can enter.
What triggers repolarisation?
Voltage-gated K+ channels open and K+ ions leave the neurone down their electrochemical gradient.
What happens to membrane potential during repolarisation?
As K+ leaves, the inside becomes more negative and moves back towards the resting potential.
What is hyperpolarisation (undershoot)?
K+ channels stay open slightly too long, so too many K+ leave and the membrane becomes more negative than the resting potential.
Why does hyperpolarisation occur?
Slow-closing K+ channels allow extra K+ efflux, driving the potential below –70 mV.
How does the neurone return to resting potential after hyperpolarisation?
Voltage-gated K+ channels close and the Na+/K+ pump plus leak channels gradually restore –70 mV.
What is the refractory period and why is it important?
The time during and after an action potential when a neurone cannot fire again; it ensures impulses travel in one direction and limits firing frequency.
