The Refractory Period
- Very shortly (about 1 ms) after an action potential has been generated in a section of the axon membrane, all the sodium ion voltage-gated channel proteins in this section close
- This stops any further sodium ions from diffusing into the axon
- Potassium ion voltage-gated channel proteins in this section of axon membrane open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
- This gradually returns the potential difference to normal (about -70mV) – a process known as repolarisation
- Once the resting potential is close to being re-established, the potassium ion voltage-gated channel proteins close and the sodium ion channel proteins in this section of the membrane become responsive to depolarisation again
- Until this occurs, this section of the axon membrane is in a period of recovery and is unresponsive
- This is known as the refractory period
Refractory Period Diagram
The refractory period begins when repolarisation starts and ends when the resting state is re-established.
The importance of the refractory period
- The refractory period is important for the following reasons:
- It ensures that action potentials are discrete events, stopping them from merging into one another
- It ensures that changes of membrane potential are generated ahead (ie. further along the axon), rather than behind the original action depolarisation, as the region behind is ‘recovering’ from the repolarisation that has just occurred
- This means that the impulse can only travel in one direction, which is essential for the successful and efficient transmission of nerve impulses along neurones
- This also means there is a minimum time between action potentials occurring at any one place along a neurone
- The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones (between 500 and 1000 per second)
