Absolute and Relative Refractory period in cardiac muscle cells | States...

The absolute and relative refractory period

Refractory means the cells are unresponsive to stimulus. It may be either absolute or relative.

In the absolute refractory period, the cells are unresponsive to any stimulus no matter how strong it is.

In the relative refractory period, the cells are unresponsive to threshold stimulus but respond to a suprathreshold stimulus that is a higher strength of the stimulus is required than usual to stimulate the cell.

Refractory period occurs due to different states of sodium channels. 

States of sodium channels 

Sodium channels can be in 3 states:
1. Closed (at RMP)
2. Open (as depolarization occurs, lot o channels open at threshold)
3. Inactivated: From the open state, channels become inactivated.

Channels go back to the closed state from the inactivated state as repolarization occurs. At RMP, all become closed. These channels can only open from the closed state and not from the inactivated state. So for the next action potential to occur, channels should go back to the closed state. So till the time, channels are in an inactivated state, the next action potential cannot occur.

Cause of Absolute/Effective refractory period 

From phase 0 to half of phase 3 of the cardiac action potential is the absolute refractory period. It occurs because sodium channels are in the inactivated state during this time.

Cause of relative refractory period 

After half of phase 3 to RMP is the relative refractory period. It occurs because some channels have changed their state to closed state so few channels are available which can open.