The refractory periods are due to the inactivation property of voltage-gated sodium channels and the lag of potassium channels in closing. ... During this time, the extra potassium conductance means that the membrane is at a higher threshold and will require a greater stimulus to cause action potentials to fire.

What causes the refractory period?

Refractory periods are caused by the inactivation gate of the Na⁺ channel. Once inactivated, the Na⁺ channel cannot respond to another stimulus until the gates are reset.

Which occurs first refractory period?

A fibre first enters the absolute refractory period directly after an action potential has been fired, then the relative refractory period. The absolute refractory period of a human muscle fibre is typically between 2.2 and 4.6 ms.