When cant the neuron fire?

A neuron cannot fire during the absolute refractory period, which occurs immediately after an action potential when the sodium channels are inactivated and cannot reopen, regardless of the stimulus strength. Additionally, during the relative refractory period, a neuron can fire only if the stimulus is strong enough to overcome the hyperpolarization caused by potassium channels remaining open. These periods ensure that action potentials are unidirectional and that neurons have time to recover before firing again.