The period after an initial stimulus when a neuron is not sensitive to another stimulus is the

Absolute refractory period

Explanation:

The absolute refractory period is short in an action potential. During the refractory period, there is no occurrence of another action potential. There would occur two types of refractory periods  

Absolute refractory periods: Absolute period is the period when another action potential won't occur immediately after a first action potential. This refractory period also called an effective refractory period. This period happened because the sodium channel is inactivated and is stable in the same way until the hyper polarization does not occur.  

Relative refractory period:

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