Final answer:
The period after an initial stimulus when a neuron is not sensitive to another stimulus is called the absolute refractory period, during which a new action potential cannot be triggered because the voltage-gated Na+ channels are inactivated.
Step-by-step explanation:
The period after an initial stimulus when a neuron is not sensitive to another stimulus is known as the absolute refractory period. During this time, an action potential cannot be initiated because the voltage-gated Na+ channel is inactivated. This period ensures that the action potential only travels in one direction along the neuron and also provides time for the ion channels to return to their resting state. Once the voltage-gated Na+ channel returns to its resting conformation, typically when the membrane potential is less than -55 mV, a new action potential could theoretically be triggered by a much stronger stimulus than usually needed due to the ongoing outflow of K+, but this occurs during the subsequent relative refractory period, not the absolute one.