Final answer:
The neuron cannot respond to a second stimulus during the refractory period, which includes the absolute and relative phases, due to the inactivation of Na+ channels and the outflow of K+.
Step-by-step explanation:
A neuron cannot respond to a second stimulus, no matter how strong, during the refractory period. This period follows immediately after the transmission of an impulse in a neuron. It's a time during which the neuron is ineffective in generating another action potential due to the inactivation of voltage-gated Na+ channels and the opening of additional K+ channels. This period is characterized by two phases: the absolute refractory period and the relative refractory period. During the absolute refractory period, the inactivation gate of the voltage-gated Na+ channel prevents any new action potential. Even after the channel returns to a resting state, a much stronger stimulus is needed to initiate a new action potential during the relative refractory period, as a result of the outflow of K+.