Final answer:
The period when an excitable membrane can respond again with a greater than threshold stimulus is the relative refractory period. After the absolute refractory period during which no action potential can be initiated, a stronger stimulus can trigger a new action potential in the relative refractory period.
Step-by-step explanation:
The period during which an excitable membrane can respond again, but only if the stimulus is greater than the threshold stimulus, is the relative refractory period. The absolute refractory period is when another action potential cannot be initiated no matter how large the stimulus, because the voltage-gated Na+ channel is inactivated. After this phase, during the relative refractory period, the membrane can respond to a new action potential, but this requires a stronger than usual stimulus due to the outward flow of K+ that is countering any incoming Na+. This ensures that the neuron or muscle cell has time to return to its resting state.
The refractory period serves to space out signals, providing the nervous system with a tool to regulate the timing and frequency of nerve impulses, which is crucial for the proper functioning of neurons and, conversely, muscle cells.