Question

For action potentials, why does the minimum amplitude needed to reach the threshold decrease when increasing the stimulus duration?

Answer 1

Initially the neuron is in resting membrane potential. Once there is a stimuli the membrane potential shoots up and due to the action of Na and K pumps the signal propagate. Since the neuron is already in a higher action potential due to the start of a stimulus If you increase the stimulus duration it requires very little threshold to bring the already high membrane potential membrane to again trigger the reaction. This is the reason why minimum amplitude needed to reach the threshold decrease when increasing the stimulus duration.