Final answer:
An action potential can have a smaller amplitude during the relative refractory period, which occurs right after the normal action potential phase when the resting state of the membrane is not yet fully restored, requiring a stronger stimulus to generate a new action potential.
Step-by-step explanation:
The text states that all action potentials are identical to one another. However, an exception where an action potential can have a smaller amplitude is during the relative refractory period. During this time, the normal resting state is not yet fully restored, and the voltage-gated Na+ channels are starting to return from their inactivated state. As a result, it is possible, though more challenging, to generate a new action potential. However, this action potential would require a stronger stimulus because the membrane potential is closer to the K+ equilibrium potential due to the ongoing outward flow of K+ ions, which hyperpolarizes the cell beyond the resting potential. Therefore, the amplitude of an action potential during this phase could be potentially smaller than normal if the stimulus is just above the increased threshold.