How does action potential change with signal intensity?

Signal intensity alters the frequency or number of action potentials. However, it does not change the size or amplitude of the action potential – these remain constant as the action potential propagates along the nerve fiber.  

Action potentials are frequency-modulated, not amplitude-modulated. Signal intensity is encoded by the frequency of action potentials. Stronger stimuli cause neurons to fire action potentials more frequently, while weaker stimuli result in a lower frequency of action potentials. This frequency coding mechanism allows the nervous system to convey information about the intensity of a stimulus.