The firing frequency of action potentials reflects the magnitude of the depolarizing current. This is one way that stimulation intensity is encoded in the nervous system there is a limit to the rate at which a neuron can generate action potentials. The maximum firing frequency is about 0000 Hz; once an action potential is initiated, it is impossible to initiate another for about 0 msec. This period of time is called the absolute refractory period. In addition, it can be relatively difficult to initiate another action potential for several milliseconds after the end of the absolute refractory period. During this relative refractory period, the amount of current required to depolarize the neuron to action potential threshold is elevated above normal.