Frequency coding in the nervous system: Threshold stimulus.

If a threshold stimulus is applied to a neuron and maintained (top, red trace), action potentials occur at a maximum frequency that is limited by the sum of the absolute and relative refractory periods (bottom, blue trace). Here, a threshold stimulus refers to that which is just strong enough to bring a resting neuron to threshold. Thus, with maintained threshold stimulus, subsequent action potentials occur only at the end of the relative refractory period of the preceding action potential. The top and bottom traces are on the same time scale. The dashed line represents the threshold voltage (Vthreshold) of approximately −50 mV. ARP, absolute refractory period; RRP, relative refractory period.