Figure 1

Experimental observation of excitability dynamics. (a) The response of a single isolated neuron to sequences of pulse stimuli delivered at 20 Hz. The responses are ordered top to bottom; every 20th response is shown for clarity. The delaying of the AP can be observed, as well as response failures when excitability is below threshold. (b) The AP latency plotted as a function of time in an experiment where the stimulation rate is changed. For low stimulation rates, the excitability stabilizes at a fixed, suprathreshold value. For a high stimulation rate (20 Hz), the excitability decreases below threshold, and the neuron responds intermittently. (c) Response latencies (solid line) in response to a stimulation sequence with slowly increasing stimulation rate (dashed line). (d) Failure (no spike) probability as a function of stimulation rate. A critical stimulation rate is clearly evident. (e) Mean response latency as a function of stimulation rate. The increase of the latency accelerates as the stimulation rate approaches the critical point. (f) The jitter (coefficient of variation) of the latency as a function of stimulation rate. (g) Scale-free fluctuations in the intermittent mode. Periodograms of the failure rate fluctuations, at five different stimulation rates above $r_0$. (h) Length distribution of spike-response sequences on a semilogarithmic plot, demonstrating an exponential behavior. Example from one neuron stimulated at 20 Hz for 24 h. (i) Length distribution of no-spike response sequences from the same neuron, on a double logarithmic plot, demonstrating a power-law-like behavior.Reuse & Permissions

Figure 2

(a) The effect of modulating $G_{\mathrm{Na}}$ in the Hodgkin-Huxley model under short pulse stimulation. As it decreases, the AP is delayed. Below a certain threshold, no AP is produced. (b) AP latency in panel (a) as a function of $G_{\mathrm{Na}}$, demonstrating the existence of a sharp threshold. Propagating APs are marked with solid circles; nonpropagating responses are marked with empty circles. (c) Simulation results of the contact process model [Eq. (2)]. Dependence of the spike failure probability on the stimulation rate, analogous to Fig. 1(d). (d) Power spectral densities of the response fluctuations at different frequencies above the $r_0$, $1/f$-type behavior. (e) Length distribution of spike-response sequences, on a semilogarithmic plot, demonstrating an exponential behavior, analogous to Fig. 1(h). (f) Length distribution of no-spike response sequences from the same neuron, on a double logarithmic plot, demonstrating a power-law-like behavior, analogous to Fig. 1(i).Reuse & Permissions