Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation
Figure 1
Hodgkin's three classes of neuronal excitability. (A) Sample responses from spinal lamina I neurons representing each of Hodgkin's three classes. Hodgkin's classification is based on the f–I curve which is continuous (class 1), discontinuous (class 2), or undefined because measurement of firing rate requires at least two spikes (class 3). Data points comprising a single spike (ss) are indicated with open symbols in (A) or gray shading in (B–D). (B) Each cell class could be reproduced in a Morris-Lecar model by varying a single parameter, in this case βw. Like in (A), rheobasic stimulation (minimum Istim eliciting ≥1 spike) elicited a single spike at short latency in class 2 and 3 neurons compared with slow repetitive spiking in class 1 neurons. Despite reproducing the discontinuous f–I curve, the 2D model could not reproduce the phasic-spiking pattern. (C) Phasic-spiking was generated by adding slow adaptation, thus giving a 3D model described by C dV/dt = Istim−g̅fast m∞(V)(V−ENa)−g̅sloww(V−EK)−gleak(V−Eleak)−gadapta(V−EK) and where a controls activation of adaptation and g̅adapt = 0.5 mS/cm2, φa = 0.05 ms−1, βa = −40 mV, and γa = 10 mV. Bottom traces show single-spike elicited by second stimulus applied shortly after the end of first stimulus, which suggests that adaptation slowly shifts the neuron from class 2 towards class 3 excitability. (D) Firing rate (color) is plotted against Istim and βw. Separable regions of the graph correspond to different classes of excitability. Neuronal classification is based on which class of excitability is predominant (i.e., exhibited over the broadest range of Istim) and is indicated above the graph.