Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation
Figure 4
Biophysical correlate of differences in βw.
(A) The w-nullcline (inset) corresponds to the voltage-dependent activation curve for Islow. Horizontal positioning of that curve is controlled by βw. Differences between class 1, 2, and 3 models may thus reflect differences in the voltage-dependency of Islow. (B) It is more likely, however, that the components of Islow vary between cells of different classes (see Results). Islow may comprise multiple currents with similar kinetics. If Islow = IK,dr+Isub, the position of the net I–V curve can be changed in qualitatively the same way as in (A) by changing the direction and magnitude of Isub (see insets) without changing the voltage-dependencies of Isub (βz = −21 mV, γz = 15 mV) or of IK,dr (βy = −10 mV, γy = 10 mV); voltage-dependencies of Isub and IK,dr are different, however, with the former being more strongly activated at subthreshold potentials. These results predict that tonic-spiking neurons express a subthreshold inward current and/or that single-spiking neurons express a subthreshold outward current.