(color online) Curves of (a) dx/dt–t and (b) x–t of the improved model. Parameters are the same as those in Fig. 6 and a stimulus of 0.3 V for 50 s and 0.01 V for 50 s is applied. (c) x–t curve, a sine stimulus is applied, vt = 0.5 V and the other parameters are the same as those in Fig. 6.