Fig. 4

Phase portrait of the Morris–Lecar model at \(I=0.075\) with \(C=1\), \(V_{k}=-0.7\), \(V_{L}=-0.5\), \(V_{\mathrm{Ca}}=1\), \(g_{\mathrm {K}}=2\), \(g_{L}=0.5\), \(V_{1}=-0.01\), \(V_{2}=0.15\), \(g_{\mathrm{Ca}}=1.33\), \(V_{3}=0.1\), \(V_{4}=0.145\) and \(\phi=1/3\). The voltage nullcline is shown in red and that of the gating variable in green. The filled black circle indicates a stable fixed point, the grey filled circle a saddle and the filled white circle an unstable fixed point. The periodic orbit is shown in blue