Folded conductance-model nullcline

For $g(v,n)={\gamma }_K(v+v_K)n^4-(1-v)(H-n)M(v)$, with rapidly increasing saturating activation $M$, the zero-current nullcline satisfies $n^4/(H-n)=(1-v)M(v)/[{\gamma }_K(v+v_K)]$ in the physical strip. The left side increases strictly, so it defines one height for each voltage. Since $g_n>0$, its slope is $-g_v/g_n$. Rapid activation can make $g_v$ negative between two positive-slope current regimes, giving two descending attracting branches and an ascending repelling branch in the voltage–recovery phase plane. Sufficient activation steepness is a parameter condition; the shape is not forced by monotonicity alone.