= 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.
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