 Three-region calcium nullcline

For $J=\beta u^2/(1+u^2)$ and $K=u^4/(1+u^4)$ with small leak $\delta$, the positive nullcline is unique at each $u$ because its derivative with respect to $v$ is strictly negative. Its asymptotic regions are a high-store balance $v\simeq (J-K)/\delta$, a crossing balance near $J(u_c)=K(u_c)$, and a negligible-leak balance $v^2/(1+v^2)\simeq J/K$. The crossing relation is $(K_c^{\prime }-J_c^{\prime })(u-u_c)\simeq K_c/v^2-\delta v$. These describe one continuous curve, not three distinct equilibria at a fixed cytoplasmic concentration.