Fig. 13.

Two perspectives of the E-cell critical manifold $C_0^E$, fold $L^E$, and the singular orbit of the limit cycle. A: Critical manifold $C_0^E$ has a cubic shape. $L^E$ separates $C_0^E$ into two outer attracting sheets and one inner repelling sheet. B: Another perspective of $C_0^E$. It gives a better idea about the position of the singular orbit relative to $C_0^E$ and $L^E$. The slow part of the orbit lies on $C_0^E$. The switch from the slow flow to the fast flow happens exactly at the fold $L^E$, which represents the physiological firing threshold. We also used $ϵ=0.001$ in (20) to approximate the singular orbit numerically (dashed magenta line). It stays very close to the singular orbit