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 \(\epsilon =0.001\) in (20) to approximate the singular orbit numerically (dashed magenta line). It stays very close to the singular orbit