Fig. 4.

Critical manifolds for $g_{\mathrm{SI}}=0.5615{\text{mS/cm}}^2$ embedded in the five-dimensional phase space of system (1). Shown are projections onto the $(h_{\mathrm{SI}},m_{\mathrm{SO}},V)$-coordinates; panels (a) and (b) show two different viewpoints of the surfaces of equilibria, coloured black when stable and red when not; from the same viewpoints, panels (c) and (d) also show maxima and minima with respect to V of the two-parameter families of periodic orbits, coloured blue when stable and magenta when not. The equilibrium manifold splits into six sheets, labelled $S_1^a$, $S_1^r$, $S_2^a$, $S_2^r$, $S_3^r$ and $S_3^a$, that are separated by four fold curves, $F_0$ (not shown), $F_1$, $F_2$ and $F_3$, and a curve of Hopf bifurcations labelled H. The saddle and attracting families of periodic orbits are labelled $P^r$ and $P^a$, respectively. See also Table 2.