Dynamics of the MLT system (Equations 6a-6c) at . (a)-(e) Time series of the voltage variable x at several values of k, with k increasing from top to bottom. (f) Bifurcation diagram, including branches of fixed points (black curve) and periodic orbits (two red curves, indicating maximal and minimal values of V over the orbit). Solid/dashed curves indicate stable/unstable solutions. The torus bifurcations at and are both supercritical. Direct numerical simulations show that the system exhibits bursting (□, indicating maximal and minimal values of V) and AM spiking (△, indicating extrema of the modulation envelope) at values of k between the two torus bifurcations.