Dynamics of the HR system (Equations 4a-4c) at . (a)-(e) Time series of the voltage variable x at several values of , with decreasing 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 x over the orbit). Solid/dashed curves indicate stable/unstable solutions. The Hopf bifurcation (H) at is supercritical. The torus bifurcations (TR) at and are also supercritical. Direct numerical simulations show that the system exhibits bursting (□, indicating maximal and minimal values of x) and AM spiking (△, indicating extrema of the modulation envelope) at values of between the two torus bifurcations.