As described in the lectures, in addition to generating periodic action potentials, it is sometimes possible for neurons to generate bursting, or the switching of the voltage between an active state (characterized by rapid oscillations) and a rest state (characterized by the lack of oscillations). Models of such bursting typically involve vector fields in which system variables are classified as being "fast" or "slow" depending on whether or not they change significantly over the duration of a single spike. The slow variables may then be thought of as slowly varying parameters for the equations describing the fast variables. As the slow variables evolve, the state of the system in the fast variables may change from a stable periodic orbit (corresponding to the active state) to a stable fixed point (corresponding to the rest state) and vice versa; such transitions are often associated with a region of bistability for the periodic orbit and the fixed point.