<p>Bursting is a fundamental firing pattern in conductance-based neuron models and is often analyzed through fast–slow decomposition. Most classical studies rely on a single slow variable, restricting the repertoire of burst dynamics. Here we extend the analysis to a model with two interacting slow inactivation variables, explicitly constructing a three-dimensional bifurcation manifold for the fast subsystem. Using the dynamic spike height (DSPK) model under modulation of the persistent sodium conductance, we identify four characteristic firing patterns: one-spike bursting, three-spike bursting, ramping bursting, and tonic spiking. Across bursting regimes, initiation occurs by a saddle-node bifurcation of equilibria in the fast subsystem, whereas termination occurs either by crossing a homoclinic boundary or by a slow-drift-induced loss of canard recurrence. In contrast, tonic spiking corresponds to confinement of the slow trajectory within the oscillatory region, preventing exit from the limit-cycle manifold. A key finding is the emergence of a mixed bursting pattern in which two-spike bursting, seven-spike bursting, and ramping bursting appear within a single trajectory. This mixed bursting arises from switching among distinct entry routes along the saddle-node boundary, driven by interburst slow drift of the two inactivation variables. These results provide a framework for linking intrinsic conductances, bifurcation manifolds, and firing variability, offering new insights into the dynamical basis of rhythmogenesis in the pre-Bötzinger complex (pre-BötC).</p>