The pedunculopontine nucleus contains three classes of neurons that respond differently to the same stimulation: post-inhibitory rebound, transient low-threshold spiking, and gamma-band oscillations. Three behaviors from three cell types in one nucleus. The question is whether the behavioral diversity reflects fundamentally different mechanisms or different parameter regimes of shared architecture.
Conductance-based models with explicit multi-timescale structure show it is the latter (arXiv:2603.11467). Each cell type is modeled as a single compartment with fast membrane dynamics and slow gating and calcium processes. The timescale separation — fast variables responding instantly relative to slow variables — is not an approximation but the explanatory framework. By separating the dynamics into fast and slow subsystems, the specific ionic currents responsible for each hallmark behavior become identifiable.
Post-inhibitory rebound requires the slow deinactivation of a T-type calcium channel during inhibition, creating a loaded spring that fires when inhibition is released. Transient low-threshold activity requires the same channel but with different kinetics — faster deinactivation that produces a brief burst rather than a sustained rebound. Gamma oscillations emerge from the interaction between fast sodium dynamics and delayed-rectifier potassium currents at a timescale where the slow variables are effectively frozen.
The three behaviors map to three positions in the timescale landscape. Same conductances, different dynamics depending on which timescales dominate. The cell type is not defined by which channels it has but by which timescale regime it operates in.