Nonlinear and ionic mechanisms for bursting patterns and subthreshold resonance in mesencephalic V neurons

The bursting and subthreshold resonance of mesencephalic trigeminal nucleus neurons play a critical role in bite force control and orofacial pain processing, yet their underlying dynamical mechanisms remain poorly understood. The nonlinear and ionic mechanisms are presented in the present paper. First, bifurcations from the resting state to bursting, modulated by persistent sodium current (I_NaP) and applied current (I_app), are obtained, including a Hopf bifurcation. Second, three typical bursting patterns are distinguished via codimension-2 bifurcations, and coexisting behaviors are acquired through fast/slow analysis. Modulated by I_NaP, the most common bursting pattern, similar to experimental observations, exhibits bursts interrupted by long quiescent states. In the fast subsystem, the limit cycle for the burst is far from the coexisting equilibrium for the quiescent state. Then, stochastic transitions between the two behaviors cannot be evoked, resulting in robustness to noise. Two other bursting patterns manifest transitions between bursts for a large limit cycle and subthreshold oscillation for a small limit cycle. Proximity between the two limit cycles allows noise to drive stochastic transitions, leading to noise-sensitive dynamics. The sensitivity is determined by the distance between the two cycles. Finally, subthreshold resonance evoked from the resting state is reproduced, which is mediated by potassium current. Enhanced I_NaP and I_app amplify the resonance amplitude and frequency by driving the resting state toward the Hopf bifurcation. The dynamics of bursting and resonance advance the understanding of sensorimotor processing and present potential targets for treating orofacial motor disorders.