Multi-timescale Rhythmic Dynamics in Rostral Ventromedial Medulla Neurons.

Neural circuits involved in sensory control must integrate fast, reflexive responses with slower, state-dependent modulation. This coordination is especially critical in the pain system, where rapid withdrawal must be balanced against slower adjustments in nociceptive sensitivity that reflect arousal, physiology, and internal state. We hypothesized that neurons of the rostral ventromedial medulla (RVM)-the principal brainstem node for descending pain control-operate across multiple temporal scales, combining fast stimulus-evoked responses with slower intrinsic dynamics. Using single-unit recordings from identified RVM ON-, OFF-, and NEUTRAL-cells and probabilistic modeling, we find that ON- and OFF-cells exhibit multi-phase population responses characterized by rapid activation and prolonged recovery dynamics lasting tens of seconds. In the absence of stimulation, the same neurons display coherent, quasi-periodic oscillations on the order of minutes, captured by Gaussian process models as predictable, low-frequency fluctuations. These rhythmic dynamics were specific to ON- and OFF-cells and occasionally coherent with autonomic parameters, suggesting coupling between nociceptive and homeostatic control loops. Together, these results identify a multi-timescale organizational principle within descending brainstem circuits, in which fast, reflex-related and slow, rhythmic processes jointly structure sensory modulation and internal state control.