A dynamic landscape of flexible respiratory control

The rhythm and pattern of breathing arises from a spatially distributed and bilaterally synchronized neuronal column in the ventrolateral medulla called the Ventral Respiratory Column (VRC). Recently, we used high density recordings using Neuropixels to show that the activity of these populations evolve through rotational dynamics on a low-dimensional latent trajectory (Bush and Ramirez, 2024). We further showed that the offset of inspiration is a temporally salient feature of that latent space such that trajectories target a conserved region of that space on each breath. This phenomenon is reminiscent of the classical idea of the inspiratory-off switch. What remains unclear is how these latent trajectories are shaped and constrained by constituent circuit components and inputs to the VRC. Here we combine high-density electrophysiology with optogenetic and physiological manipulations to dissect the responses of the population trajectories to these perturbations. Multiple region-specific cell types within the brainstem are known to alter respiratory rate. Activation of NMB + neurons in the retrotrapezoid nucleus (RTN) mediates the hypercapnic ventilatory response to increase breathing rate, activation of inhibitory preBötC populations can slow or speed up breathing in a phase specific manner, and activation of mechanoreceptive vagal afferents causes abrupt apneas (i.e., the Hering-Breuer reflex). Stimulation of NMB + RTN neurons evokes an increase in the speed of the latent trajectory to increase respiratory rate, most notably during inspiration, with minimal effect on the trajectory path. In contrast, the expected respiratory rate increase during inspiratory-timed stimulation of inhibitory preBötC neurons is mediated by a short-cutting of the trajectory path rather than increased speed. This alteration of trajectory is characterized by forcing the network into a basin of attraction that inexorably leads to the offset of inspiration and advancement to the next breath. In conjunction, prolonged inhibitory stimulation clamps the network in that inspiration-off region. Importantly, this trajectory short-cut and clamp near the inspiratory-off region is also observed when triggering the Hering-Breuer reflex with positive airway pressure. We then show that optogenetic activation of inhibitory neurons in the Nucleus of the Solitary Tract (NTS), putative pump cells, recapitulates the phase-dependent breathing modulation observed during direct preBötC inhibitory activation. This inhibitory NTS activation evokes near-identical responses in the VRC population trajectories as both the positive airway pressure and the direct preBötC inhibitory activation, suggesting that these different perturbations trigger similar or identical response in the VRC population. Together, these data point to convergent, network-level control mechanisms that allow for flexible breathing, and highlight the critical involvement of ongoing sensory feedback for maintenance of normal, physiological respiratory activity. R01 HL126523 F32HL159904 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.