Raman, Nima2026-02-262026-02-262026-02-26http://hdl.handle.net/10393/51414https://doi.org/10.20381/ruor-31776The vestibular system is critical for regulating balance and postural maintenance in many vertebrates. The medial vestibular nuclei (MVN) in the brainstem are known to serve as a hub for receiving and relaying sensory information that is important for vestibular function. While considerable research has been dedicated to understanding how MVN regulate gaze stabilization and spatial orientation/perception, little is known about its connections with other brainstem nuclei that play critical roles for fine-tuning posture and balance. In this thesis, we identified a population of vasoactive intestinal peptide (VIP)-expressing neurons in the facial motor nucleus (FMN) of the brainstem that provide long-range inputs to MVN. Lower motoneurons in FMN have been shown to mediate specific orofacial and vibrissal muscle movements, but these FMN^VIP neurons have never been described. We employed advanced viral tracing methods, in vivo fiber photometry, and cell-type specific ablation and chemogenetic manipulations to study whether and how FMN^VIP neurons contribute to vestibular-mediated balance maintenance. Ablation or acute silencing of FMN^VIP neurons altered the animals' balance performance without affecting their gaze stabilization. Intriguingly, we found that these FMN^VIP neurons are cholinergic, rather than glutamatergic or GABAergic, and using anterograde monosynaptic labeling, we further identified that the major postsynaptic targets of FMN^VIP neurons are the GABAergic neurons in MVN. In vivo Ca²⁺ imaging of FMN^VIP neurons and ACh sensor recordings in MVN demonstrated that FMN^VIP neurons are activated only when the animal's balance is challenged, and they release ACh to modulate MVN neuronal activity and fine-tune body posture for balance maintenance. Lastly, cell-type specific rabies tracing revealed that the major input regions to FMN^VIP neurons arise from the reticular formation, a phylogenetically old brain structure that is known to regulate balance. Altogether, the work from this thesis unveils a new brainstem circuit pathway that bridge the reticular with vestibular nuclei to regulate balance and postural maintenance.enAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-nd/4.0/BrainstemFacial Motor NucleusVestibular NucleusVestibular BalanceVasoactive Intestinal Peptide (VIP) NeuronsThesis