PROJECT SUMMARY/ABSTRACTTongue muscles which are innervated by hypoglossal motoneurons (XIIMNs) are critical for survival given theirrole in suckling swallowing mastication breathing and more advanced functions such as human speech. Thehypoglossal motor nucleus is a bilateral collection of seven separate motoneuron pools with motoneurons ineach pool innervating one of the seven different tongue muscles. We recently showed that XIIMNs innervatingthe superior longitudinalis and genioglossus tongue muscles of neonatal rats have significantly different restingmembrane potentials action potential firing thresholds and f-I curves i.e. the change in firing rate as a functionof injected current. These findings raise three very important questions: 1) what is the extent and nature ofphenotypic diversity both within and between individual XIIMN pools? 2) what are the anatomic and ionicmechanisms that underlie this phenotypic diversity? 3) do structural and functional differences among XIIMNs ineach pool map to unique gene expression profiles? We propose a rational and robust approach to address thesequestions: specifically to describe the morphology intrinsic membrane properties and the transcriptome ofmuscle specific XIIMNs. Our initial targets are XIIMNs innervating the genioglossus hyoglossus and superiorlongitudinalis muscles as each muscle has different effects on tongue movement. Muscle-specific XIIMNs willbe identified by injecting each of the muscles with a retrograde tracer conjugated to a fluorescent reporter. Allexperiments use brain tissue from neonatal rats 5-12 days of age. Key techniques include neuroanatomic tracingto define neuron morphology immunohistochemistry whole cell patch clamp electrophysiology and next-generation RNA sequencing. These basic science studies will identify unique molecular targets associated withfunctional and/or structural differences between the motoneuron pools. Without this fundamental informationinterventions aimed at stimulating or inhibiting the activity of specific tongue muscles will be imprecise and mayresult in unintended outcomes. In contrast specific knowledge of unique molecular targets will focus thedevelopment of therapeutic approaches aimed at stimulation and/or inhibition of specific tongue muscles.Preliminary data show several pool-specific differences in motoneuron function and gene expression stronglysuggesting that the proposed work will provide truly novel data on the anatomic physiologic and molecularunderpinnings of phenotypic diversity within and between muscle-specific hypoglossal motoneuron pools. Thisin turn will lead to a major leap in our understanding of how the tongue muscles perform complex coordinatedbehaviors such as suckling swallowing and defense of the upper airway during sleep and will lay the foundationfor the development of therapies aimed at controlling the activity of specific tongue muscles.