FROM SOUND TO MOVEMENT: THE NEURAL BACKBONE OF THE ACOUSTIC STARTLE REFLEX

The acoustic startle reflex (ASR) is a fast, reactive body movement in response to a loud and unexpected auditory stimulus that has been observed in all mammalian species. In rats, cochlear root neurons (CRNs) – located between auditory periphery and cochlear nucleus (CN) – have been identified as the first neural substrate of the underlying reflex circuit. CRNs receive excitatory input from primary afferent spiral ganglion neurons (SGNs). Their axons mainly target neurons of the contralateral pontine reticular nucleus (PnC) formation, which project onto motor neurons. CRNs are subject to diverse modulatory inputs from higher brain areas. Despite their central role in a reflex circuit directly implicated with individual survival, CRNs are poorly characterized in regards to their cellular excitability and activity patterns, as well as their molecular composition and morphology in situ. Therefore, we set out to characterize CRNs in different species regarding their morphology, their pattern and identity of afferent as well as efferent somatic innervation, as well as their counts and spatial distribution between modiolar base and CN. For these purposes, we combined immunofluorescent labelings with either whole cochlea tissue clearing or high-resolution confocal microscopy using a novel intact cochlea+CN preparation. We further employed genetic reporter mice, neuronal tracer dyes, and immunohistochemical stainings to visualize the afferent and efferent functional inputs onto CRNs soma. Finally, we are currently establishing tissue preparations for functional analyses. In this way, we hope to shed light onto the primary neural base of the ASR.