Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale
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Medial olivocochlear (MOC) neurons project to outer hair cells (OHC), forming the efferent arm of a reflex that affects sound processing and offers protection from acoustic overstimulation. The central pathways that trigger the MOC reflex in response to sound are poorly understood. Insight into these pathways can be obtained by examining the responses of single MOC neurons recorded from anesthetized guinea pigs. ⋯ The MOC reflex interneurons are thus likely to be chopper units of PVCN. Also supporting this conclusion, chopper units and MOC neurons both have sharp frequency tuning. Thus, the most likely pathway for the sound-evoked MOC reflex begins with the responses of hair cells, proceeds with type I auditory-nerve fibers, PVCN chopper units, and MOC neurons, and ends with the MOC terminations on OHC.
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Fos-like immunoreactivity (FLI) was evaluated in auditory and nonauditory brain structures in hamsters that had been exposed previously to intense sound and tested behaviorally for tinnitus. The immunocytochemical results demonstrated a significant increase in exposed animals of FLI in auditory brain structures such as the lateral lemniscus, central nucleus of inferior colliculus, and auditory cortex, as well as in some nonauditory brain structures such as the locus coeruleus, lateral parabrachial nucleus, certain subregions of the hypothalamus, and amygdala. ⋯ This is consistent with the hypothesis that FLI induced by intense sound exposure might represent a neural correlate of tinnitus or of plasticity associated with tinnitus. The possibility and the mechanisms underlying the increased FLI are discussed.