The mammalian ear is connected to the brain via four types of nerve fibers: two afferent types carry sensory input to the brain, while two efferent types carry feedback control from the brain. The overall goal of our research effort is to understand the role(s) of each fiber type in audition: our current understanding of these roles is rudimentary in many areas. The present application includes one aim directed at each of the four fiber types. 1) Type-I afferents are subdivided into spontaneous-rate (SR) groups, which differ in threshold sensitivity. We will use intracellular labeling to study the central projections of these fibers, testing the hypothesis that an SR-based spatial organization within the cochlear nucleus (CN) is superimposed on its frequency-based organization, thereby allowing CN cell-types (and the higher centers to which they project) to sample activity from different SR groups according to the nature of the decoding operations performed. 2) Response properties of type-11 afferents from outer hair cells (OHCS) have never been studied, both because their axons are small and because they are few in number. We will use a drug (carboplatin) to selectively eliminate the large type-I population and then apply electrode technology for recording from unmyelinated fibers. The resultant type-II recordings will allow a test of the hypothesis that type-11's mediate the sensation of auditory pain. 3) Olivocochlear (OC) efferent fibers to OHCs comprise a sound-evoked reflex, and chronic de-efferentation greatly increases vulnerability to permanent noise-induced hearing loss (NIHL). Interestingly, the strength of the OC reflex varies among normal individuals, and so does the vulnerability to NIHL. We will test the hypothesis that """"""""tough"""""""" ears are those with the most active OC reflex and that """"""""toughening"""""""" of the ear seen with long-term, moderate-level acoustic exposure arises via an amplification of the OC reflex. Since OC reflex strength can be measured non-invasively, a NIHL vulnerability screen for humans could be devised based on our findings. 4) The OC fibers to the inner hair cell area are unmyelinated, and their response properties and functional effects are completely unknown, largely because their axons are difficult to excite electrically. Our preliminary results suggest that this part of the OC pathway may be excitable via electric activation of the inferior colliculus. We will pursue this approach to elucidate the peripheral effects these OC efferents. Such information is fundamental to an understanding of the functional role of the entire OC system.
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