In the mammalian auditory system, information pertaining to hearing enters the brain by way of the auditory nerve. The axons of two types of primary neurons, type I and type II spiral ganglion cells, compose the nerve and terminate in the cochlear nucleus. In a general way, the function of the cochlear nucleus is to receive incoming auditory nerve activity, to preserve or transform the signals, and to distribute outgoing activity to higher brain centers. Our long-term objectives are (1) to describe the neuronal circuitry in the cochlear nucleus and (2) to understand the mechanisms underlying the early stages of acoustic signal processing in the brain. Knowledge of these issues will depend substantially on the organization of auditory nerve input to the cochlear nucleus. Intracellular recording and staining methods will be used to label individual type I auditory nerve fibers. The marking of single fibers with horseradish peroxidase (HRP) after first characterizing their response properties allows a direct comparison between a fiber's response features, its axonal and synaptic morphology and its connections with other neurons. Because of the difficulty in recording from type II auditory nerve fibers, we will concentrate our analysis on their light and electron microscopic features and compare them to those of type I fibers. Type II fibers will be labeled using extracellular injections of HRP in the auditory nerve or spiral ganglion. We will test the hypothesis that type II fibers exhibit systematic differences in their synaptic morphology and connections compared to type I fibers. The proposed studies should generate new information regarding the anatomical foundations of stimulus coding and neural circuitry in the auditory nerve and cochlear nucleus. The data will also have relevance to broader issues in sensory neurobiology and could be especially important for the design of prosthetic devices that replace nonfunctioning cochleas or auditory nerves.
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