This research will identify the innervation patterns and synaptic organization that underlie binaural and monaural processing in the auditory brainstem. The present proposal will focus specifically on the pathways that underlie binaural hearing, those from the cochlear nucleus (CN) to the inferior colliculus (IC) via the superior olivary complex (SOC). Each experiment will use intracellular-like anterograde filling and/or intracellular injections of identified neurons in a brain slice. Each experiment will identify structure/function relationships in the medial superior olive (MSO), in the IC, or in the covering ascending pathways to the IC.
Aim 1. To determine how the innervation and synaptic organization of MSO create binaural time sensitivity. The neural circuity from the anteroventral CN (AVCN) to the MSO will be visualized. The experiments will: (A) Completely fill the axons from the AVCN with dextran and completely reconstruct their projections to the SOC. (B) Use a brain slice to fill neurons in MSO whose axons project to the IC and study their innervation by AVCN axons labeled with dextran. There may be rostra-caudal differences in axon branching, length, diameter, myelination, and topography that may underlie the delay-line mechanism.
Aim 2. To determine how the MSO transfer intermural time differences (ITD) information to the IC. A topographical projection from MSO to the IC is hypothesized. Within this target zone, the synaptic organization of the IC may play an important role in maintaining ITD sensitivity. To test this: (A) Injections of anterograde markers in the MSO will determine the topography of the projection to the IC. Recordings of IC neurons in vivo will relate the responses to the terminal fields of MSO axons. (B) Intracellular filling will identify specific neuron types in the IC and their innervation by MSO afferent will be determined in a brain slice preparation. The projection from MSO may provide anatomical evidence for a spatial map of azimuth in the IC. This projection may be segregated into only part of the low-frequency IC, and it may not innervate each cell type similarly.
Aim 3. To establish how the innervation and synaptic organization of IC integrates the MSO inputs with the binaural and monaural inputs from other sources. Injections of two anterograde markers will be made: one in MSO and a second in another brainstem nucleus. MSO inputs may remain separate from other inputs from contralateral LSO, contralateral AVCN, and contralateral DNLL. These experiments will demonstrate which afferent systems coverage and which remain segregated in the IC.
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