An anatomical description of descending auditory projections from cortex

This dissertation provides an anatomical description of descending corticofugal projections to the inferior colliculus (IC) and superior olivary complex (SOC) of the guinea pig. Two aspects of corticofugal projections to the brainstem are examined: (1) experiments to identify brainstem circuits contacted by cortical axons, and (2) experiments to identify corticofugal collaterals to multiple targets.; In the first series of experiments, injections of an anterograde tracer into temporal cortex labeled axons bilaterally in the SOC. Corticofugal terminations were observed in almost all of the SOC nuclei. Additional combined anterograde and retrograde tracing experiments indicated that cortical axons are likely to contact olivary cells that project to the inferior colliculus.; The IC is also a direct target of cortical projections. We used multiple retrograde tracers to determine whether ascending and descending projections from the IC arise from the same cells. The results indicate that different populations of IC cells project to the thalamus and to the cochlear nucleus. We then combined anterograde and retrograde tracing techniques to demonstrate that cortical projections to the IC are likely to contact colliculogeniculate cells. The results of this first series of experiments suggest that cortical projections to the IC, as well as cortical projections to the SOC, directly contact ascending auditory pathways in the brainstem.; The last series of experiments identified cortical cells that project to multiple targets. Injection of different tracers into the left and right IC labeled many cortical cells. The results suggest two patterns of projections: (1) an ipsilateral projection that originates from many cortical cells and, (2) a bilateral projection that originates from a smaller number of cells and sends axonal branches to both colliculi. It is likely that these two projection patterns serve different functions.