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theses

Many structural features of spiral ganglion neurons are specifically designed to carry out frequency-specific coding. Specific morphological properties correspond to tonotopic position along the cochlea contour, which include soma area and axon length. To examine how these features are regulated, we used various preparations. Thus, soma area is altered in a predictable manner corresponding to the frequency location of its peripheral target tissue. The sensory endorgan alters the axon characteristics of the spiral ganglion neurons. The regenerated axons have peripheral nervous system-like (PNS) and central nervous system-like (CNS) axon profiles that enable the examination of putative PNS/CNS axon length ratios. Similar to their in vivo patterns, the putative peripheral axons regenerated from low frequency neurons extend their processes a longer distance than high frequency neurons. This pattern is altered when the neuronal explants are paired with peripheral target tissue isolated from different cochlear regions. To identify the regulatory factors that control these structural features, we investigated brain derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3). These neurotrophins control the ion channel composition that mediates the neuronal firing patterns (Adamson et al., 2002) and the synaptic protein levels (Flores-Otero et al., 2007) in spiral ganglion neurons. For example, the exposure to BDNF transformed neurons isolated from the apical or basal region phenotypic properties to resemble basal neurons. In contrast, the exposure to NT3 transformed the phenotypic properties of both apical and basal neurons to resemble apical neurons. We investigated whether soma area and axon length conforms to the same regulatory mechanism. In the neuronal explant cultures, exposure to BDNF or NT3 had limited or no influence on cell size. Moreover, the effect of the peripheral target tissue on axon length is modulated by BDNF and NT3. However, both neurotrophins have a differential result on the axon length of spiral ganglion neurons isolated from distinct cochlear regions. In all, frequency specific signals within the organ of Corti direct the structural phenotype of spiral ganglion neurons. These results indicate a different regulatory mechanism is needed to alter the structural phenotype within the spiral ganglion.

ETD_5930

Ph.D.

Includes bibliographical references

by Felicia L. Smith

doi:10.7282/T31V5GKB

ETD doctoral

The author owns the copyright to this work.