Topographic organization of the cochlear spiral ganglion demonstrated by restricted lesions of the anteroventral cochlear nucleus.

The morphological organization of the central projections of the cat cochlear spiral ganglion into the cochlear nucleus has been investigated by creating restricted lesions in the anteroventral cochlear nucleus (AVCN) in order to ablate selectively either the lateral or the medial aspect of isofrequency projection laminae. Such lesions induced highly selective retrograde degeneration of spiral ganglion cells. Ablation of the lateral part of the AVCN resulted in degeneration of cells within the scala tympani portion of the ganglion, whereas medial lesions within the AVCN induced degeneration of the scala vestibuli portion of the ganglion. Since most, if not all, of the primary afferent axons of the cochlear nerve bifurcate into ascending and descending branches as they enter the brainstem, it is noteworthy that selective damage to the ascending branch in the AVCN was sufficient to induce retrograde degeneration of the spiral ganglion cell somata. The peripheral and central axons also degenerated, and the losses of both the radial nerve fibers in the osseous spiral lamina and the central axons passing into the modiolus displayed selective topographies that paralleled the cell loss within the spiral ganglion. The results of this study support our previous hypothesis, based upon earlier horseradish peroxidase labeling experiments, that there is a topographic organization to the projection of the spiral ganglion within the isofrequency laminae that is orthogonal to the frequency representation within the ventral cochlear nuclei (VCN). That is, in addition to the spiral frequency organization of the ganglion, represented by the dorsal-to-ventral frequency map in the VCN, there is also an orderly and sequential distribution of inputs from the vertical (scala tympani-to-scala vestibuli) dimension of the spiral ganglion across the lateral-to-medial axis of the VCN. The interaction of these two topographic representations, distributed across the three dimensions of the VCN, must partly define the selective and/or integrative neuronal response properties at this first level of central nervous system processing of auditory signals within the cochlear nuclei.