Central trajectories of type II spiral ganglion cells from various cochlear regions in mice.

Type II spiral ganglion cells provide the afferent pathway from outer hair cells in the mammalian cochlea to neurons in the cochlear nucleus. The present study compares the projection patterns of type II fibers originating from spiral ganglion cells of apical, middle and basal cochlear regions in mice. Fibers were labeled by extracellular injections of horseradish peroxidase into the spiral ganglion. Type II fibers from all regions displayed many 'en passant' swellings (mean = 95) and had very few terminal swellings (mean = 6); fibers from the base had significantly more swellings than those from the apex. Type II fibers traveled into the cochlear nucleus together with type I fibers labeled by the same injection, and both types bifurcated in a cochleotopic manner. The bifurcations formed ascending and descending branches that traveled initially with type I branches in the magnocellular regions of the cochlear nucleus. Type II fibers differed from type I branches in that many fibers subsequently distributed collaterals and terminals to granule cell regions and to the boundaries of these regions that typically do not receive type I input. This projection into the granule cell regions depended on cochlear origin: ascending branches of type II fibers from the cochlea apex did not usually terminate in granule cell regions, whereas those from the base often ended in these regions. Descending branches of type II fibers from all regions, however, projected to the granule cell regions, particularly the granule cell lamina between ventral and dorsal cochlear nucleus. These observations suggest that afferent information from outer hair cells reaches a wide area of the magnocellular parts of the cochlear nucleus in a cochleotopic fashion, and reaches granule cell regions with a less distinct cochleotopic mapping.