Figure 7.

Reconstructed auditory nerve fibers. A–C: Fluorescent photomicrographs show three coronal sections through the PVCN (#45; Table 1). AN fibers labeled with rhodamine (magenta; 27.0 kHz injection) or fluorescein (green; 48.5 kHz injection) can be seen passing through the dorsal region of the PVCN in each section. The location of fibers in each section was approximated by outlining the extent of visible label (rhodamine: dashed-yellow; fluorescein: dashed-cyan). Solid lines show the computed medial axes, which bisect each outline and are used to represent the location of labeled AN fibers. Values at bottom right designate relative distance from the posterior edge of the CN. D: 3D reconstruction of AN fibers from all sections, aligned and positioned within the template CN. Each “ribbon” corresponds to the medial axis computed from a single section. Ribbons for both rhodamine (magenta; 27.0 kHz) and fluorescein (green; 48.5 kHz) are shown. Black arrowheads indicate the positions of the three sections shown in A–C; leftmost arrowheads correspond to panel A. The fibers follow a stereotyped trajectory, entering as part of the auditory nerve root from the ventral surface and traversing dorsally before bifurcating, giving rise to the ascending and descending branches of the AN. There is a systematic relationship between AN position and frequency, with fibers tuned to higher frequencies (in this case, green), having a more dorsomedial position compared to those fibers tuned to lower frequencies. The CN is presented from a medial viewpoint.