The course and distribution of medial efferent fibers in the cochlea of the mustached bat

Abstract

The course and distribution of medial olivocochlear (MOC) nerve fibers were studied in the cochlea of the mustached bat. This animal is of interest because of the very sharp tuning of the ear and fine frequency resolution in small frequency bands near 60 and 90 kHz. The MOC fibers arise from about 400 cells in the dorsomedial periolivary (DMPO) nucleus and they are distributed to approximately 4500 outer hair cells (OHCs), resulting in an average OHC unit size of 11.25. Individual fibers appear to have a small number of branches and each branch entering the tunnel of Corti terminates on a patch of OHCs. The patch size is typically 1–3 OHCs with the smallest average patch sizes in the regions tuned to 60 and 90 kHz. The majority of the MOC terminals are derived from the contralateral DMPO. Contralateral vs. ipsilateral projecting fibers are not preferentially distributed within any of the three rows of OHCs or within specific regions throughout most of the cochlea. It can be concluded that the main differences between the mustached bat's MOC system and that of most other mammals are: (1) origin from a single nucleus; (2) relatively small sizes of the patches; (3) a single terminal on each OHC; (4) a gradient in the size of the terminals but not in the number of terminals from row to row or from base to apex.