Origin of cochlea efferents in some gerbil species. A comparative anatomical study with fluorescent tracers

The distribution and the projection patterns of olivo-cochlear neurons (OCN) was studied in three species of gerbils (Meriones unguiculatus, Meriones tristrami, Pachyuromys duprasi) using fluorescent tracers (Fast blue, Diamidino yellow) in double labeling experiments. In all gerbil species up to 1600 retrogradely labeled neurons were found in the superior olivary complex. Small OCN were located inside or at the margins of the lateral superior olive. A second set of larger OCN was seen bilaterally in the periolivary region of the superior olivary complex forming an independent cell population within the trapezoid body. The exact locations of small and large OCN varied among the particular species. Quantitative results suggested that a gradual loss of large OCN might be a general feature of species with highly specialized frequency selectivity.