Frequency representation and spiral ganglion cell density in the cochlea of the gerbil Pachyuromys duprasi.

The tonotopic map of the cochlea in the gerbil Pachyuromys duprasi was analysed by local iontophoretic HRP-application into physiologically defined regions of the cochlear nucleus and mapping of subsequent HRP transport patterns in cochlear spiral ganglion cells. Furthermore the spiral ganglion cell density along the cochlear duct was determined. The cochlear tonotopic map was established in the frequency range between 0.6 and 17.5 kHz. These frequencies corresponded to locations between 86 and 3% basilar membrane length (0% = cochlear base). It was found that the slope of the place-frequency map varied with frequency, the maximum slope being found between 1 and 4 kHz. This frequency range corresponds to the frequency range of highest auditory sensitivity as determined from cochlear microphonic recordings (Plassmann et al., 1987). The density of spiral ganglion cells also varied along the cochlear duct. A pronounced maximum (1927 cells/mm) was located at around 70% basilar membrane length, compared to values of 800 cell per mm near the cochlear apex and base. This region of high ganglion cell density also corresponds to the frequency range of highest auditory sensitivity.