Spontaneous Discharge Patterns in Cochlear Spiral Ganglion Cells Before the Onset of Hearing in Cats

Abstract

Spontaneous neural activity has been recorded in the auditory nerve of cats as early as 2 days postnatal (P2), yet individual auditory neurons do not respond to ambient sound levels <90–100 dB SPL until about P10. Significant refinement of the central projections from the spiral ganglion to the cochlear nucleus occurs during this neonatal period. This refinement may be dependent on peripheral spontaneous discharge activity. We recorded from single spiral ganglion cells in kittens aged P3–P9. The spiral ganglion was accessed through the round window through the spiral lamina. A total of 112 ganglion cells were isolated for study in nine animals. Spike rates in neonates were very low, ranging from 0.06 to 56 spikes/s, with a mean of 3.09 ± 8.24 spikes/s. Ganglion cells in neonatal kittens exhibited remarkable repetitive spontaneous bursting discharge patterns. The unusual patterns were evident in the large mean interval CV (CVi = 2.9 ± 1.6) and burst index of 5.2 ± 3.5 across ganglion cells. Spontaneous bursting patterns in these neonatal mammals were similar to those reported for cochlear ganglion cells of the embryonic chicken, suggesting this may be a general phenomenon that is common across animal classes. Rhythmic spontaneous discharge of retinal ganglion cells has been shown to be important in the development of central retinotopic projections and normal binocular vision. Bursting rhythms in cochlear ganglion cells may play a similar role in the auditory system during prehearing periods.