Cochlear Nerve Activity After Intense Sound Exposure in Neonatal Chicks. Saunders, James C., Daryl E. Doan, Christopher P. Poje and Kimberly A. Fisher. Department of Otorhinolaryngology: Head and Neck Surgery and Department of Bioengineering, University of Pennsylvania, Division of Otolaryngology, Children's, Hospital of Philadelphia, Philadelphia, Pennsylvania l9l04.
APStracts 3:0059N, 1996.
SUMMARY AND CONCLUSIONS
1. Single neuron behavior in the cochlear nerve of neonatal (3-day old) chicks was examined following exposure to a 120 dB SPL pure tone (0.9 kHz) for 48 hours. Exposed animals were tested after 0-days or 12-days of recovery. Non- exposed chicks, age matched to the exposed animals, formed two control groups. 2. Spectral response plots were obtained from each cell. These plots described the neuron discharge rates in response to 1767 tone burst stimuli, each with a unique frequen cy/intensity combination. The tone bursts were presented at frequencies between 0.1 and 4.5 kHz and for intensities between 0 and 100 dB SPL. From these plots the characteristic frequency (CF), CF threshold, and Q 10 dB were derived for each cell. Frequency response area functions at selected stimulus levels, and rate-intensity functions at the CF were also constructed from the spectral response plots. In addition, spontaneous activity was deter mined. Data were obtained from 903 cells. 3. Neuron activity in the control cells revealed no differences between CF thresholds, Q 10 dB , or spontaneous activity in the two age groups. However, age differences at all frequencies were noted in the rate-intensity functions. 4. A frequency dependent loss in CF threshold was observed in the 0-day recovered cells. The threshold shift (relative to age-matched control cells) was 55-65 dB between 0.8-1.5 kHz, but only 10-15 dB between 0.1-0.4 kHz and 2.5-3.5 kHz. The exposed cells showed no loss in frequency selectivity (Q 10 dB ) below 0.5 kHz, while above this frequency an increasing deterioration in tuning was noted. Spontaneous activity in the 0-day cells was suppressed across the entire range of CFs. The rate-intensity function of exposed cells had a steeper growth rate than that of control cells. 5. At 12-days of recovery CF threshold, Q 10 dB , and spontaneous activity all recov ered to the levels exhibited by age-matched control cells. However, the rate-intensity function for cells with CF's between 0.8 and 1.0 kHz showed abnormal growth and higher discharge rates at saturation than the control cells. Outside of this frequency range the rate-intensity functions of control and exposed cells were similar to each other. 6. Recovery of function in the sound-damaged chick ear is accompanied by almost complete repair of the basilar papilla. The tectorial membrane, however, retains a major defect and only the lower layer of this membrane regenerates. An important observation in this presentation was the abnormal rate-intensity functions (in the 12-day recovered cells) reported for frequencies served by that region of the sensory epithelium where the tectorial membrane defect was found. This observation may be related to sustained struc tural damage to the short hair cell region of the papilla and/or alterations in the efferent control of papilla function mediated by the short hair cells.

Received 26 July 1995; accepted in final form 15 March 1996.
APS Manuscript Number J482-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 27 March 96