[Activities of single cochlear nerve fibers in rats]

Activities of 325 single cochlear nerve fibers in response to pure tones at various frequencies including ultrasonic sounds were observed in rats. The stimulus sounds were measured with a probe tube inserted into an ear canal and a microphone and expressed in dB SPL. The best frequency (BF, characteristic frequency) and the threshold at BF were determined in every fiber. The measured BF ranged from 0.58 kHz to 62.6 kHz. BF and the threshold at BF of a fiber with the lowest BF-threshold were 27.49kHz and 6dB SPL respectively. The highest sensitivity was found in fibers with BF from 20 to 50 kHz. The tuning curve rose from BF and its slope was steeper for the high side than the low one as observed in other animals. The sharpness of the tuning curve was expressed by QN values, where QN is the ratio of BF to the band width at N dB above the BF threshold. Mean and the variance of Q10dB, Q20dB, Q30dB, Q40dB, Q50dB, all increased as the BF did. Almost all fibers had spontaneous discharge. The discharge rate increased with the increase in sound intensity and saturated at about 30 dB above the threshold. The auditory threshold curve obtained from distribution of the lowest BF-threshold of 325 fibers was consistent with the audiograms determined behaviorally.