Guinan J J, Gifford M L
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge.
Hear Res. 1988 Dec;37(1):29-45. doi: 10.1016/0378-5955(88)90075-5.
In order to study the effects of efferent activity, olivocochlear efferents were stimulated with an electrode in the fourth ventricle at the decussation of the crossed olivocochlear bundle (midline-OCB stimulation) or with an electrode at the brainstem origin of medial efferents (MOC stimulation). Tuning curves, or similar measures of threshold, were obtained from auditory-nerve fibers in the presence or absence of efferent stimulation. Efferent stimulation raised the thresholds of fibers for tones at the characteristic frequency (CF) by an amount which varied with the spontaneous rate (SR) of the auditory-nerve fiber. On the average, high-SR fibers had the smallest threshold shifts, and low-SR fibers had the largest threshold shifts. The distribution of threshold shifts as a function of CF peaked at CFs of 3-8 kHz for high-SR and medium-SR fibers but appeared to peak at higher CFs for low-SR fibers. Within the high-SR or medium-SR groups, the fibers with the lowest thresholds had the largest threshold shifts. Efferent stimulation decreased the Q20 of the tuning curves from most fibers (i.e. it made the tuning curves wider), but increased the Q20 from some fibers with CFs below 2 kHz. For fibers with CFs above 4 kHz, efferent stimulation shifted the tuning-curve tails to higher sound levels by about 1 dB on the average. The qualitative patterns of the effects due to midline-OCB stimulation or to MOC stimulation were similar. The distribution of high-SR threshold shifts vs. CF appears to be displaced apically in the cochlea compared to the distribution of MOC endings on outer hair cells. This can be understood in terms of efferent activity depressing basilar membrane motion and affecting regions at, and apical to, the activated efferent synapses. To explain the low-SR threshold shifts, an additional way in which efferent activity inhibits responses appears to be required. The data are consistent with one function of the medial efferents being to raise the thresholds of auditory-nerve fibers and thereby adjust the effective range of the auditory system.
为了研究传出活动的影响，用位于第四脑室交叉橄榄耳蜗束交叉处的电极刺激橄榄耳蜗传出神经（中线 - 橄榄耳蜗束刺激），或用位于内侧传出神经脑干起源处的电极刺激（MOC刺激）。在有或没有传出刺激的情况下，从听神经纤维获取调谐曲线或类似的阈值测量值。传出刺激使听神经纤维在特征频率（CF）处对纯音的阈值升高，升高幅度随听神经纤维的自发放电率（SR）而变化。平均而言，高SR纤维的阈值变化最小，低SR纤维的阈值变化最大。高SR和中SR纤维的阈值变化分布作为CF的函数在3 - 8 kHz的CF处达到峰值，但低SR纤维似乎在更高的CF处达到峰值。在高SR或中SR组内，阈值最低的纤维阈值变化最大。传出刺激降低了大多数纤维调谐曲线的Q20（即使调谐曲线变宽），但增加了一些CF低于2 kHz的纤维的Q20。对于CF高于4 kHz的纤维，传出刺激使调谐曲线尾部平均向更高声级移动约1 dB。中线 - 橄榄耳蜗束刺激或MOC刺激产生的效应的定性模式相似。与外毛细胞上MOC末梢的分布相比，高SR阈值变化与CF的分布在耳蜗中似乎向顶端移位。这可以从传出活动抑制基底膜运动并影响激活的传出突触处及顶端区域来理解。为了解释低SR阈值变化，似乎需要一种传出活动抑制反应的额外方式。这些数据与内侧传出神经的一个功能是提高听神经纤维的阈值从而调整听觉系统的有效范围相一致。
