Art J J, Crawford A C, Fettiplace R, Fuchs P A
J Physiol. 1985 Mar;360:397-421. doi: 10.1113/jphysiol.1985.sp015624.
Intracellular recordings were made from turtle cochlear hair cells in order to study the changes in their tuning properties resulting from electrical stimulation of the efferent axons. Efferent stimulation caused a reduction in the amplitude of the receptor potential at the hair cell's most sensitive or characteristic frequency, an increased amplitude at frequencies more than an octave below the characteristic frequency, and no change at very high frequencies. These differential effects resulted in a broadening of each cell's tuning curve, which, during maximal efferent stimulation degenerated from a sharply tuned resonance to a critically damped low-pass filter. Efferent alterations in tuning were also inferred from the oscillations in membrane potential produced by acoustic clicks or extrinsic currents. The quality factor (Q) of tuning, derived from the decay of the oscillations, was progressively reduced with synaptic hyperpolarizations up to about 5 mV in amplitude. A consequence of efferent action was that the wave forms of transient pressure changes were more faithfully encoded as changes in hair cell membrane potential. Hyperpolarization of a hair cell by steady current injection resulted in a lowering of its characteristic frequency and quality factor, and an increase in steady-state resistance. By comparison, for a given reduction in quality factor, efferent stimulation was associated with a smaller change in characteristic frequency. This difference is expected if the resonance is also damped by the shunting action of the synaptic conductance. Perfusion with perilymphs containing 0.5-15 mM of the potassium channel blocker, tetraethylammonium bromide (TEA) reduced the hair cell's frequency selectivity, whether assayed acoustically or with extrinsic currents. Lower TEA concentrations abolished the efferent inhibitory post-synaptic potential with only a minor change in tuning. TEA produced other effects different from efferent stimulation including (i) a lowering of the characteristic frequency, and (ii) a highly asymmetric receptor potential. These observations suggest that the efferents do not simply block membrane conductances associated with tuning. We conclude that the efferent modification of the shape of the tuning curve may be a composite result of the synaptic conductance and the hyperpolarization of the hair cell membrane.
为了研究传出轴突电刺激引起的龟耳蜗毛细胞调谐特性的变化，对其进行了细胞内记录。传出刺激导致毛细胞最敏感或特征频率处的感受器电位幅度降低，在低于特征频率一个倍频程以上的频率处幅度增加，而在非常高的频率处没有变化。这些差异效应导致每个细胞的调谐曲线变宽，在最大传出刺激期间，调谐曲线从尖锐调谐的共振退化为临界阻尼的低通滤波器。传出调谐的改变也可从声点击或外部电流产生的膜电位振荡中推断出来。从振荡衰减得出的调谐品质因数（Q）随着突触超极化幅度增加到约5mV而逐渐降低。传出作用的一个结果是，瞬态压力变化的波形更忠实地编码为毛细胞膜电位的变化。通过稳定电流注入使毛细胞超极化会导致其特征频率和品质因数降低，以及稳态电阻增加。相比之下，对于品质因数的给定降低，传出刺激与特征频率的较小变化相关。如果共振也因突触电导的分流作用而被阻尼，那么这种差异是可以预期的。用含有0.5 - 15 mM钾通道阻滞剂溴化四乙铵（TEA）的外淋巴灌注会降低毛细胞的频率选择性，无论是通过声学检测还是外部电流检测。较低浓度的TEA消除了传出抑制性突触后电位，而调谐仅有轻微变化。TEA产生了与传出刺激不同的其他效应，包括（i）特征频率降低，以及（ii）高度不对称的感受器电位。这些观察结果表明，传出神经并非简单地阻断与调谐相关的膜电导。我们得出结论，调谐曲线形状的传出修饰可能是突触电导和毛细胞膜超极化的综合结果。
