Kuba Hiroshi, Yamada Rei, Fukui Iwao, Ohmori Harunori
Department of Physiology, Faculty of Medicine, Kyoto University, Kyoto 606-8501, Japan.
J Neurosci. 2005 Feb 23;25(8):1924-34. doi: 10.1523/JNEUROSCI.4428-04.2005.
The interaural time difference (ITD) is a cue for localizing a sound source along the horizontal plane and is first determined in the nucleus laminaris (NL) in birds. Neurons in NL are tonotopically organized, such that ITDs are processed separately at each characteristic frequency (CF). Here, we investigated the excitability and coincidence detection of neurons along the tonotopic axis in NL, using a chick brainstem slice preparation. Systematic changes with CF were observed in morphological and electrophysiological properties of NL neurons. These properties included the length of dendrites, the input capacitance, the conductance of hyperpolarization-activated current, and the EPSC time course. In contrast to these gradients, the conductance of low-threshold K+ current and the expression of Kv1.2 channel protein were maximal in the central (middle-CF) region of NL. As a result, the middle-CF neuron had the smallest input resistance and membrane time constant, and consequently the fastest EPSP, and exhibited the most accurate coincidence detection. The specialization of middle-CF neurons as coincidence detectors may account for the high resolution of sound-source localization in the middle-frequency range observed in avians.
双耳时间差（ITD）是在水平面定位声源的一个线索，并且首先在鸟类的层状核（NL）中确定。NL中的神经元呈音频拓扑组织，这样ITD在每个特征频率（CF）处被分别处理。在这里，我们使用鸡脑干切片标本研究了NL中沿音频拓扑轴的神经元的兴奋性和重合检测。在NL神经元的形态和电生理特性中观察到了随CF的系统性变化。这些特性包括树突长度、输入电容、超极化激活电流的电导以及兴奋性突触后电流（EPSC）的时程。与这些梯度相反，低阈值钾电流的电导和Kv1.2通道蛋白的表达在NL的中央（中频CF）区域最大。结果，中频CF神经元具有最小的输入电阻和膜时间常数，因此其兴奋性突触后电位（EPSP）最快，并且表现出最精确的重合检测。中频CF神经元作为重合检测器的特化可能解释了在鸟类中观察到的中频范围内声源定位的高分辨率。
