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Axonal site of spike initiation enhances auditory coincidence detection

机译：刺突起始轴突部位增强听觉符合检测

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Neurons initiate spikes in the axon initial segment or at the first node in the axon(1-4). However, it is not yet understood how the site of spike initiation affects neuronal activity and function. In nucleus laminaris of birds, neurons behave as coincidence detectors for sound source localization and encode interaural time differences (ITDs) separately at each characteristic frequency (CF)(5-7). Here we show, in nucleus laminaris of the chick, that the site of spike initiation in the axon is arranged at a distance from the soma, so as to achieve the highest ITD sensitivity at each CF. Na+ channels were not found in the soma of high-CF (2.5-3.3 kHz) and middle-CF (1.0-2.5 kHz) neurons but were clustered within a short segment of the axon separated by 20-50 mu m from the soma; in low-CF (0.4-1.0 kHz) neurons they were clustered in a longer stretch of the axon closer to the soma. Thus, neurons initiate spikes at a more remote site as the CF of neurons increases. Consequently, the somatic amplitudes of both orthodromic and antidromic spikes were small in high-CF and middle-CF neurons and were large in low-CF neurons. Computer simulation showed that the geometry of the initiation site was optimized to reduce the threshold of spike generation and to increase the ITD sensitivity at each CF. Especially in high-CF neurons, a distant localization of the spike initiation site improved the ITD sensitivity because of electrical isolation of the initiation site from the soma and dendrites, and because of reduction of Na+-channel inactivation by attenuating the temporal summation of synaptic potentials through the low-pass filtering along the axon.
机译：神经元在轴突起始节段或轴突的第一个节点（1-4）处引发尖峰。然而，尚不清楚刺突起始位点如何影响神经元活动和功能。在鸟类的椎板层核中，神经元充当声源定位的巧合检测器，并分别在每个特征频率（CF）处对耳间时间差（ITD）进行编码（5-7）。在这里，我们显示出，在雏鸡的椎板核中，轴突的刺突起始位置与躯体相距一定距离，从而在每个CF处获得最高的ITD敏感性。在高CF（2.5-3.3 kHz）和中CF（1.0-2.5 kHz）神经元的体中未发现Na +通道，但聚集在轴突的短段中，该轴突与体的间隔为20-50μm。在低CF（0.4-1.0 kHz）神经元中，它们聚集在轴突较长的轴突附近，靠近躯体。因此，随着神经元CF的增加，神经元会在更远的位置启动尖峰。因此，在高CF和中CF神经元中，正畸和反峰峰值的体细胞振幅均较小，而在低CF神经元中则较大。计算机仿真表明，对起始位点的几何形状进行了优化，以降低尖峰生成的阈值并提高每个CF处的ITD灵敏度。尤其是在高CF神经元中，尖峰起始位点的远处定位提高了ITD灵敏度，这是因为起始位点与躯体和树突之间的电隔离，以及由于通过削弱突触电位的时间总和而减少了Na +通道失活。通过沿轴突的低通滤波。

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