Neubauer Heinrich, Köppl Christine, Heil Peter
Leibniz Institute for Neurobiology, 39118 Magdeburg, Germany.
J Neurophysiol. 2009 Jun;101(6):3169-91. doi: 10.1152/jn.90779.2008. Epub 2009 Apr 8.
In vertebrate auditory systems, the conversion from graded receptor potentials across the hair-cell membrane into stochastic spike trains of the auditory nerve (AN) fibers is performed by ribbon synapses. The statistics underlying this process constrain auditory coding but are not precisely known. Here, we examine the distributions of interspike intervals (ISIs) from spontaneous activity of AN fibers of the barn owl (Tyto alba), a nocturnal avian predator whose auditory system is specialized for precise temporal coding. The spontaneous activity of AN fibers, with the exception of those showing preferred intervals, is commonly thought to result from excitatory events generated by a homogeneous Poisson point process, which lead to spikes unless the fiber is refractory. We show that the ISI distributions in the owl are better explained as resulting from the action of a brief refractory period ( approximately 0.5 ms) on excitatory events generated by a homogeneous stochastic process where the distribution of interevent intervals is a mixture of an exponential and a gamma distribution with shape factor 2, both with the same scaling parameter. The same model was previously shown to apply to AN fibers in the cat. However, the mean proportions of exponentially versus gamma-distributed intervals in the mixture were different for cat and owl. Furthermore, those proportions were constant across fibers in the cat, whereas they covaried with mean spontaneous rate and with characteristic frequency in the owl. We hypothesize that in birds, unlike in mammals, more than one ribbon may provide excitation to most fibers, accounting for the different proportions, and that variation in the number of ribbons may underlie the variation in the proportions.
在脊椎动物听觉系统中，毛细胞膜上的分级受体电位转换为听神经（AN）纤维的随机尖峰序列是由带状突触完成的。这一过程背后的统计学原理限制了听觉编码，但尚不完全清楚。在这里，我们研究了仓鸮（Tyto alba）听神经纤维自发放电的峰峰间隔（ISI）分布，仓鸮是一种夜间捕食性鸟类，其听觉系统专门用于精确的时间编码。除了那些具有偏好间隔的纤维外，听神经纤维的自发放电通常被认为是由均匀泊松点过程产生的兴奋性事件导致的，这些事件会引发尖峰，除非纤维处于不应期。我们发现，猫头鹰的ISI分布更适合解释为一个短暂不应期（约0.5毫秒）对由均匀随机过程产生的兴奋性事件的作用结果，其中事件间隔的分布是指数分布和形状因子为2的伽马分布的混合，两者具有相同的缩放参数。之前已证明相同的模型适用于猫的听神经纤维。然而，猫和猫头鹰混合中指数分布与伽马分布间隔的平均比例不同。此外，猫的纤维中这些比例是恒定的，而在猫头鹰中它们与平均自发放电率和特征频率共同变化。我们推测，与哺乳动物不同，在鸟类中，可能有不止一个带状突触为大多数纤维提供兴奋，这解释了比例的差异，并且带状突触数量的变化可能是比例变化的基础。
