Asymmetric excitatory synaptic dynamics underlie interaural time difference processing in the auditory system.
Asymmetric excitatory synaptic dynamics underlie interaural time difference processing in the auditory system.
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DOI：
10.1371/journal.pbio.1000406
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发表时间：
2010-06-29
期刊：
影响因子：
9.8
通讯作者：
Rinzel J
中科院分区：
文献类型：
--
作者：
Jercog PE;Svirskis G;Kotak VC;Sanes DH;Rinzel J
为了定位环境中的声音，听觉系统检测并编码每只耳朵之间的信号差异。听觉脑干神经元对来自双耳的激励信号的上升时间的差异的灵敏度允许神经元对微秒级的耳间时间差进行编码。低频声音定位依赖于耳间时间差（ITD）的神经计算，并依赖于听觉脑干中的神经元，这些神经元整合了从双耳开始的同侧和对侧听觉通路传递的突触输入。对ITD有选择性反应的第一听觉神经元位于内侧上级橄榄核（MSO）。我们确定了一个新的机制ITD编码使用大脑切片准备，保留双耳输入的MSO。两个兴奋性通路存在内部潜伏期差异，如果不进行补偿，则ITD反应函数的位置远远超出生理范围，无法用于估计ITD。我们证明，并支持使用基于生物药理学的计算模型，兴奋性突触后电位（EPSP）斜率的双边不对称性提供了一个强大的补偿延迟机制，由于这些输入的低阈值钾电导的差异激活，并允许MSO神经元编码生理ITD。我们建议，更一般地说，尖峰概率的依赖率的去极化，在这些听觉神经元，提供了一个机制的时间顺序之间的歧视EPSP。动物可以通过检测声音到达两只耳朵的时间的微秒差异来定位声音的来源。编码这些耳间时间差（ITD）的神经元从每只耳朵接收兴奋性突触输入。它们可以用具有毫秒时间尺度的兴奋性突触执行微秒计算，因为它们对输入的“上升时间”非常敏感，即达到突触输入峰值所需的时间。目前的理论假设这两种输入的生物物理性质是相同的。我们挑战这一假设表明，由同侧耳驱动的兴奋性突触电位的上升时间比由对侧耳驱动的更快。此外，我们提出了一个计算模型，表明这种差异的上升时间，加上神经元的敏感性，兴奋的上升时间，可以赋予ITD编码微秒分辨率在生物相关的范围。我们的分析还解决了时间不匹配问题。对侧和同侧LATERAL之间的差异远大于相关ITD范围。我们展示了如何上升时间的差异补偿这种不匹配。概括地说，我们认为，相位放电神经元-那些迅速作出反应，但不是缓慢，不断变化的刺激-是有选择性的短暂输入的时间顺序。在重合检测计算中，当一个较快的输入领先于一个较慢的输入时，神经元将做出更稳健的响应，即使输入很短并且具有相似的幅度。
In order to localize sounds in the environment, the auditory system detects and encodes differences in signals between each ear. The exquisite sensitivity of auditory brain stem neurons to the differences in rise time of the excitation signals from the two ears allows for neuronal encoding of microsecond interaural time differences. Low-frequency sound localization depends on the neural computation of interaural time differences (ITD) and relies on neurons in the auditory brain stem that integrate synaptic inputs delivered by the ipsi- and contralateral auditory pathways that start at the two ears. The first auditory neurons that respond selectively to ITD are found in the medial superior olivary nucleus (MSO). We identified a new mechanism for ITD coding using a brain slice preparation that preserves the binaural inputs to the MSO. There was an internal latency difference for the two excitatory pathways that would, if left uncompensated, position the ITD response function too far outside the physiological range to be useful for estimating ITD. We demonstrate, and support using a biophysically based computational model, that a bilateral asymmetry in excitatory post-synaptic potential (EPSP) slopes provides a robust compensatory delay mechanism due to differential activation of low threshold potassium conductance on these inputs and permits MSO neurons to encode physiological ITDs. We suggest, more generally, that the dependence of spike probability on rate of depolarization, as in these auditory neurons, provides a mechanism for temporal order discrimination between EPSPs. Animals can locate the source of a sound by detecting microsecond differences in the arrival time of sound at the two ears. Neurons encoding these interaural time differences (ITDs) receive an excitatory synaptic input from each ear. They can perform a microsecond computation with excitatory synapses that have millisecond time scale because they are extremely sensitive to the input's “rise time,” the time taken to reach the peak of the synaptic input. Current theories assume that the biophysical properties of the two inputs are identical. We challenge this assumption by showing that the rise times of excitatory synaptic potentials driven by the ipsilateral ear are faster than those driven by the contralateral ear. Further, we present a computational model demonstrating that this disparity in rise times, together with the neurons' sensitivity to excitation's rise time, can endow ITD-encoding with microsecond resolution in the biologically relevant range. Our analysis also resolves a timing mismatch. The difference between contralateral and ipsilateral latencies is substantially larger than the relevant ITD range. We show how the rise time disparity compensates for this mismatch. Generalizing, we suggest that phasic-firing neurons—those that respond to rapidly, but not to slowly, changing stimuli—are selective to the temporal ordering of brief inputs. In a coincidence-detection computation the neuron will respond more robustly when a faster input leads a slower one, even if the inputs are brief and have similar amplitudes.
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