Young E D, Sachs M B
Department of Biomedical Engineering and Center for Hearing Sciences, 505 Traylor Building, Johns Hopkins University, 720 Rutland Avenue, Baltimore, MD 21205, USA.
Neuroscience. 2008 Jun 12;154(1):127-38. doi: 10.1016/j.neuroscience.2008.01.036. Epub 2008 Feb 5.
The strength of synapses between auditory nerve (AN) fibers and ventral cochlear nucleus (VCN) neurons is an important factor in determining the nature of neural integration in VCN neurons of different response types. Synaptic strength was analyzed using cross-correlation of spike trains recorded simultaneously from an AN fiber and a VCN neuron in anesthetized cats. VCN neurons were classified as chopper, primarylike, and onset using previously defined criteria, although onset neurons usually were not analyzed because of their low discharge rates. The correlograms showed an excitatory peak (EP), consistent with monosynaptic excitation, in AN-VCN pairs with similar best frequencies (49% 24/49 of pairs with best frequencies within +/-5%). Chopper and primarylike neurons showed similar EPs, except that the primarylike neurons had shorter latencies and shorter-duration EPs. Large EPs consistent with end bulb terminals on spherical bushy cells were not observed, probably because of the low probability of recording from one. The small EPs observed in primarylike neurons, presumably spherical bushy cells, could be derived from small terminals that accompany end bulbs on these cells. EPs on chopper or primarylike-with-notch neurons were consistent with the smaller synaptic terminals on multipolar and globular bushy cells. Unexpectedly, EPs were observed only at sound levels within about 20 dB of threshold, showing that VCN responses to steady tones shift from a 1:1 relationship between AN and VCN spikes at low sound levels to a more autonomous mode of firing at high levels. In the high level mode, the pattern of output spikes seems to be determined by the properties of the postsynaptic spike generator rather than the input spike patterns. The EP amplitudes did not change significantly when the presynaptic spike was preceded by either a short or long interspike interval, suggesting that synaptic depression and facilitation have little effect under the conditions studied here.
听神经（AN）纤维与蜗腹侧核（VCN）神经元之间突触的强度，是决定不同反应类型的VCN神经元神经整合性质的一个重要因素。利用在麻醉猫中同时记录的AN纤维和VCN神经元的峰电位序列的互相关分析突触强度。VCN神经元根据先前定义的标准被分类为斩波型、初级样和起始型，不过起始型神经元由于放电率低通常未被分析。相关图显示，在最佳频率相似的AN - VCN对中（最佳频率在±5%范围内的对中有49%，即24/49），存在一个与单突触兴奋一致的兴奋性峰值（EP）。斩波型和初级样神经元表现出相似的EP，只是初级样神经元的潜伏期更短，EP持续时间更短。未观察到与球形布什细胞上终球终末一致的大EP，可能是因为记录到一个的概率很低。在初级样神经元（推测为球形布什细胞）中观察到的小EP，可能源自这些细胞上伴随终球的小终末。斩波型或有切迹的初级样神经元上的EP与多极和球形布什细胞上较小的突触终末一致。出乎意料的是，仅在阈值约20 dB范围内的声级下观察到EP，这表明VCN对稳态音调的反应从低声级时AN和VCN峰电位之间的1:1关系转变为高声级时更自主的发放模式。在高声级模式下，输出峰电位的模式似乎由突触后峰电位发生器的特性而非输入峰电位模式决定。当突触前峰电位之前有短或长的峰电位间隔时，EP幅度没有显著变化，这表明在此研究条件下突触抑制和易化作用很小。
