Department of Neuroscience, Center for Learning and Memory, The University of Texas at Austin Austin, TX, USA.
Front Neural Circuits. 2014 May 15;8:49. doi: 10.3389/fncir.2014.00049. eCollection 2014.
The medial superior olive (MSO) senses microsecond differences in the coincidence of binaural signals, a critical cue for detecting sound location along the azimuth. An important component of this circuit is provided by inhibitory neurons of the medial and lateral nuclei of the trapezoid body (MNTB and LNTB, respectively). While MNTB neurons are fairly well described, little is known about the physiology of LNTB neurons. Using whole cell recordings from gerbil brainstem slices, we found that LNTB and MNTB neurons have similar membrane time constants and input resistances and fire brief action potentials, but only LNTB neurons fire repetitively in response to current steps. We observed that LNTB neurons receive graded excitatory and inhibitory synaptic inputs, with at least some of the latter arriving from other LNTB neurons. To address the relative timing of inhibition to the MSO from the LNTB versus the MNTB, we examined inhibitory responses to auditory nerve stimulation using a slice preparation that retains the circuitry from the auditory nerve to the MSO intact. Despite the longer physical path length of excitatory inputs driving contralateral inhibition, inhibition from both pathways arrived with similar latency and jitter. An analysis of paired whole cell recordings between MSO and MNTB neurons revealed a short and reliable delay between the action potential peak in MNTB neurons and the onset of the resulting IPSP (0.55 ± 0.01 ms, n = 4, mean ± SEM). Reconstructions of biocytin-labeled neurons showed that MNTB axons ranged from 580 to 858 μm in length (n = 4). We conclude that while both LNTB and MNTB neurons provide similarly timed inhibition to MSO neurons, the reliability of inhibition from the LNTB at higher frequencies is more constrained relative to that from the MNTB due to differences in intrinsic properties, the strength of excitatory inputs, and the presence of feedforward inhibition.
中脑上橄榄复合体（MSO）感知双侧信号的微秒差异，这是检测沿方位角的声音位置的关键线索。该电路的一个重要组成部分由梯形体的内侧核和外侧核（分别为 MNTB 和 LNTB）的抑制性神经元提供。虽然 MNTB 神经元已经得到了相当详细的描述，但关于 LNTB 神经元的生理学知之甚少。我们使用来自沙鼠脑干切片的全细胞膜片钳记录，发现 LNTB 和 MNTB 神经元具有相似的膜时间常数和输入电阻，并产生短暂的动作电位，但只有 LNTB 神经元在响应电流步长时会重复放电。我们观察到 LNTB 神经元接收分级兴奋性和抑制性突触输入，其中至少有一些来自其他 LNTB 神经元。为了确定来自 LNTB 而非 MNTB 的抑制作用相对于 MSO 的相对时间，我们使用保留了听觉神经到 MSO 完整回路的切片制备物检查了对听觉神经刺激的抑制反应。尽管驱动对侧抑制的兴奋性输入的物理路径长度较长，但来自这两种途径的抑制作用具有相似的潜伏期和抖动。对 MSO 和 MNTB 神经元之间的成对全细胞膜片钳记录的分析表明，MNTB 神经元中的动作电位峰值与随后产生的 IPSP 的起始之间存在短暂而可靠的延迟（0.55 ± 0.01 ms，n = 4，平均值 ± SEM）。对生物胞素标记神经元的重建表明，MNTB 轴突的长度范围为 580 至 858 μm（n = 4）。我们得出的结论是，尽管 LNTB 和 MNTB 神经元都向 MSO 神经元提供了定时相同的抑制作用，但由于内在特性、兴奋性输入的强度以及前馈抑制的存在，来自 LNTB 的抑制作用在较高频率下的可靠性受到限制。
