Phase Locking Asymmetries at Flexor-Extensor Transitions during Fictive Locomotion

The motor output for walking is produced by a network of neurons termed the spinal central pattern generator (CPG) for locomotion. The basic building block of this CPG is a half-center oscillator composed of two mutually inhibitory sets of interneurons, each controlling one of the two dominant phases of locomotion: flexion and extension. To investigate symmetry between the two components of this oscillator, we analyzed the statistics of natural variation in timing during fictive locomotion induced by stimulation of the midbrain locomotor region in the cat. As a complement to previously published analysis of these data focused on burst and cycle durations, we present a new analysis examining the strength of phase locking at the transitions between flexion and extension. Across our sample of nerve pairs, phase locking at the transition from extension to flexion (E to F) is stronger than at the transition from flexion to extension (F to E). This pattern did not reverse when considering bouts of fictive locomotion that were flexor vs. extensor dominated, demonstrating that asymmetric locking at the transitions between phases is dissociable from which phase dominates cycle duration. We also find that the strength of phase locking is correlated with the mean latency between burst offset and burst onset. These results are interpreted in the context of a hypothesis where network inhibition and intrinsic oscillatory mechanisms make distinct contributions to flexor-extensor alternation in half-center networks.行走的运动输出由被称为脊髓中枢模式发生器（spinal central pattern generator, CPG）的神经元网络所产生。该CPG的基本构成单元为半中心振荡器（half-center oscillator），其由两组相互抑制的中间神经元集群组成，分别控制运动的两个主要时相：屈曲（flexion）与伸展（extension）。为探究该振荡器两个组成部分间的对称性，我们分析了猫中脑运动区刺激诱导的拟运动活动（fictive locomotion）期间，时序自然波动的统计特征。作为此前针对该数据集、聚焦于爆发电位（burst）与周期时长的已发表分析的补充，我们开展了一项新的分析，用于检测屈曲与伸展转换阶段的相位锁相（phase locking）强度。在我们的神经配对样本中，从伸展到屈曲的转换阶段的相位锁相强度高于从屈曲到伸展的转换阶段（即E→F强于F→E）。当考虑以屈肌为主导与伸肌为主导的拟运动活动时段时，该模式并未发生反转，这表明相位转换阶段的不对称锁相与主导周期时长的时相是相互独立的。我们还发现，相位锁相强度与爆发电位终止至下一次爆发电位起始的平均潜伏期呈相关关系。我们结合相关假说对上述结果进行了解释：该假说认为，网络抑制与内在振荡机制对半中心网络中屈肌-伸肌交替活动具有不同的调控作用。