Laurent G, Hustert R
Department of Zoology, University of Cambridge, England.
J Neurosci. 1988 Nov;8(11):4349-66. doi: 10.1523/JNEUROSCI.08-11-04349.1988.
During walking, the muscles of a leg undergo a typical sequence of activity, which is partly shaped by phasic sensory feedback. To assess the role played by such feedback, we characterized intracellularly the receptive fields of tarsal motor neurons in the locust Schistocerca gregaria and considered these receptive fields within the context of a step cycle. The depressor motor neurons, active during the stance phase, are excited by ventral tarsal contact or an imposed levation and are inhibited by dorsal contact or an imposed depression. Partial deafferentation of the anterior tarsus reduces this stance phase depressor activity. The levator motor neuron, active during the swing phase, has the opposite receptive field. The retractor unguis motor neurons, synergistic to the depressors, are, like them, excited by ventral contact but, like the levator, are inhibited by afferents which can signal the end of the stance phase. The inhibition of the retractors could constitute a preparation for the swing phase, by reducing the grip on the substrate. The motor neuronal receptive fields thus appear to support the patterns of muscular activity recorded during walking. Excitation of the motor neurons by extero- and proprioceptors is usually direct: hair, canal, campaniform, and chordotonal afferents all evoke 1:1 EPSPs in motor neurons after a central latency of 1-1.5 msec. Inhibition is indirect, as IPSPs occur at least 2 msec later than the EPSPs. The motor neurons of one pool have overlapping but not necessarily identical, receptive fields. Parallel, supplementary excitatory and inhibitory pathways involving nonspiking local interneurons also exist, which can allow gain control of a specific reflex. The weight given to a reflex response will thus depend, first, on the number of motor neurons in a pool affected by the stimulus and, second, on the existence and state of intercalated interneurons.
在行走过程中，腿部肌肉会经历一系列典型的活动，其中部分活动由阶段性感觉反馈塑造。为了评估这种反馈所起的作用，我们在细胞水平上对沙漠蝗（Schistocerca gregaria）跗节运动神经元的感受野进行了特征描述，并在一个步周期的背景下考虑这些感受野。在站立期活跃的降肌运动神经元，会因腹侧跗节接触或施加的抬起动作而兴奋，并因背侧接触或施加的下压动作而受到抑制。前跗节部分去传入神经会降低这种站立期降肌的活动。在摆动期活跃的提肌运动神经元，其感受野则相反。与降肌协同的收爪肌运动神经元，和降肌一样，会因腹侧接触而兴奋，但和提肌一样，会被能够指示站立期结束的传入神经抑制。收爪肌的抑制可能通过减少对底物的抓握，为摆动期做准备。因此，运动神经元的感受野似乎支持了行走过程中记录到的肌肉活动模式。外感受器和本体感受器对运动神经元的兴奋通常是直接的：毛感受器、管感受器、钟形感受器和弦音器传入神经在1 - 1.5毫秒的中枢潜伏期后，都会在运动神经元中引发1:1的兴奋性突触后电位（EPSP）。抑制是间接的，因为抑制性突触后电位（IPSP）比EPSP至少晚2毫秒出现。一个神经元池中的运动神经元具有重叠但不一定相同的感受野。还存在涉及非爆发性局部中间神经元的平行、补充性兴奋性和抑制性通路，这可以实现对特定反射的增益控制。因此，反射反应的权重首先取决于受刺激影响的神经元池中运动神经元的数量，其次取决于插入中间神经元的存在和状态。
