Armstrong D M, Marple-Horvat D E
Department of Physiology, School of Medical Sciences, University of Bristol, England, U.K.
Can J Physiol Pharmacol. 1996 Apr;74(4):443-55.
An account is given of the current state of knowledge of the contributions of the cerebellum and the forelimb motor cortex (MC) to the neural control of walking movements in the cat. The main emphasis is on information obtained by recording from single MC and cerebellar neurones in chronically instrumented cats engaged in walking on the rungs of a horizontal ladder, a form of locomotion that is heavily dependent on visual input and for which the integrity of MC is essential. Evidence from the authors' laboratory and from other studies is presented which establishes that MC neurones, including pyramidal tract neurones, show higher levels of activity during ladder walking than during overground walking (i.e., when less constraint exists over the locus of footfall) and that this increase is greatest in late swing-early stance in the contralateral forelimb, consistent with one role of MC being to help determine the locus of footfall. However, many MC neurones develop peak activity at other times in the step cycle, and a comparison with recordings during treadmill walking suggests MC may also help regulate stance duration when walking speed is an important performance variable. Recordings from Purkinje cells and cerebellar nuclear neurones show that during ladder walking step-related activity is widespread in the vermal, paravermal, and crural regions of cortex and in the interposed and dentate nuclei. Nuclear cell activity is so timed that it could be contributing to producing the locomotor rhythms evident in MC cells, although this is not yet proven. Results are also presented and discussed relating to MC and cerebellar neuronal responses that occur when a step onto an unstable rung results in an unexpected external perturbation of the forelimb step cycle. MC responses begin with onset latency as short as 20 ms so that MC may assist spinal reflex mechanisms to produce a post hoc compensatory change in motor output. However, work in progress suggests that corresponding responses in paravermal cerebellum are weak and infrequent, so provisionally it seems that the MC responses are initiated via pathways that do not pass through the cerebellum. By contrast, current work involving a paradigm in which a ladder rung is motor driven to a new position as the animal approaches (thereby providing a visual cue that an adaptive change in gait will soon be required) is revealing in lateral cerebellar neurones, including dentate neurones, changes in discharge that are time locked to the execution of an adapted pace. In addition, there are prominent earlier responses, which begin at short latency after the onset of rung movement. These apparently visual responses have characteristics that encourage the speculation that they may represent a cerebellar signal that "primes for action" other more directly motor regions of the central nervous system.
本文阐述了小脑和前肢运动皮层（MC）对猫行走运动神经控制作用的当前知识状态。主要重点是通过对长期植入仪器的猫进行水平梯级行走时，记录单个MC和小脑神经元所获得的信息，这种运动形式严重依赖视觉输入，且MC的完整性至关重要。作者实验室和其他研究的证据表明，包括锥体束神经元在内的MC神经元，在梯级行走时的活动水平高于地面行走（即对落脚点限制较少时），且这种增加在对侧前肢的摆动后期至站立早期最为显著，这与MC的一个作用是帮助确定落脚点一致。然而，许多MC神经元在步周期的其他时间达到峰值活动，与跑步机行走时的记录比较表明，当行走速度是一个重要的性能变量时，MC也可能有助于调节站立持续时间。浦肯野细胞和小脑核神经元的记录表明，在梯级行走期间，与步相关的活动广泛分布于小脑蚓部、旁蚓部和小腿区域以及间位核和齿状核。核细胞活动的时间安排可能有助于产生MC细胞中明显的运动节律，尽管这尚未得到证实。本文还展示并讨论了有关当踏上不稳定梯级导致前肢步周期意外外部扰动时，MC和小脑神经元反应的结果。MC反应的起始潜伏期短至20毫秒，因此MC可能协助脊髓反射机制在事后产生运动输出的补偿性变化。然而，正在进行的研究表明，旁蚓部小脑的相应反应微弱且不常见，所以暂时看来，MC反应是通过不经过小脑的途径启动的。相比之下，目前涉及一种范式的研究，即当动物接近时将梯级电机驱动到新位置（从而提供一个视觉线索，表明很快需要对步态进行适应性改变），正在揭示外侧小脑神经元（包括齿状神经元）的放电变化，这些变化与适应步伐的执行时间锁定。此外，还有明显更早的反应，在梯级运动开始后短潜伏期就开始。这些明显的视觉反应具有一些特征，促使人们推测它们可能代表一种小脑信号，该信号“为行动做好准备”，作用于中枢神经系统其他更直接的运动区域。
