Baker S N, Olivier E, Lemon R N
Sobell Department of Neurophysiology, Institute of Neurology, London, UK.
Exp Brain Res. 1998 Dec;123(4):397-411. doi: 10.1007/s002210050585.
The motor cortex contains a distributed map of muscles, with a single muscle represented over a wide cortical area. We have searched for inter-connections between distant sites projecting to common muscles by delivering pairs of 20-microA single-pulse intracortical microstimuli (ICMS) to sites separated by 1.5-2 mm in the hand-area primary motor cortex of two macaque monkeys performing a precision grip task. The facilitation of hand- and forearm-muscle rectified EMG was measured. When stimuli were delivered simultaneously, responses were quantified using a technique to correct for non-linearities inherent in the use of averaged, rectified EMG. A spatial facilitation was seen for such simultaneous stimuli; however, it was of the same magnitude as that occurring when ICMS was paired with stimulation of corticospinal axons in the pyramidal tract (PT), so that it was likely to be spinal in origin. When two such distant sites were stimulated separated by a 10- or 20-ms delay, the second response scaled with the level of background EMG in the same way as a response to the PT stimulus. By contrast, when the same site was stimulated twice with these delays, the second response showed a facilitation compared with a similarly timed PT response. There would therefore appear to be a local facilitation of the cortical output at these intervals, which is not seen between distant sites. Antidromically identified pyramidal-tract neurones (PTNs) were recorded whilst stimuli were delivered to a cortical site, with a distance between stimulating and recording electrodes of also 1.5-2 mm. The most common response was a facilitation followed by a suppression. Six of eleven PTNs showed a facilitation in their discharge following this stimulation (maximum connection strength s=0.19), 8/11 showed a suppression (maximum s=0.16). It is concluded that powerful inter-connections do exist between distributed parts of the motor output and that there is widespread cortical activation after even a single ICMS pulse. However, these inter-connections do not lead to interactions between cortical outputs following stimulation, as assessed from the EMG. It is proposed that this is likely to reflect differences in the summation of output cells to local versus remote stimulation.
运动皮层包含一个肌肉的分布式图谱，单一肌肉在广泛的皮层区域中有所表征。我们通过向两只执行精确抓握任务的猕猴手部区域初级运动皮层中相距1.5 - 2毫米的位点施加成对的20微安单脉冲皮质内微刺激（ICMS），来寻找投射至共同肌肉的远距离位点之间的相互连接。测量了手部和前臂肌肉整流肌电图（EMG）的易化情况。当同时施加刺激时，使用一种技术对响应进行量化，以校正使用平均整流EMG时固有的非线性。对于这种同时施加的刺激，观察到了空间易化；然而，其幅度与ICMS与锥体束（PT）中皮质脊髓轴突刺激配对时出现的幅度相同，因此其起源可能是脊髓性的。当以10或20毫秒的延迟刺激两个这样的远距离位点时，第二个响应与背景EMG水平的缩放方式与对PT刺激的响应相同。相比之下，当以这些延迟对同一位点进行两次刺激时，与类似定时的PT响应相比，第二个响应显示出易化。因此，在这些间隔似乎存在皮质输出的局部易化，而在远距离位点之间未观察到。在将刺激施加到皮层位点时，记录了逆向识别的锥体束神经元（PTN），刺激电极与记录电极之间的距离也为1.5 - 2毫米。最常见的响应是先易化后抑制。11个PTN中有6个在此刺激后放电出现易化（最大连接强度s = 0.19），8/11显示出抑制（最大s = 0.16）。得出的结论是，运动输出的分布式部分之间确实存在强大的相互连接，并且即使在单个ICMS脉冲后也存在广泛的皮质激活。然而，从EMG评估来看，这些相互连接在刺激后不会导致皮质输出之间的相互作用。有人提出，这可能反映了输出细胞对局部与远程刺激总和的差异。
