Liang F, Rouiller E M, Wiesendanger M
Institute of Physiology, University of Fribourg, Switzerland.
Somatosens Mot Res. 1993;10(1):51-61. doi: 10.3109/08990229309028823.
In rats, a rostral and a caudal forelimb motor area (RFA and CFA, respectively) have been distinguished on the basis of intracortical microstimulation effects (see Neafsey et al., 1986, for a review). The goal of the present study was to assess and compare their relative connectional strength with target motor units of the forelimb. This was achieved by averaging modulation responses of sustained electromyographic (EMG) activity triggered by single intracortical microstimuli (S-ICMS) of relatively low intensity (mostly below 35 microA) to minimize both direct and transsynaptic current spread. In chronically prepared and ketamine-sedated rats, this method produced prominent peaks and troughs in the averaged EMG at short latencies with S-ICMS currents as low as 5 microA. S-ICMS at 30-50 microA in CFA sometimes even elicited visible twitches and an EMG burst of the contralateral wrist or digits following each stimulation pulse. Increasing S-ICMS currents to about 1.5 mA revealed a sudden shortening of EMG response latencies, which was most likely induced by current spread to brainstem motor centers. S-ICMS at near-threshold intensity in the majority of effective sites in both CFA and RFA produced modulation responses in more than one group of forelimb muscles, frequently also including muscles of the ipsilateral forelimb. Usually the ipsilateral responses were weaker, as were the suppression effects. Comparison of CFA and RFA revealed similar effects in terms of the number of modulated muscle groups and the response latencies. In contralateral wrist/digit muscles, facilitation responses were elicited at latencies of 9.7 +/- 1.8 msec (CFA) and 9.6 +/- 1.9 msec (RFA), with the shortest latencies around 6 msec. However, modulations by S-ICMS in RFA had significantly smaller amplitudes, had slower rates of buildup, and required higher thresholds than those obtained from S-ICMS in CFA. It is concluded, on the basis of the S-ICMS method, that both the CFA and the RFA exert a prominent and relatively direct influence on forelimb motoneurons. The present findings, together with calculations of conduction time, suggest that a contingent of corticospinal axons of the rat has oligosynaptic and possibly even monosynaptic connections with forelimb motoneurons. The recruitment of a relatively large number of muscles, including those of the ipsilateral forelimb, by S-ICMS in both areas may be explained by the prominent divergence of corticospinal axons. Further investigations are required to understand the relative positions and roles of the two areas in motor control and their possible homology with primary and nonprimary motor areas of primates.
在大鼠中，根据皮质内微刺激效应，已区分出一个嘴侧和一个尾侧前肢运动区（分别为RFA和CFA）（有关综述，请参阅Neafsey等人，1986年）。本研究的目的是评估并比较它们与前肢目标运动单位的相对连接强度。这是通过对由相对低强度（大多低于35微安）的单个皮质内微刺激（S - ICMS）触发的持续肌电图（EMG）活动的调制反应进行平均来实现的，以尽量减少直接和经突触的电流扩散。在长期制备且用氯胺酮镇静的大鼠中，该方法在S - ICMS电流低至5微安时，能在平均EMG中产生短潜伏期的明显峰谷。CFA中30 - 50微安的S - ICMS有时甚至在每次刺激脉冲后引发可见的抽搐以及对侧手腕或手指的EMG爆发。将S - ICMS电流增加到约1.5毫安时，EMG反应潜伏期突然缩短，这很可能是由电流扩散到脑干运动中枢所致。在CFA和RFA的大多数有效位点，接近阈值强度的S - ICMS在不止一组前肢肌肉中产生调制反应，通常还包括同侧前肢的肌肉。同侧反应通常较弱，抑制效应也是如此。CFA和RFA的比较显示，在调制肌肉群数量和反应潜伏期方面有相似的效应。在对侧手腕/手指肌肉中，促进反应在9.7±1.8毫秒（CFA）和9.6±1.9毫秒（RFA）的潜伏期出现，最短潜伏期约为6毫秒。然而，RFA中S - ICMS产生的调制幅度明显较小，建立速率较慢，且比CFA中S - ICMS所需的阈值更高。基于S - ICMS方法得出结论，CFA和RFA对前肢运动神经元都有显著且相对直接的影响。本研究结果，连同传导时间的计算，表明大鼠的一部分皮质脊髓轴突与前肢运动神经元有少突触甚至可能是单突触连接。两个区域中S - ICMS对包括同侧前肢肌肉在内的相对大量肌肉的募集，可能由皮质脊髓轴突的显著发散来解释。需要进一步研究以了解这两个区域在运动控制中的相对位置和作用，以及它们与灵长类动物初级和非初级运动区可能的同源性。
