Baldissera Fausto G, Tesio Luigi
RetiredMilan, Italy.
Department of Biomedical Sciences for Health, Università degli Studi di MilanoMilan, Italy.
Front Hum Neurosci. 2017 Mar 31;11:152. doi: 10.3389/fnhum.2017.00152. eCollection 2017.
When rhythmically moving two limbs in either the same or in opposite directions, one coupling mode meets constraints that are absent in the other mode. Isodirectional (ISO) flexion-extensions of the ipsilateral hand and foot can be easily performed with either the hand prone or supine. Instead, antidirectional (ANTI) movements require attentive effort and irresistibly tend to reverse into ISO when frequency increases. Experimental evidence indicates that the direction dependent easy-difficult dichotomy is caused by interference of the anticipatory postural commands associated to movements of one limb with voluntary commands to the other limb. Excitability of the resting wrist muscles is subliminally modulated at the period of ipsilateral foot oscillations, being phase-opposite in the antagonists and distributed so as to facilitate ISO and obstacle ANTI coupling of the hand (either prone or supine) with the foot. Modulation is driven by cortical signals dispatched to the forearm simultaneously with the voluntary commands moving the foot. If right foot oscillations are performed when standing on the left foot with the right hand touching a fixed support, the subliminal excitability modulation is replaced by overt contractions of forearm muscles conforming the APAs features. This suggests that during hand-foot ANTI coupling the voluntary commands to forearm muscles are contrasted by APAs commands of opposite sign linked to foot oscillations. Correlation between the easy-difficult dichotomy and the APAs distribution is also found in coupled adduction-abduction of the arms or hands in the transverse plane and in coupled flexion-extension of the arms in the parasagittal plane. In all these movements, APAs commands linked to the movement of each limb reach the motor pathways to the contralateral muscles homologous to the prime movers and can interfere during coupling with their voluntary activation. APAs are also generated in postural muscles of trunk and lower limbs and size-increase when the movement frequency is incremented. The related increase in postural effort apparently contributes in destabilizing the difficult coupling mode. Motor learning may rely upon more effective APAs. APAs and focal contraction are entangled within the same voluntary action. Yet, neural diseases may selectively impair APAs, which represent a potential target for rehabilitation.
当有节奏地向相同或相反方向移动双侧肢体时，一种耦合模式会遇到另一种模式中不存在的限制。同侧手和脚的同向（ISO）屈伸动作，无论手是掌心向下还是向上都能轻松完成。相反，反向（ANTI）动作需要集中注意力，并且当频率增加时不可避免地倾向于转变为同向动作。实验证据表明，这种方向依赖性的难易二分法是由与一侧肢体运动相关的预期姿势指令对另一侧肢体的自主指令产生干扰所致。在同侧足部摆动期间，静息腕部肌肉的兴奋性会受到阈下调制，拮抗剂中的调制相位相反，且分布方式有助于手部（掌心向下或向上）与足部的同向耦合以及反向耦合。调制由与移动足部的自主指令同时发送到前臂的皮质信号驱动。当用右手触摸固定支撑物并以左脚站立时进行右脚摆动，阈下兴奋性调制会被符合预期姿势调整（APAs）特征的前臂肌肉明显收缩所取代。这表明在手足反向耦合期间，对前臂肌肉的自主指令会与与足部摆动相关的相反符号的预期姿势调整指令相互对抗。在横平面内手臂或手部的内收 - 外展耦合以及矢状旁平面内手臂的屈伸耦合中，也发现了难易二分法与预期姿势调整分布之间的相关性。在所有这些运动中，与每个肢体运动相关的预期姿势调整指令会到达与原动肌同源的对侧肌肉的运动通路，并且在耦合过程中可能会干扰其自主激活。预期姿势调整也会在躯干和下肢的姿势肌肉中产生，并且当运动频率增加时其大小会增加。姿势努力的相关增加显然有助于破坏困难的耦合模式。运动学习可能依赖于更有效的预期姿势调整。预期姿势调整和局部收缩在同一自主动作中相互交织。然而，神经疾病可能会选择性地损害预期姿势调整，而预期姿势调整是康复的潜在目标。
