Sumbre Germán, Fiorito Graziano, Flash Tamar, Hochner Binyamin
Department of Neurobiology, Institute of Life Sciences, Hebrew University, Jerusalem, Israel.
Curr Biol. 2006 Apr 18;16(8):767-72. doi: 10.1016/j.cub.2006.02.069.
One of the key problems in motor control is mastering or reducing the number of degrees of freedom (DOFs) through coordination. This problem is especially prominent with hyper-redundant limbs such as the extremely flexible arm of the octopus. Several strategies for simplifying these control problems have been suggested for human point-to-point arm movements. Despite the evolutionary gap and morphological differences, humans and octopuses evolved similar strategies when fetching food to the mouth. To achieve this precise point-to-point-task, octopus arms generate a quasi-articulated structure based on three dynamic joints. A rotational movement around these joints brings the object to the mouth . Here, we describe a peripheral neural mechanism-two waves of muscle activation propagate toward each other, and their collision point sets the medial-joint location. This is a remarkably simple mechanism for adjusting the length of the segments according to where the object is grasped. Furthermore, similar to certain human arm movements, kinematic invariants were observed at the joint level rather than at the end-effector level, suggesting intrinsic control coordination. The evolutionary convergence to similar geometrical and kinematic features suggests that a kinematically constrained articulated limb controlled at the level of joint space is the optimal solution for precise point-to-point movements.
运动控制中的关键问题之一是通过协调来掌握或减少自由度（DOF）的数量。这个问题在超冗余肢体中尤为突出，比如章鱼极其灵活的手臂。对于人类的点对点手臂运动，已经提出了几种简化这些控制问题的策略。尽管存在进化差距和形态差异，但人类和章鱼在将食物送到嘴边时进化出了相似的策略。为了完成这个精确的点对点任务，章鱼的手臂基于三个动态关节生成了一种准关节结构。围绕这些关节的旋转运动将物体带到嘴边。在这里，我们描述了一种外周神经机制——两波肌肉激活相互向对方传播，它们的碰撞点确定了中间关节的位置。这是一种根据物体被抓握的位置来调整节段长度的非常简单的机制。此外，与某些人类手臂运动类似，在关节水平而非末端执行器水平观察到了运动学不变量，这表明存在内在的控制协调。向相似几何和运动学特征的进化趋同表明，在关节空间层面受运动学约束的关节肢体是精确点对点运动的最优解决方案。
