Earhart G M, Stein P S
Department of Biology and Program in Movement Science, Washington University, St. Louis, Missouri 63130, USA.
J Neurophysiol. 2000 Nov;84(5):2181-90. doi: 10.1152/jn.2000.84.5.2181.
The turtle generates a variety of coordinated hindlimb movements, including different forms of locomotion and scratching. The intact turtle produces forward step, forward swim, and backpaddle. Following spinal cord transection, rostral, pocket, and caudal scratches can be evoked by mechanical stimulation of the shell. Comparisons of the kinematics and motor patterns of these six behaviors provide insights regarding neuronal mechanisms underlying their production. All six behaviors were characterized by alternating hip flexion and extension and by an event during which force was exerted against a substrate. The portion of the cycle occupied by hip flexion or extension movement varied across behaviors. Hip extension occupied well over half the cycle period in the forward step and the caudal scratch. The cycle was split into approximately half hip flexion and half hip extension for the forward swim, the backpaddle, and the rostral scratch. Hip flexion occupied over half the cycle in the pocket scratch. The swim and scratch forms had curvilinear, crescent-shaped toe trajectories and a single burst of monoarticular knee extensor activity during each cycle. The forward step had a linear toe trajectory and two bursts of knee extensor activity during each cycle, one during swing and one during stance. Timing of monoarticular knee extensor onset was similar for: the forward swim, the rostral scratch, and the swing phase burst of forward step; the pocket scratch and the stance phase burst of forward step; and the backpaddle and the caudal scratch. Amplitudes of muscle activity varied among the six behaviors; high amplitudes of activity were associated with events during which force was exerted against a substrate. These times of force exertion were: stance phase in the forward step, powerstroke in the forward swim and the backpaddle, and rubs of the limb against the shell in the scratch forms. The six behaviors studied represent a range of parameter values, as evidenced by relative durations of hip flexion to hip extension, knee extensor phasing, and electromyogram (EMG) amplitudes. This range of behaviors could be produced by assembling different combinations of neurons from a common pool, with all six behaviors likely sharing some basic circuitry. The extent of shared circuitry may be greater between behaviors with similar timing, e.g., backpaddle and caudal scratch.
乌龟会产生多种协调的后肢运动，包括不同形式的 locomotion 和抓挠动作。完整的乌龟会做出向前迈步、向前游动和向后划水动作。脊髓横断后，通过对龟壳进行机械刺激可诱发头部、口袋状和尾部抓挠动作。对这六种行为的运动学和运动模式进行比较，有助于深入了解其产生的神经元机制。所有这六种行为的特征都是髋部交替屈伸，以及存在肢体对底物施加力的过程。髋部屈伸运动在整个周期中所占的比例因行为而异。在向前迈步和尾部抓挠动作中，髋部伸展占据了超过半个周期的时间。对于向前游动、向后划水和头部抓挠动作，整个周期大致被分为髋部屈伸各占一半。在口袋状抓挠动作中，髋部屈曲占据了超过半个周期的时间。游动和抓挠动作的脚趾轨迹呈曲线形、月牙状，且每个周期中有一次单关节伸膝肌的爆发式活动。向前迈步的脚趾轨迹呈直线形，且每个周期中有两次伸膝肌的爆发式活动，一次在摆动期，一次在支撑期。单关节伸膝肌开始活动的时间在以下几种情况中相似：向前游动、头部抓挠以及向前迈步的摆动期爆发；口袋状抓挠以及向前迈步的支撑期爆发；向后划水和尾部抓挠。六种行为的肌肉活动幅度各不相同；高活动幅度与肢体对底物施加力的过程相关。这些施加力的时刻分别为：向前迈步的支撑期、向前游动和向后划水的动力冲程，以及抓挠动作中肢体与龟壳的摩擦。所研究的这六种行为代表了一系列参数值，髋部屈伸的相对持续时间、伸膝肌的相位以及肌电图（EMG）幅度都证明了这一点。通过组合来自共同神经元池的不同组合，可能产生这一系列行为，所有这六种行为可能共享一些基本的神经回路。在时间安排相似的行为之间，例如向后划水和尾部抓挠，共享神经回路的程度可能更高。
