Earhart G M, Stein P S
Department of Biology, Program in Movement Science, Washington University, St. Louis, Missouri 63130, USA.
J Neurophysiol. 2000 Jan;83(1):156-65. doi: 10.1152/jn.2000.83.1.156.
Turtles with a complete transection of the spinal cord just posterior to the forelimb enlargement at the D2-D3 segmental border produced coordinated rhythmic hindlimb movements. Ipsilateral stimulation of cutaneous afferents in the midbody shell bridge evoked a rostral scratch. Electrical stimulation of the contralateral dorsolateral funiculus (DLF) at the anterior cut face of the D3 segment activated a forward swim. Simultaneous stimulation of the ipsilateral shell bridge and the contralateral DLF elicited a scratch-swim hybrid: a behavior that blended features of both rostral scratch and forward swim into each cycle of rhythmic movement. This is the first demonstration of a scratch-locomotion hybrid in a spinal vertebrate. The rostral scratch and the forward swim shared some characteristics: alternating hip flexion and extension, similar timing of knee extensor activity within the hip cycle, and a behavioral event during which force was exerted against a substrate. During each cycle, each behavior exhibited three sequential stages, preevent, event, and postevent. The rostral scratch event was a rub of the foot against the stimulated shell site. The forward swim event was a powerstroke, a hip extension movement with the foot held in a vertical position with toes and webbing spread. The two behaviors differed with respect to several features: amount of hip flexion and extension, electromyogram (EMG) amplitudes, and EMG duty cycles. Scratch-swim hybrids displayed two events, the scratch rub and the swim powerstroke, within each cycle. Hybrid hip flexion excursion, knee extensor EMGs, and hip flexor EMGs were similar to those of the scratch; hybrid hip extension excursion and hip extensor EMGs were similar to those of the swim. The hybrid also had three sequential stages during each cycle: 1) a combined scratch prerub and swim postpowerstroke, 2) a scratch rub that also served as a swim prepowerstroke, and 3) a swim powerstroke that also served as a scratch postrub. Merging of the rostral scratch with the forward swim was possible because of similarities between the sequential stages of the two forms, making them biomechanically compatible for hybrid formation. Kinematic and myographic similarities between the rostral scratch and the forward swim support the hypothesis that the two behaviors share common neural circuitry. The common features of the sequential stages of each behavior and the production of scratch-swim hybrids provide additional support for the hypothesis of a shared core of spinal cord neurons common to both rostral scratch and forward swim.
在D2 - D3节段边界处前肢膨大后方脊髓完全横断的海龟产生了协调性的有节奏后肢运动。对中体甲桥处的同侧皮肤传入神经进行刺激会引发头向搔抓动作。在D3节段前切面处对侧背外侧索（DLF）进行电刺激会激活向前游动动作。同时刺激同侧甲桥和对侧DLF会引发搔抓 - 游动混合动作：一种将头向搔抓和向前游动的特征融合到每个有节奏运动周期中的行为。这是在脊髓脊椎动物中首次证明存在搔抓 - 运动混合动作。头向搔抓和向前游动有一些共同特征：髋部交替屈伸、在髋部运动周期内膝伸肌活动的时间相似以及存在一个向底物施加力的行为事件。在每个周期中，每种行为都表现出三个连续阶段，即事件前、事件中和事件后。头向搔抓事件是脚摩擦受刺激的甲桥部位。向前游动事件是以蹬水动作，即髋部伸展动作，此时脚保持垂直位置，脚趾和蹼展开。这两种行为在几个特征方面存在差异：髋部屈伸程度、肌电图（EMG）幅度以及EMG占空比。搔抓 - 游动混合动作在每个周期内表现出两个事件，即搔抓摩擦和游动蹬水动作。混合动作的髋部屈曲幅度、膝伸肌EMG以及髋屈肌EMG与搔抓动作相似；混合动作的髋部伸展幅度和髋伸肌EMG与游动动作相似。混合动作在每个周期内也有三个连续阶段：1）搔抓前摩擦和游动蹬水后阶段的组合，2）既是搔抓摩擦又是游动蹬水前阶段的动作，3）既是游动蹬水又是搔抓摩擦后阶段的动作。头向搔抓与向前游动能够融合是因为这两种形式的连续阶段存在相似性，使得它们在生物力学上适合形成混合动作。头向搔抓和向前游动之间的运动学和肌电图相似性支持了这两种行为共享共同神经回路的假说。每种行为连续阶段的共同特征以及搔抓 - 游动混合动作的产生为头向搔抓和向前游动共享共同脊髓神经元核心的假说提供了额外支持。
