Grandhe S, Abbas J J, Jung R
Center for Biomedical Engineering, University of Kentucky, Lexington, KY 40506-0070, USA.
Biomed Sci Instrum. 1999;35:175-80.
Motor networks within the spinal cord of vertebrates are capable of generating rhythmic locomotor output even in the absence of phasic sensory input. In an intact animal these spinal pattern generators are affected by descending inputs from the brain and by sensory inputs. The role of the feedforward-feedback (FF-FB) loops between the brain and the spinal cord in the control of locomotion are not well understood. We hypothesized that the dynamic interaction between the brain and the spinal cord would affect the response of the neural system to external perturbation. We investigated this hypothesis in an in-vitro brain-spinal cord fictive locomotion preparation of a primitive vertebrate, lamprey. In tandem, we analyzed the behavior of a neural network model representing the brain and multiple segments of the spinal cord. Our experimental results indicate that with intact FF-FB loops, phase locked entrainment of the spinal motor activity can be obtained on direct stimulation of the spinal cord. However, the effect is localized with minimal influence on distal spinal segments. The intersegmental coupling strength is strong as indicated by a fast recovery of the perturbed rhythm to the natural frequency on termination of the perturbation. With the FF-FB loop interrupted, the perturbation was capable of altering the motor activity from multiple sites in the spinal cord. Also, upon termination of the perturbation there was a prolonged period before recovery of the original natural frequency. Model analyses support our interpretation of the experimental results. In the neural network model with the brain-spinal cord loops closed there was a localized effect on the oscillatory rhythm and strong intersegmental coupling. Also, the analysis indicated the presence of a smaller entrainment range and many more periodic orbits than with the loops open. The results suggest that the increased variability in the locomotor rhythm and decreased sensitivity to perturbation observed in the presence of intact brain spinal cord connections may be a reflection of a higher dimensional system with many periodic orbits. The higher dimension could allow the system to collectively remain within the attractor space of one of these periodic orbits and thus remain resilient to perturbation.
脊椎动物脊髓内的运动网络即使在没有阶段性感觉输入的情况下也能够产生有节奏的运动输出。在完整的动物体内，这些脊髓模式发生器会受到来自大脑的下行输入和感觉输入的影响。大脑与脊髓之间的前馈 - 反馈（FF - FB）回路在运动控制中的作用尚未得到充分理解。我们假设大脑与脊髓之间的动态相互作用会影响神经系统对外部扰动的反应。我们在一种原始脊椎动物七鳃鳗的体外脑 - 脊髓虚拟运动准备实验中研究了这一假设。同时，我们分析了一个代表大脑和脊髓多个节段的神经网络模型的行为。我们的实验结果表明，在完整的FF - FB回路情况下，直接刺激脊髓可实现脊髓运动活动的锁相夹带。然而，这种效应是局部性的，对脊髓远端节段的影响最小。节段间耦合强度很强，这表现为扰动终止后，受扰节律能快速恢复到自然频率。当FF - FB回路中断时，扰动能够改变脊髓多个部位的运动活动。而且，扰动终止后，恢复到原始自然频率之前会有一段较长的时间。模型分析支持了我们对实验结果的解释。在脑 - 脊髓回路闭合的神经网络模型中，对振荡节律有局部效应且节段间耦合很强。此外，分析表明与回路开放时相比，夹带范围更小且周期轨道更多。结果表明，在存在完整的脑脊髓连接时观察到的运动节律变异性增加和对扰动的敏感性降低，可能反映了一个具有许多周期轨道的高维系统。更高的维度可能使系统能够集体保持在这些周期轨道之一的吸引子空间内，从而对扰动保持弹性。
