Rhythmic behaviors such as breathing, walking, and scratching are vital to many species. Such behaviors can emerge from groups of neurons, called central pattern generators, in the absence of rhythmic inputs. In vertebrates, the identification of the cells that constitute the central pattern generator for particular rhythmic behaviors is difficult, and often, its existence has only been inferred. For example, under experimental conditions, intact turtles generate several rhythmic scratch motor patterns corresponding to non-rhythmic stimulation of different body regions. These patterns feature alternating phases of motoneuron activation that occur repeatedly, with different patterns distinguished by the relative timing and duration of activity of hip extensor, hip flexor, and knee extensor motoneurons. While the central pattern generator network responsible for these outputs has not been located, there is hope to use motoneuron recordings to deduce its properties. To this end, this work presents a model of a previously proposed central pattern generator network and analyzes its capability to produce two distinct scratch rhythms from a single neuron pool, selected by different combinations of tonic drive parameters but with fixed strengths of connections within the network. We show through simulation that the proposed network can achieve the desired multi-functionality, even though it relies on hip unit generators to recruit appropriately timed knee extensor motoneuron activity, including a delay relative to hip activation in rostral scratch. Furthermore, we develop a phase space representation, focusing on the inputs to and the intrinsic slow variable of the knee extensor motoneuron, which we use to derive sufficient conditions for the network to realize each rhythm and which illustrates the role of a saddle-node bifurcation in achieving the knee extensor delay. This framework is harnessed to consider bistability and to make predictions about the responses of the scratch rhythms to input changes for future experimental testing.

中文翻译：

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受Turtle Scratching启发的节奏生成网络中多功能性的条件。

有节奏的行为，如呼吸，行走和抓挠对许多物种至关重要。在没有节奏输入的情况下，此类行为可能会从称为中央模式生成器的神经元组中出现。在脊椎动物中，很难确定构成特定节奏行为的中央模式生成器的细胞，而且常常只能推断出它的存在。例如，在实验条件下，完整的海龟会产生几种有节奏的刮擦运动模式，对应于不同身体部位的非有节奏刺激。这些模式的特征是反复发生的运动神经元激活交替相，其不同的模式以髋部伸肌，髋屈肌和膝部伸肌运动神经元的相对活动时间和持续时间为特征。尽管没有找到负责这些输出的中央模式发生器网络，但希望能使用运动神经元记录来推导其特性。为此，这项工作提出了一个先前提出的中央模式生成器网络的模型，并分析了其从单个神经元池产生两个不同的抓痒节奏的能力，该神经元池是由不同的滋补驱动参数组合选择的，但网络中的连接强度是固定的。通过仿真，我们证明了所提出的网络可以实现所需的多功能性，即使它依靠臀部单位生成器来招募适当的定时膝盖伸肌运动神经元活动，包括相对于在延髓头端的臀部激活产生延迟。此外，我们开发了相空间表示，着重于膝盖伸肌神经元的输入和内在的慢变量，我们使用它来为网络实现每个节奏获取足够的条件，并说明了鞍节点分叉在实现膝盖伸肌延迟中的作用。利用该框架来考虑双稳态，并就从临时节奏输入变化以供将来进行实验测试的响应做出预测。