Functional organization of a reticulospinal motor connectome

描述(申请人提供)：运动控制领域的一个中心挑战是了解下行系统如何控制脊髓间神经元(INS)。然而，下行轴突以数千个脊髓IN为靶标，形成复杂的神经回路。这种复杂性限制了传统调查技术的有用性和效率。为了解决这一障碍并加快进展，我们最近开发了替代方法，将功能连接性研究转移到体外小鼠身上。我们使用光学成像来记录刺激皮质下下行系统后的个体和群体神经元活动。这一新的方法使我们能够研究下行网络中的突触连接，其效率和吞吐量是以前在哺乳动物中无法实现的。在这项建议中，我们将重点放在运动协调中的一个主要角色--网状脊髓(RS)下行系统。我们建议确定RS神经元和一群带有下行轴突投射的连合INS之间的功能联系，称为dCIN。CINS在RS输入和运动输出之间具有战略意义，在运动节律的产生和肢体间的协调中发挥着重要作用。然而，它们在递质表型、输入-输出连接和运动过程中的功能作用方面是不同的。我们已经开始阐明RS-DCIN系统的组织原理，并取得了两个重要的初步发现。我们发现：(I)两组不同的延髓RS神经元分别激活轴向和后肢运动神经元，以及(Ii)对RS组或两者都有反应的dCIN的三个亚群。因此，RS神经元被组织成不同的组，选择性地招募dCIN。在这里，在目标1中，我们建议测试RS的两种形式的选择性 神经元可能用来招募dCIN。首先，我们将利用已知的脊髓节段轴向、屈肌和伸肌运动神经元的组成差异来测试节段选择性。然后，我们将使用转基因方法来测试递质的表型选择性。在目标2和目标3中，我们将确定RS-DCIn招募在多大程度上取决于脊髓运动网络的兴奋性和行为状态。这些目标的成功实现将极大地促进我们对RS系统功能组织的理解，无论是在静止期还是在复杂运动行为的积极参与期间。我们的长期目标是确定脑干下行系统通过脊髓间神经元控制运动输出的组织原则。这些知识无疑将提供对损伤后回路功能障碍的更多洞察，并有助于确定合理的MOTO恢复策略。

DESCRIPTION (provided by applicant): A central challenge in the field of motor control is to understand how descending systems control spinal cord interneurons (INs). However, descending axons target thousands of spinal INs and form complex neural circuits. This complexity limits the usefulness and efficiency of conventional investigation techniques. To solve this roadblock and accelerate progress, we recently developed alternative approaches that transfer functional connectivity studies to the in vitro mouse. We use optical imaging to record individual and population neuronal activity following stimulation of subcortical descending systems. This novel approach allows us to study synaptic connections in descending networks with an efficiency and throughput that has not been possible previously in mammals. In this proposal, we focus on a major player in motor coordination, the reticulospinal (RS) descending system. We propose to identify the functional connections between RS neurons and a population of commissural INs with descending axonal projections called dCINs. CINs are strategically interposed between RS input and motor output to play an important role in locomotor rhythm generation and interlimb coordination. However, they are heterogeneous with respect to transmitter phenotype, input-output connectivity and functional role during movement. We have begun to elucidate organizational principles of RS-dCIN system and made two important preliminary findings. We found; (i) two distinct groups of medullary RS neurons that differentially activate axial and hindlimb motoneurons, and (ii) three subpopulations of dCINs that respond to either RS group or to both. Thus, RS neurons are organized in discrete groups that recruit dCINs selectively. Here, in Aim 1, we propose to test two forms of selectivity that RS neurons may use to recruit dCINs. First, we will test segmental selectivity by taking advantage of the known spinal segmental differences in composition of axial, flexor and extensor motoneurons. Then, we will test transmitter phenotype selectivity using transgenic approaches. In Aims 2 and 3, we will determine the extent to which RS-dCIN recruitment depends on the excitability and behavioral state of the spinal motor network. Successful completion of these aims will significantly advance our understanding of the functional organization of the RS system both during quiescence and during the active engagement of a complex motor behavior. Our long-term goal is to define organizational principles by which brainstem descending systems control motor output via spinal interneurons. Such knowledge will undoubtedly provide greater insight into circuit dysfunction after injury and aid identify rational strategies for moto recovery.