Pre-motor neuronal networks, from connectivity to function

人类中枢神经系统包含超过1000亿个神经元，每个神经元通过称为突触的小接触传递和接收来自数千个其他神经元的信息。破译如此复杂的机器的确切接线图是现代神经科学中最令人着迷的任务之一，可以说，我们的认知和运动功能，我们是谁，我们做什么，与我们神经细胞之间的连接模式及其连接的强度严格相关，就像计算机的性能由其微电路的连接决定一样。直到几年前，对于复杂的生物体来说，破译神经密码的任务似乎是难以完成的，事实上，这项任务只在小动物身上取得了一些成功，这些动物的大脑由几十个甚至最多数百个神经元组成。过去四年，由于引入了附着在荧光蛋白上的新型修饰病毒化合物，这种化合物可以传播到与最初感染的细胞或细胞组进行突触接触的所有神经元，并且只传播到这些细胞，对所谓的“连接体”，或者换句话说，大规模大脑连接图谱的研究取得了巨大进展。我们计划使用这种新颖而强大的工具来感染脊髓中特定群体的运动神经元。运动神经元是中枢神经系统中唯一兴奋非神经细胞(肌肉)的细胞，它们负责启动和协调我们所做的每一个动作。在感染了与特定肌肉相关的运动神经元后，我们将能够追踪到与它们通信的所有细胞，并确定它们相对于目标运动神经元的位置。我们还将记录从每个连接的细胞传输到运动神经元的电信号，我们将能够测量它们对执行运动任务的相对贡献。一次绘制一个细胞的连接性图是非常准确的，但这可能很耗时，而且不是很有效，所以我们将使用另一种最近开发的技术并将一种蛋白质附加到病毒构建中，这种蛋白质一旦在目标细胞中表达，就可以在暴露在强烈蓝光下时激发它，从而模仿神经元之间的正常传输模式。在这种光激活蛋白质的帮助下，我们将能够使用蓝色激光从一个细胞快速跳到另一个细胞，并绘制出单个运动神经元的整个连接。我们将从这项大规模连通性研究中收集的信息将告诉我们，脊髓中的马达电路是如何连接在一起的，以产生我们每天执行的复杂和多方面的运动任务。我们提出的研究将使我们对控制运动的回路有前所未有的了解。这当然是设计旨在修复因受伤或疾病造成的损害的干预措施的先决条件之一。此外，我们将开发新的方法来将特定的编码基因转移到细胞群体中，并用光来控制它们的活动。这些方法已经在癫痫动物模型中成功地控制了癫痫发作，它们在脊髓上的应用可能是未来改善运动控制受损受试者生活质量的一条途径。

The human central nervous system contains more than 100 billion neurons, each one of them transmitting and receiving information from thousands of other neurons through small contacts called synapses. Decoding the exact wiring diagram of such a complicated machine is one of the most fascinating tasks in modern neuroscience and it could be said that our cognitive and motor functions, who we are and what we do, are strictly related to the pattern of connectivity between our nerve cells and the strength of their connections, in the same way as the performance of a computer is determined by the wiring of its microcircuits. Until a few years ago the task of deciphering the neural code might have seemed unapproachable in the case of complex organisms and in fact it was achieved with some success only on small animals whose brain is composed of a few tens or at most hundreds of neurons. The study of the so called "connectome", or in other words the map of the brain connections over a large scale, has made giant steps in the past four years thanks to the introduction of novel modified viral compounds attached to fluorescent proteins that can spread to all of the neurons that make synaptic contacts with the cell or group of cells that were initially infected, and only to those cells. We plan to use this novel and powerful tool to infect specific populations of motoneurons in the spinal cord. Motoneurons are the only cells in the central nervous system that excite non neuronal cells (the muscles) and they are responsible for the initiation and coordination of every single movement we make. Following infection of motoneurons associated with specific muscles, we will be able to trace down all of the cells that communicate with them and determine their positions relative to their target motoneurons. We will also record the electrical signal transmitted to the motoneuron from each of the connected cells and we will be able to measure their relative contribution to the execution of motor tasks. Mapping the connectivity one cell at a time is extremely accurate, but it could be time consuming and not very effective, so we will use another recently developed technique and attach to the viral construct a protein that once expressed in the target cell, can excite it when exposed to an intense blue light and therefore imitate the normal mode of transmission between neurons. With the aid of this light activated protein we will be able to hop quickly from one cell to the other using a blue laser beam and map the entire connectivity of single motoneurons. The information that we will collect from this large scale connectivity study will tell us how the motor circuits in the spinal cord are wired together to produce the complex and multifaceted motor tasks that we execute every day. Our proposed research will give us an unprecedented level of knowledge of the circuits underlying the control of movement. This is certainly one of the prerequisites to design interventions aimed at repairing damages that occurs due to injuries or diseases. Furthermore, we will exploit new methods for transferring specific coding genes into population of cells and for controlling their activity with light. These methods have been already used with spectacular success for controlling seizures in an animal model of epilepsy and it is possible that their application to the spinal cord could be a future avenue towards an improvement in the quality of life of subjects with impaired motor control.