Optogenetics has become an integral tool for studying and dissecting the neural circuitries of the brain using optical control. Recently, it has also begun to be used in the investigation of the spinal cord and peripheral nervous system. However, information on these regions' optical properties is sparse. Moreover, there is a lack of data on the dependence of light propagation with respect to neural tissue organization and orientation. This information is important for effective simulations and optogenetic planning, particularly in the spinal cord where the myelinated axons are highly organized. To this end, we report experimental measurements for the scattering coefficient, validated with three different methods in both the longitudinal and radial directions of multiple mammalian spinal cords. In our analysis, we find that there is indeed a directional dependence of photon propagation when interacting with organized myelinated axons. Specifically, light propagating perpendicular to myelinated axons in the white matter of the spinal cord produced a measured reduced scattering coefficient ( μ s ' ) of 3.52 ± 0.1    mm - 1 , and light that was propagated along the myelinated axons in the white matter produced a measured μ s ' of 1.57 ± 0.03    mm - 1 , across the various species considered. This 50% decrease in scattering power along the myelinated axons is observed with three different measurement strategies (integrating spheres, observed transmittance, and punch-through method). Furthermore, this directional dependence in scattering power and overall light attenuation did not occur in the gray matter regions where the myelin organization is nearly random. The acquired information will be integral in preparing future light-transport simulations and in overall optogenetic planning in both the spinal cord and the brain.

中文翻译：

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脊髓中髓鞘状轴突的各向异性光散射。

光遗传学已经成为使用光学控制研究和解剖大脑神经回路的不可或缺的工具。最近，它也已开始用于研究脊髓和周围神经系统。然而，关于这些区域的光学性质的信息很少。而且，缺乏关于光传播相对于神经组织组织和取向的依赖性的数据。该信息对于有效的模拟和光遗传学计划非常重要，特别是在髓鞘轴突高度组织化的脊髓中。为此，我们报告了散射系数的实验测量结果，并通过三种不同方法在多条哺乳动物脊髓的纵向和径向方向上对其进行了验证。在我们的分析中 我们发现与有组织的髓鞘轴突相互作用时，确实存在光子传播的方向依赖性。具体而言，垂直于脊髓白质中髓鞘轴突传播的光产生的测得的降低的散射系数（μs'）为3.52±0.1 mm-1，沿白质中髓鞘轴突传播的光产生了在所考虑的各种物种中测得的μs'为1.57±0.03 mm-1。使用三种不同的测量策略（积分球，观察到的透射率和穿通法）观察到沿有髓神经轴突的散射能力降低了50％。此外，在髓磷脂组织几乎是随机的灰质区域中，这种散射功率和总光衰减的方向依赖性没有发生。所获得的信息将在准备未来的光传输模拟以及脊髓和大脑的整体光遗传学计划中不可或缺。