The suprachiasmatic nucleus of the hypothalamus is the chief circadian pacemaker in the brain, and is entrained to day-night cycles by visual afferents from melanopsin containing retinal ganglion cells via the inferior accessory optic tract. Tracer studies have demonstrated efferents from the suprachiasmatic nucleus projecting to the paraventricular nucleus of the hypothalamus, which in turn project to first-order sympathetic neurons in the intermedio-lateral grey of the spinal cord. Sympathetic projections to the pineal gland trigger the secretion of the sleep inducing hormone melatonin. The current study reports the first demonstration of potential sympathopetal hypothalamic projections involved in circadian regulation in humans with in vivo virtual white matter dissections using probabilistic diffusion tensor imaging (DTI) tractography. Additionally, our data shows a correlation between individual differences in white matter microstructure (measured with fractional anisotropy) and increased daytime sleepiness [measured with the Epworth Sleepiness Scale (ESS, Johns, 1991)]. Sympathopetal connections with the hypothalamus were virtually dissected using designated masks on the optic chiasm, which served as an anatomical landmark for retinal fibres projecting to the suprachiasmatic nucleus, and a waypoint mask on the lateral medulla, where hypothalamic projections to the sympathetic nervous system traverse in humans. Sympathopetal projections were demonstrated in each hemisphere in twenty-six subjects. The tract passed through the suprachiasmatic nucleus of the hypothalamus and its trajectory corresponds to the dorsal longitudinal fasciculus traversing the periaqueductal region and the lateral medulla. White matter microstructure (FA) in the left hemisphere correlated with high scores on the ESS, suggesting an association between circadian pathway white matter microstructure, and increased daytime sleepiness.

中文翻译：

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人类的昼夜节律：白质微观结构可预测白天的嗜睡情况。

下丘脑的视交叉上核是大脑中的昼夜节律性起搏器，并通过下视神经束从含有黑色素的视网膜神经节细胞的视觉传入带入昼夜周期。示踪剂研究表明，射出上眼交叉核投射至下丘脑室旁核，继而投射至脊髓中外侧灰色的一级交感神经元。松果体的交感投射触发睡眠诱导激素褪黑激素的分泌。本研究报道了使用概率扩散张量成像（DTI）图像进行体内虚拟白质解剖的人类在昼夜节律调控中潜在的交感神经下丘脑投射的首次证明。此外，我们的数据显示白质微观结构的个体差异（用分数各向异性测量）与白天嗜睡增加之间的相关性（用Epworth嗜睡量表（ESS，Johns，1991）衡量）。使用视交叉上的指定面罩解剖解剖与下丘脑的交感神经连接，该面罩用作投射到视交叉上核的视网膜纤维的解剖学界标，而在外侧延髓上充当视点罩，其中下丘脑向交感神经系统的下垂投射横越人类。在每个半球的26位受试者中均显示出交感神经投射。穿过下丘脑上睑上突核的束及其轨迹对应于横穿导水管周围区域和外侧延髓的背纵筋膜。左半球的白质微结构（FA）与ESS的高分相关，表明昼夜节律途径的白质微结构与白天嗜睡现象之间存在关联。