Department of Clinical and Movement Neurosciences, Queen Square Institute of Neurology, UCL, London WC1N 3BG, UK.
Division of Basic Biomedical Sciences, Laboratory of Neurological Sciences, The University of South Dakota, Sanford School of Medicine, Vermillion, SD 57069, USA.
Brain. 2023 May 2;146(5):1791-1803. doi: 10.1093/brain/awac496.
We review the spatial organization of corticospinal outputs from different cortical areas and how this reflects the varied functions mediated by the corticospinal tract. A long-standing question is whether the primate corticospinal tract shows somatotopical organization. Although this has been clearly demonstrated for corticofugal outputs passing through the internal capsule and cerebral peduncle, there is accumulating evidence against somatotopy in the pyramidal tract in the lower brainstem and in the spinal course of the corticospinal tract. Answering the question on somatotopy has important consequences for understanding the effects of incomplete spinal cord injury. Our recent study in the macaque monkey, using high-resolution dextran tracers, demonstrated a great deal of intermingling of fibres originating from primary motor cortex arm/hand, shoulder and leg areas. We quantified the distribution of fibres belonging to these different projections and found no significant difference in their distribution across different subsectors of the pyramidal tract or lateral corticospinal tract, arguing against somatotopy. We further demonstrated intermingling with corticospinal outputs derived from premotor and supplementary motor arm areas. We present new evidence against somatotopy for corticospinal projections from rostral and caudal cingulate motor areas and from somatosensory areas of the parietal cortex. In the pyramidal tract and lateral corticospinal tract, fibres from the cingulate motor areas overlap with each other. Fibres from the primary somatosensory cortex arm area completely overlap those from the leg area. There is also substantial overlap of both these outputs with those from posterior parietal sensorimotor areas. We argue that the extensive intermingling of corticospinal outputs from so many different cortical regions must represent an organizational principle, closely related to its mediation of many different functions and its large range of fibre diameters. The motor sequelae of incomplete spinal injury, such as central cord syndrome and 'cruciate paralysis', include much greater deficits in upper than in lower limb movement. Current teaching and text book explanations of these symptoms are still based on a supposed corticospinal somatotopy or 'lamination', with greater vulnerability of arm and hand versus leg fibres. We suggest that such explanations should now be finally abandoned. Instead, the clinical and neurobiological implications of the complex organization of the corticospinal tract need now to be taken into consideration. This leads us to consider the evidence for a greater relative influence of the corticospinal tract on upper versus lower limb movements, the former best characterized by skilled hand and digit movements.
我们回顾了来自不同皮质区域的皮质脊髓输出的空间组织，以及这如何反映皮质脊髓束介导的各种功能。一个长期存在的问题是，灵长类动物的皮质脊髓束是否表现出躯体感觉组织。尽管这已经在穿过内囊和大脑脚的皮质传出物中得到了明确的证明，但越来越多的证据表明，在下脑桥的锥体束和皮质脊髓束的脊髓中不存在躯体感觉组织。回答躯体感觉组织的问题对于理解不完全性脊髓损伤的影响具有重要意义。我们最近在猕猴中的研究使用高分辨率葡聚糖示踪剂，证明了源自初级运动皮质臂/手、肩部和腿部区域的纤维大量混合。我们量化了属于这些不同投射的纤维的分布，并且在锥体束或外侧皮质脊髓束的不同亚区之间，没有发现它们的分布有显著差异，这表明不存在躯体感觉组织。我们进一步证明了与来自运动前区和辅助运动区的皮质脊髓输出的混合。我们提供了新的证据，表明来自扣带运动区的皮质脊髓投射和来自顶叶感觉区的皮质脊髓投射都没有躯体感觉组织。在锥体束和外侧皮质脊髓束中，扣带运动区的纤维相互重叠。初级体感皮质臂区的纤维与腿部区域的纤维完全重叠。这两个输出与后顶叶感觉运动区的输出也有很大的重叠。我们认为，来自如此多的不同皮质区域的皮质脊髓输出的广泛混合必须代表一种组织原则，与它介导的许多不同功能及其广泛的纤维直径密切相关。不完全性脊髓损伤的运动后遗症，如中央索综合征和“十字形瘫痪”，上肢运动比下肢运动的缺陷更大。目前关于这些症状的教学和课本解释仍然基于假设的皮质脊髓躯体感觉组织或“分层”，上肢和手纤维比下肢纤维更容易受损。我们建议现在应该最终放弃这种解释。相反，现在需要考虑皮质脊髓束复杂组织的临床和神经生物学意义。这使我们考虑了皮质脊髓束对上肢和下肢运动的相对影响较大的证据，前者以熟练的手部和手指运动为特征。
