Department of Clinical and Movement Sciences, Queen Square Institute of Neurology, UCL, London WC1N 3BG, UK.
Clin Neurophysiol. 2024 Apr;160:56-67. doi: 10.1016/j.clinph.2024.02.001. Epub 2024 Feb 9.
Corticospinal neurons located in motor areas of the cerebral neocortex project corticospinal axons which synapse with the spinal network; a parallel corticobulbar system projects to the cranial motor network and to brainstem motor pathways. The primate corticospinal system has a widespread cortical origin and an extensive range of different fibre diameters, including thick, fast-conducting axons. Direct cortico-motoneuronal (CM) projections from the motor cortex to arm and hand alpha motoneurons are a recent evolutionary feature, that is well developed in dexterous primates and particularly in humans. Many of these projections originate from the caudal subdivision of area 4 ('new' M1: primary motor cortex). They arise from corticospinal neurons of varied soma size, including those with fast- and relatively slow-conducting axons. This CM system has been shown to be involved in the control of skilled movements, carried out with fractionation of the distal extremities and at low force levels. During movement, corticospinal neurons are activated quite differently from 'lower' motoneurons, and there is no simple or fixed functional relationship between a so-called 'upper' motoneuron and its target lower motoneuron. There are key differences in the organisation and function of the corticospinal and CM system in primates versus non-primates, such as rodents. These differences need to be recognized when making the choice of animal model for understanding disorders such as amyotrophic lateral sclerosis (ALS). In this neurodegenerative brain disease there is a selective loss of fast-conducting corticospinal axons, and their synaptic connections, and this is reflected in responses to non-invasive cortical stimuli and measures of cortico-muscular coherence. The loss of CM connections influencing distal limb muscles results in a differential loss of muscle strength or 'split-hand' phenotype. Importantly, there is also a unique impairment in the coordination of skilled hand tasks that require fractionation of digit movement. Scores on validated tests of skilled hand function could be used to assess disease progression.
位于大脑新皮质运动区的皮质脊髓神经元投射皮质脊髓轴突，与脊髓网络形成突触；平行的皮质延髓系统投射到颅运动网络和脑干运动途径。灵长类皮质脊髓系统具有广泛的皮质起源和广泛的不同纤维直径范围，包括粗而快速传导的轴突。来自运动皮层的直接皮质运动神经元（CM）投射到手臂和手部α运动神经元是最近进化的特征，在灵巧的灵长类动物中特别是在人类中得到了很好的发展。这些投射中的许多来自 4 区的尾部分支（“新”M1：初级运动皮层）。它们源自具有不同大小胞体的皮质脊髓神经元，包括具有快速和相对缓慢传导轴突的神经元。已经表明，该 CM 系统参与了精细运动的控制，这些运动涉及到远端肢体的分割和低力水平。在运动过程中，皮质脊髓神经元的激活方式与“较低”运动神经元非常不同，并且所谓的“较高”运动神经元与其目标“较低”运动神经元之间没有简单或固定的功能关系。在灵长类动物与非灵长类动物（如啮齿动物）之间，皮质脊髓和 CM 系统的组织和功能存在关键差异。在选择用于理解肌萎缩侧索硬化症（ALS）等疾病的动物模型时，需要认识到这些差异。在这种神经退行性脑疾病中，快速传导的皮质脊髓轴突及其突触连接选择性丧失，这反映在对非侵入性皮质刺激的反应和皮质肌肉相干性的测量中。影响远端肢体肌肉的 CM 连接的丧失导致肌肉力量的差异丧失或“分裂手”表型。重要的是，在需要手指运动分割的精细手部任务的协调中也存在独特的障碍。经过验证的手部技能测试的得分可用于评估疾病进展。
