Departments of Physiology & Biomedical Engineering and.
Anaesthesiology and Perioperative Medicine, Mayo Clinic, Rochester, MN.
J Physiol. 2020 Oct;598(20):4693-4711. doi: 10.1113/JP280130. Epub 2020 Aug 19.
Motor units, comprising a motor neuron and the muscle fibre it innervates, are activated in an orderly fashion to provide varying amounts of force. A unilateral C2 spinal hemisection (C2SH) disrupts predominant excitatory input from medulla, causing cessation of inspiratory-related diaphragm muscle activity, whereas higher force, non-ventilatory diaphragm activity persists. In this study, we show a disproportionately larger loss of excitatory glutamatergic innervation to small phrenic motor neurons (PhMNs) following C2SH, as compared with large PhMNs ipsilateral to injury. Our data suggest that there is a dichotomy in the distribution of inspiratory-related descending excitatory glutamatergic input to small vs. large PhMNs that reflects their differential recruitment.
Excitatory glutamatergic input mediating inspiratory drive to phrenic motor neurons (PhMNs) emanates primarily from the ipsilateral ventrolateral medulla. Unilateral C2 hemisection (C2SH) disrupts this excitatory input, resulting in cessation of inspiratory-related diaphragm muscle (DIAm) activity. In contrast, after C2SH, higher force, non-ventilatory DIAm activity persists. Inspiratory behaviours require recruitment of only smaller PhMNs, whereas with more forceful expulsive/straining behaviours, larger PhMNs are recruited. Accordingly, we hypothesize that C2SH primarily disrupts glutamatergic synaptic inputs to smaller PhMNs, whereas glutamatergic synaptic inputs to larger PhMNs are preserved. We examined changes in glutamatergic presynaptic input onto retrogradely labelled PhMNs using immunohistochemistry for VGLUT1 and VGLUT2. We found that 7 days after C2SH there was an ∼60% reduction in glutamatergic inputs to smaller PhMNs compared with an ∼35% reduction at larger PhMNs. These results are consistent with a more pronounced impact of C2SH on inspiratory behaviours of the DIAm, and the preservation of higher force behaviours after C2SH. These results indicate that the source of glutamatergic synaptic input to PhMNs varies depending on motor neuron size and reflects different functional control - perhaps separate central pattern generator and premotor circuits. For smaller PhMNs, the central pattern generator for inspiration is located in the pre-Bötzinger complex and premotor neurons in the ventrolateral medulla, sending predominantly ipsilateral projections via the dorsolateral funiculus. C2SH disrupts this glutamatergic input. For larger PhMNs, a large proportion of excitatory inputs appear to exist below the C2 level or from contralateral regions of the brainstem and spinal cord.
运动单位由一个运动神经元及其支配的肌纤维组成，它们以有序的方式被激活，以提供不同的力量。单侧 C2 脊髓半切（C2SH）破坏了来自延髓的主要兴奋性传入，导致与吸气相关的膈肌无力活动停止，而更高强度的非通气膈肌无力活动持续存在。在这项研究中，我们发现与损伤侧的大膈神经运动神经元（PhMNs）相比，C2SH 后小 PhMNs 接受的兴奋性谷氨酸能传入明显减少。我们的数据表明，与吸气相关的下行兴奋性谷氨酸能传入到小 PhMNs 和大 PhMNs 的分布存在二分法，这反映了它们的差异募集。
介导膈神经运动神经元（PhMNs）吸气驱动的兴奋性谷氨酸能传入主要来自同侧腹外侧延髓。单侧 C2 半切（C2SH）破坏了这种兴奋性传入，导致与吸气相关的膈肌（DIAm）活动停止。相比之下，C2SH 后，更高强度的非通气 DIAm 活动持续存在。吸气行为只需要募集较小的 PhMNs，而在更有力的呼气/紧张行为中，较大的 PhMNs 被募集。因此，我们假设 C2SH 主要破坏较小 PhMNs 的谷氨酸能突触传入，而较大 PhMNs 的谷氨酸能突触传入则被保留。我们使用 VGLUT1 和 VGLUT2 的免疫组织化学方法检查了逆行标记的 PhMNs 上谷氨酸能突触前输入的变化。我们发现，与 C2SH 后较大 PhMNs 相比，C2SH 后 7 天，较小 PhMNs 的谷氨酸能传入减少了约 60%，而减少了约 35%。这些结果与 C2SH 对 DIAm 吸气行为的更明显影响以及 C2SH 后更高强度行为的保留一致。这些结果表明，PhMNs 的谷氨酸能突触传入的来源取决于运动神经元的大小，并反映了不同的功能控制-也许是单独的中枢模式发生器和运动前回路。对于较小的 PhMNs，吸气的中枢模式发生器位于 Pre-Bötzinger 复合体中，运动前神经元位于腹外侧延髓中，通过背外侧束主要发出同侧投射。C2SH 破坏了这种谷氨酸能传入。对于较大的 PhMNs，兴奋性传入的很大一部分似乎存在于 C2 以下水平或来自脑干和脊髓的对侧区域。
