Evidence for plateau potentials in tail motoneurons of awake chronic spinal rats with spasticity
Evidence for plateau potentials in tail motoneurons of awake chronic spinal rats with spasticity
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DOI：
10.1152/jn.2001.86.4.1972
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发表时间：
2001-10-01
影响因子：
2.5
通讯作者：
Gorassini, M
中科院分区：
文献类型：
--
作者：
Bennett, DJ;Li, YR;Gorassini, M
在急性（1-2天）和慢性（>30天）骶髓横断后，记录清醒大鼠节段性尾肌的运动单位，以确定平台电位是否有助于损伤后持续的运动单位放电。这项研究的动机是一项伴随的体外研究，该研究表明，慢性脊髓损伤后，骶尾脊髓的尾运动神经元表现出持续性内向电流（I-PIC），导致内在持续的去极化（平台电位）和放电（自我维持放电）。重要的是，在这项伴随研究中，高原在招募时被完全激活，随后有助于维持放电，而不会引起放电的突然非线性。也就是说，在募集和平台激活后，放电率用注入电流相对线性地调制，因此尽管有平台，但仍提供了对运动神经元的输入的良好近似。因此，在本研究中，从同一块肌肉同时记录成对的运动单位，并使用最低阈值单位（控制单位）的放电率（F）作为对两个单位的突触输入的估计。然后，我们通过确定是否需要更多的突触输入来招募测试单元，而不是维持其放电，来检查高阈值单元（测试单元）的放电是否本质上由高原维持。在测试单元的募集和去募集时估计的突触输入的差异（即，控制单元速率的变化Δ F）作为固有地维持点火的平台电流（I-PIC）的估计值。使用缓慢分级的手动皮肤刺激来招募，然后去招募单元。当急性和慢性脊髓大鼠的对照单位频率平均分别为17.8和18.9 Hz时，招募试验单位。在慢性脊髓大鼠中，当对照单位频率（重新估计的突触输入）与募集时（Δ F = -5.5 Hz）相比显著降低时，测试单位被去募集，因此平台参与维持放电。在最低阈值的运动单位中，即使是短暂的刺激也会触发非常持久的放电（几秒到几小时;自我持续放电）。更高阈值的单位需要连续刺激（或自发痉挛）来引起放电，但是再次需要更多的突触输入来募集单位而不是维持其放电（即，存在平台）。相比之下，在急性脊髓大鼠中，刺激通常不会触发可归因于平台期的持续运动单位放电，因为DeltaF与零没有显著差异。这些结果表明，高原发挥重要作用，在清醒的慢性脊髓大鼠维持运动单位放电，从而有助于反射亢进和高张力与慢性损伤。
Motor units of segmental tail muscles were recorded in awake rats following acute (1-2 days) and chronic (>30 days) sacral spinal cord transection to determine whether plateau potentials contributed to sustained motor-unit discharges after injury. This study was motivated by a companion in vitro study that indicated that after chronic spinal cord injury, the tail motoneurons of the sacrocaudal spinal cord exhibit persistent inward currents (I-PIC) that cause intrinsically sustained depolarizations (plateau potentials) and firing (self-sustained firing). Importantly, in this companion study, the plateaus were fully activated at recruitment and subsequently helped sustain the firing without causing abrupt nonlinearities in firing. That is, after recruitment and plateau activation, the firing rate was modulated relatively linearly with injected current and therefore provided a good approximation of the input to the motoneuron despite the plateau. Thus in the present study, pairs of motor units were recorded simultaneously from the same muscle, and the firing rate (F) of the lowest-threshold unit (control unit) was used as an estimate of the synaptic input to both units. We then examined whether firing of the higher-threshold unit (test unit) was intrinsically maintained by a plateau, by determining whether more synaptic input was required to recruit the test unit than to maintain its firing. The difference in the estimated synaptic input at recruitment and de-recruitment of the test unit (i.e., change in control unit rate, DeltaF) was taken as an estimate of the plateau current (I-PIC) that intrinsically sustained the firing. Slowly graded manual skin stimulation was used to recruit and then de-recruit the units. The test unit was recruited when the control unit rate was on average 17.8 and 18.9 Hz in acute and chronic spinal rats, respectively. In chronic spinal rats, the test unit was de-recruited when the control unit rate (re: estimated synaptic input) was significantly reduced, compared with at recruitment (DeltaF = -5.5 Hz), and thus a plateau participated in maintaining the firing. In the lowest-threshold motor units, even a brief stimulation triggered very long-lasting firing (seconds to hours; self-sustained firing). Higher-threshold units required continuous stimulation (or a spontaneous spasm) to cause firing, but again more synaptic input was needed to recruit the unit than to maintain its firing (i.e., plateau present). In contrast, in acute spinal rats, the stimulation did not usually trigger sustained motor-unit firing that could be attributed to plateaus because DeltaF was not significantly different from zero. These results indicate that plateaus play an important role in sustaining motor-unit firing in awake chronic spinal rats and thus contribute to the hyperreflexia and hypertonus associated with chronic injury.