Khan Serajul I, Taylor Janet L, Gandevia Simon C
Neuroscience Research Australia and University of New South Wales, Randwick, NSW, Australia.
J Physiol. 2016 May 15;594(10):2707-17. doi: 10.1113/JP272164. Epub 2016 Apr 24.
The output of human motoneurone pools decreases with fatiguing exercise, but the mechanisms involved are uncertain. We explored depression of recurrent motoneurone discharges (F-waves) after sustained maximal voluntary contractions (MVCs). MVC depressed the size and frequency of F-waves in a hand muscle but a submaximal contraction (at 50% MVC) did not. Surprisingly, activation of the motoneurones antidromically by stimulation of the ulnar nerve (at 20 or 40 Hz) did not depress F-wave area or persistence. Furthermore, a sustained (3 min) MVC of a hand muscle depressed F-waves in its antagonist but not in a remote hand muscle. Our findings suggest that depression of F-waves after voluntary contractions is not simply due to repetitive activation of the motoneurones but requires descending voluntary drive. Furthermore, this effect may depress nearby, but not distant, spinal motoneurone pools.
There are major spinal changes induced by repetitive activity and fatigue that could contribute to 'central' fatigue but the mechanisms involved are poorly understood in humans. Here we confirmed that the recurrent motoneuronal discharge (F-wave) is reduced during relaxation immediately after a sustained maximal voluntary contraction (MVC) of an intrinsic hand muscle (abductor digiti minimi, ADM) and explored the relationship between motoneurone firing and the depression of F-waves in three ways. First, the depression (in both F-wave area and F-wave persistence) was present after a 10 s MVC (initial decrease 36.4 ± 19.1%; mean ± SD) but not after a submaximal voluntary contraction at 50% maximum. Second, to evoke motoneurone discharge without volitional effort, 10 s tetanic contractions were produced by supramaximal ulnar nerve stimulation at the elbow at physiological frequencies of 25 and 40 Hz. Surprisingly, neither produced depression of F-waves in ADM to test supramaximal stimulation of the ulnar nerve at the wrist. Finally, a sustained MVC (3 min) of the antagonist to ADM (4th palmar interosseous) depressed F-waves in the anatomically close ADM (20 ± 18.2%) but not in the more remote first dorsal interosseous on the radial side of the hand. We argue that depression of F-waves after voluntary contractions may not be due to repetitive activation of the motoneurones but requires descending voluntary drive. Furthermore, this effect may depress nearby, but not distant, spinal motoneurone pools and it reveals potentially novel mechanisms controlling the output of human motoneurones.
人类运动神经元池的输出在疲劳运动时会减少，但其涉及的机制尚不清楚。我们探究了持续最大自主收缩（MVC）后，回返性运动神经元放电（F波）的抑制情况。MVC使手部肌肉中F波的大小和频率降低，但次最大收缩（50%MVC）则不会。令人惊讶的是，通过刺激尺神经（20或40Hz）逆向激活运动神经元，并不会降低F波面积或持续时间。此外，手部肌肉持续3分钟的MVC会抑制其拮抗肌中的F波，但不会抑制远处手部肌肉中的F波。我们的研究结果表明，自主收缩后F波的抑制并非仅仅由于运动神经元的重复激活，而是需要下行自主驱动。此外，这种效应可能会抑制附近而非远处的脊髓运动神经元池。
重复性活动和疲劳会引起脊髓的重大变化，这可能导致“中枢”疲劳，但其涉及的机制在人类中尚不清楚。在此，我们证实了在手部固有肌肉（小指展肌，ADM）持续最大自主收缩（MVC）后立即放松期间，回返性运动神经元放电（F波）减少，并通过三种方式探究了运动神经元放电与F波抑制之间的关系。首先，在10秒的MVC后出现了F波抑制（F波面积和F波持续时间均有抑制）（初始下降36.4±19.1%；平均值±标准差），但在50%最大收缩力的次最大自主收缩后未出现。其次，为了在无自主努力的情况下诱发运动神经元放电，在肘部通过超最大尺神经刺激以25和40Hz的生理频率进行了10秒的强直收缩。令人惊讶的是，在腕部对尺神经进行超最大刺激以测试ADM中的F波时，两者均未产生F波抑制。最后，ADM（第四掌骨间肌）的拮抗肌持续MVC（3分钟）会抑制解剖学上相邻的ADM中的F波（20±18.2%），但不会抑制手部桡侧更远处的第一背侧骨间肌中的F波。我们认为，自主收缩后F波的抑制可能并非由于运动神经元的重复激活，而是需要下行自主驱动。此外，这种效应可能会抑制附近而非远处的脊髓运动神经元池，并且揭示了控制人类运动神经元输出的潜在新机制。
