Discipline of Physiology, School of Biomedicine, The University of Adelaide, Adelaide, Australia.
Department of Neurology & Stroke, Eberhard Karls University of Tübingen, Tübingen, Germany.
J Physiol. 2023 Jul;601(14):2959-2974. doi: 10.1113/JP284204. Epub 2023 May 29.
Previous research using transcranial magnetic stimulation (TMS) has demonstrated weakened connectivity between dorsal premotor cortex (PMd) and motor cortex (M1) with age. While this alteration is probably mediated by changes in the communication between the two regions, the effect of age on the influence of PMd on specific indirect (I) wave circuits within M1 remains unclear. The present study therefore investigated the influence of PMd on early and late I-wave excitability in M1 of young and older adults. Twenty-two young (mean ± SD, 22.9 ± 2.9 years) and 20 older (66.6 ± 4.2 years) adults participated in two experimental sessions involving either intermittent theta burst stimulation (iTBS) or sham stimulation over PMd. Changes within M1 following the intervention were assessed with motor-evoked potentials (MEPs) recorded from the right first dorsal interosseous muscle. We applied posterior-anterior (PA) and anterior-posterior (AP) current single-pulse TMS to assess corticospinal excitability (PA ; AP ; PA , early; AP , late), and paired-pulse TMS short intracortical facilitation for I-wave excitability (PA SICF, early; AP SICF, late). Although PMd iTBS potentiated PA and AP MEPs in both age groups (both P < 0.05), the time course of this effect was delayed for AP in older adults (P = 0.001). Furthermore, while AP , PA SICF and AP SICF were potentiated in both groups (all P < 0.05), potentiation of PA was only apparent in young adults (P < 0.0001). While PMd influences early and late I-wave excitability in young adults, direct PMd modulation of the early circuits is specifically reduced in older adults. KEY POINTS: Interneuronal circuits responsible for late I-waves within primary motor cortex (M1) mediate projections from dorsal premotor cortex (PMd), but this communication probably changes with advancing age. We investigated the effects of intermittent theta burst stimulation (iTBS) to PMd on transcranial magnetic stimulation (TMS) measures of M1 excitability in young and older adults. We found that PMd iTBS facilitated M1 excitability assessed with posterior-anterior (PA, early I-waves) and anterior-posterior (AP, late I-waves) current TMS in young adults, with a stronger effect for AP TMS. M1 excitability assessed with AP TMS also increased in older adults following PMd iTBS, but there was no facilitation for PA TMS responses. We conclude that changes in M1 excitability following PMd iTBS are specifically reduced for the early I-waves in older adults, which could be a potential target for interventions that enhance cortical excitability in older adults.
先前使用经颅磁刺激 (TMS) 的研究表明，背侧运动前皮层 (PMd) 与运动皮层 (M1) 之间的连接随年龄的增长而减弱。虽然这种改变可能是由两个区域之间的通信变化介导的，但 PMd 对 M1 内特定间接 (I) 波回路的影响随年龄的变化仍不清楚。因此，本研究调查了 PMd 对年轻和老年成年人 M1 中早期和晚期 I 波兴奋性的影响。22 名年轻成年人（平均年龄 ± 标准差，22.9 ± 2.9 岁）和 20 名老年人（66.6 ± 4.2 岁）参加了两项实验，涉及 PMd 上的间歇性 theta 爆发刺激 (iTBS) 或假刺激。干预后 M1 内的变化通过从右第一背间骨间肌记录的运动诱发电位 (MEP) 进行评估。我们应用后前 (PA) 和前后 (AP) 电流单脉冲 TMS 来评估皮质脊髓兴奋性 (PA ; AP ; PA, 早期；AP, 晚期)，并应用成对脉冲 TMS 短程皮质内易化来评估 I 波兴奋性 (PA SICF, 早期；AP SICF, 晚期)。尽管 PMd iTBS 增强了两组的 PA 和 AP MEPs（均 P<0.05），但这种效应在老年人中的时间进程延迟了（P=0.001）。此外，尽管两组的 AP 、PA SICF 和 AP SICF 均增强（均 P<0.05），但仅在年轻成年人中观察到 PA 的增强（P<0.0001）。尽管 PMd 影响年轻成年人的早期和晚期 I 波兴奋性，但直接 PMd 调节早期回路的作用在老年人中特异性降低。关键点：负责初级运动皮层 (M1) 内晚期 I 波的中间神经元回路介导来自背侧运动前皮层 (PMd) 的投射，但这种通信可能随年龄的增长而发生变化。我们研究了间歇性 theta 爆发刺激 (iTBS) 对 PMd 对年轻和老年成年人 M1 兴奋性的经颅磁刺激 (TMS) 测量的影响。我们发现，PMd iTBS 促进了年轻成年人的 M1 兴奋性，通过后前 (PA ，早期 I 波) 和前后 (AP ，晚期 I 波) 电流 TMS 进行评估，AP TMS 的效果更强。PMd iTBS 后，老年人的 M1 兴奋性也增加，但 PA TMS 反应没有增强。我们得出结论，PMd iTBS 后 M1 兴奋性的变化在老年人中特异性降低了早期 I 波，这可能是增强老年人皮质兴奋性的干预措施的潜在靶点。
