Facultad de Ciencias Biologicas, Benemérita Universidad Autónoma de Puebla, C.P. 72000 Puebla, Mexico.
Faculty of Medical Sciences, Newcastle University, Newcastle upon Tyne NE2 4HH, United Kingdom.
J Neurosci. 2022 Oct 5;42(40):7634-7647. doi: 10.1523/JNEUROSCI.2473-21.2022. Epub 2022 Sep 2.
Reaction time is accelerated if a loud (startling) sound accompanies the cue-the "StartReact" effect. Animal studies revealed a reticulospinal substrate for the startle reflex; StartReact may similarly involve the reticulospinal tract, but this is currently uncertain. Here we trained two female macaque monkeys to perform elbow flexion/extension movements following a visual cue. The cue was sometimes accompanied by a loud sound, generating a StartReact effect in electromyogram response latency, as seen in humans. Extracellular recordings were made from antidromically identified corticospinal neurons in primary motor cortex (M1), from the reticular formation (RF), and from the spinal cord (SC; C5-C8 segments). After loud sound, task-related activity was suppressed in M1 (latency, 70-200 ms after cue), but was initially enhanced (70-80 ms) and then suppressed (140-210 ms) in RF. SC activity was unchanged. In a computational model, we simulated a motoneuron pool receiving input from different proportions of the average M1 and RF activity recorded experimentally. Motoneuron firing generated simulated electromyogram, allowing reaction time measurements. Only if ≥60% of motoneuron drive came from RF (≤40% from M1) did loud sound shorten reaction time. The extent of shortening increased as more drive came from RF. If RF provided <60% of drive, loud sound lengthened the reaction time-the opposite of experimental findings. The majority of the drive for voluntary movements is thus likely to originate from the brainstem, not the cortex; changes in the magnitude of the StartReact effect can measure a shift in the relative importance of descending systems. Our results reveal that a loud sound has opposite effects on neural spiking in corticospinal cells from primary motor cortex, and in the reticular formation. We show that this fortuitously allows changes in reaction time produced by a loud sound to be used to assess the relative importance of reticulospinal versus corticospinal control of movement, validating previous noninvasive measurements in humans. Our findings suggest that the majority of the descending drive to motoneurons producing voluntary movement in primates comes from the reticulospinal tract, not the corticospinal tract.
如果伴随提示出现响亮的（惊吓的）声音，则反应时间会加快——即“StartReact”效应。动物研究表明，网状脊髓系统是惊吓反射的基础；StartReact 可能同样涉及网状脊髓束，但目前尚不确定。在这里，我们训练了两只雌性猕猴，让它们在视觉提示后进行肘部屈伸运动。提示有时伴随着响亮的声音，这会在肌电图反应潜伏期中产生 StartReact 效应，就像在人类中看到的那样。我们从初级运动皮层 (M1)、网状结构 (RF) 和脊髓 (C5-C8 节段) 中对逆行识别的皮质脊髓神经元进行了细胞外记录。在响亮的声音之后，M1 中的任务相关活动受到抑制（提示后 70-200 毫秒），但最初增强（70-80 毫秒），然后抑制（140-210 毫秒）在 RF 中。SC 活动保持不变。在计算模型中，我们模拟了一个运动神经元池，该池接收来自不同比例的平均 M1 和 RF 实验记录的活动的输入。运动神经元的放电产生了模拟的肌电图，从而可以测量反应时间。只有当≥60%的运动神经元驱动来自 RF（M1 驱动≤40%）时，响亮的声音才会缩短反应时间。随着来自 RF 的驱动增加，缩短的程度也会增加。如果 RF 提供的驱动<60%，响亮的声音会延长反应时间——与实验结果相反。因此，用于自愿运动的大部分驱动力很可能来自脑干，而不是大脑皮层；StartReact 效应幅度的变化可以衡量下行系统相对重要性的变化。我们的结果表明，响亮的声音对来自初级运动皮层的皮质脊髓细胞和网状结构中的神经尖峰有相反的影响。我们表明，这偶然允许通过响亮的声音产生的反应时间变化用于评估网状脊髓与皮质脊髓对运动的控制的相对重要性，验证了以前在人类中的非侵入性测量。我们的发现表明，在灵长类动物中产生自愿运动的运动神经元的大部分下行驱动来自网状脊髓束，而不是皮质脊髓束。
