Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 13, 359
eCollection

Excitability of Upper Layer Circuits Relates to Torque Output in Humans

Affiliations

Excitability of Upper Layer Circuits Relates to Torque Output in Humans

Alexander Kurz et al. Front Hum Neurosci.

Abstract

The relation between primary motor cortex (M1) activity and (muscular) force output has been studied extensively. Results from previous studies indicate that activity of a part of yet unidentified neurons in M1 are positively correlated with increased force levels. One considerable candidate causing this positive correlation could be circuits at supragranular layers. Here we tested this hypothesis and used the combination of H-reflexes with transcranial magnetic stimulation (TMS) to investigate laminar associations with force output in human subjects. Excitability of different M1 circuits were probed at movement onset and at peak torque while participants performed auxotonic contractions of the wrist with different torque levels. Only at peak torque we found a significant positive correlation between excitability of M1 circuits most likely involving neurons at supragranular layers and joint torque level. We argue that this finding may relate to the special role of upper layer circuits in integrating (force-related) afferent feedback and their connectivity with task-relevant pyramidal and also extrapyramidal pathways projecting to motoneurones in the spinal cord.

Keywords: M1; TMS; force output; human; laminar; motor control; nerve stimulation.

Figures

Figure 1
Figure 1
(A) Principle of conditioning an H-reflex evoked by peripheral nerve stimulation (PNS) with transcranial magnetic stimulation (TMS). TMS and PNS were applied together, so that TMS-triggered volleys and the afferent volleys from PNS coincided at the spinal motoneurons. This leads to an increased recruitment of spinal motoneurons (middle part) and a corresponding increase in the size of the electromyographic response in the flexor carpi radialis (FCR) H-reflex (lower part). At early facilitation delay (EFD) 0 ms, the fastest conducting corticospinal volley from TMS (blue arrow) arrive at the same time at the spinal motoneurons as the fastest conducting afferent volley from PNS. At EFD +0.6 ms, subsequent volleys (orange arrow) arrive at the same time as the fastest afferent volley. Figure adapted from Kurz et al. (2019). (B) EFD 0 ms was determined by a two-step procedure in each individual: we first tested delays between the application of TMS and the application of PNS from −5 ms to −2 ms, in steps of 0.5 ms (negative delays indicate that TMS was triggered after PNS). EFD 0 ms in this example was at a delay of −3.5 ms; conditioned H-reflexes at this delay and at the subsequent delays were higher than unconditioned test H-reflexes. The gray rectangle illustrates the time window tested in the second step of the procedure, shown in (C). (C) Delays from −4.5 ms to −3.5 ms were again tested to denote EFD 0 ms in 0.1 ms steps (*P < 0.05). Figure adapted from Kurz et al. (2019).
Figure 2
Figure 2
(A) Schematic of the experimental setup. (B) Mean traces of the torque signal and its first derivative for the four torque levels (solid line: Level 1, dashed lines: Level 3–4). Solid arrows indicate stimulus timing. Red circles indicate Torquemax and rate of torque development (RTD) respectively. Dashed arrow indicates progression of Torquemax over the course of the torque levels. (C) The bar plots illustrate mean Torquemax of all subjects across the four torque levels for the stimulation at movement onset and peak torque (*p < 0.05). (D) Unconditioned H-reflexes of all subjects (mean ± SEM) across the four torque levels for the stimulation at movement onset and peak torque. (E) Mean traces of the unconditioned (gray) and conditioned (black) H-reflex. Gray and red shaded area illustrate the area used to quantify H-reflex facilitation. The dashed rectangle illustrates the time window which is used in (F) as an expanded view for more detail. (F) Expanded view showing H-reflex facilitation for the stimulation at peak torque for EFD 0 ms and EFD 0.6 ms across the torque levels. Gray and red dashed lines indicate progression of unconditioned and conditioned H-reflex over the course of the torque levels.
Figure 3
Figure 3
(A) Mean H-reflex facilitation (conditioned H-reflex/unconditioned H-reflex × 100%) and Torquemax (Nm) of all subjects across the four torque levels for the stimulation at movement onset and peak torque. Color code represents means of single subjects. (B,C) Correlations. In all graphs, z-scores of the four torque levels of each individual are plotted. The ordinate shows the z-score of the magnitude of H-reflex facilitation, the abscissa shows the z-score of the magnitude of Torquemax (B) and RTD (C). Color code represents data of single subjects. Least squares fit for each condition is indicated with the red line. Asterisks indicate significant correlation (p < 0.01).

Similar articles

See all similar articles

References

    1. Arber S., Costa R. M. (2018). Connecting neuronal circuits for movement. Science 360, 1403–1404. 10.1126/science.aat5994 - DOI - PubMed
    1. Baker S. N., Perez M. A. (2017). Reticulospinal contributions to gross hand function after human spinal cord injury. J. Neurosci. 37, 9778–9784. 10.1523/JNEUROSCI.3368-16.2017 - DOI - PMC - PubMed
    1. Cheney P. D., Fetz E. E. (1980). Functional classes of primate corticomotoneuronal cells and their relation to active force. J. Neurophysiol. 44, 773–791. 10.1152/jn.1980.44.4.773 - DOI - PubMed
    1. Cheney P. D., Fetz E. E. (1984). Corticomotoneuronal cells contribute to long-latency stretch reflexes in the rhesus monkey. J. Physiol. 349, 249–272. 10.1113/jphysiol.1984.sp015155 - DOI - PMC - PubMed
    1. Conrad B., Wiesendanger M., Matsunami K., Brooks V. B. (1977). Precentral activity related to control of arm movements. Exp. Brain Res. 29, 85–95. 10.1007/bf00236877 - DOI - PubMed

LinkOut - more resources

Feedback