Corticospinal output and cortical excitation‐inhibition balance in distal hand muscle representations in nonprimary motor area

Abstract

Transcranial magnetic stimulation (TMS) of the superior frontal gyrus in the non-primary motor area (NPMA) can evoke motor-evoked potentials (MEPs) at 20 ms latency range in contralateral distal hand muscles similar to stimulation of M1 and indicating monosynaptic corticospinal tracts. We compared the intracortical inhibitory and excitatory balance in primary motor cortex (M1) and in NPMA by navigated single- and paired-pulse TMS (ppTMS). We also evaluated the spatial stability of muscle representations in M1 and NPMA by remapping 11 healthy subjects one year after the initial mapping. Resting motor threshold (rMT) was higher in NPMA than in M1 as were the MEP amplitudes evoked by 120% rMT stimulation intensity of the local MT. Short-interval intracortical inhibition (SICI) was significantly weaker in NPMA than in M1 at ISI of 2 ms and conditioning stimulus (CS) 80% rMT. Our findings suggest that the cortical hand representations in NPMA 1) are connected to lower motoneurons monosynaptically, 2) are less strictly organized, i.e. motoneuron population representing a discrete hand muscle is sparser and less dense than in M1 and 3) have the capacity to generate powerful, rapid muscle contraction if sufficient number of motoneurones are activated. In NPMA, local intracortical inhibitory and excitatory activity is mainly similar to that in M1. The lower SICI in NPMA at an ISI of 2 ms may reflect less strict topographic organization and readiness to reorganization of neural circuits during motor learning or after motor deficits.

Evolution of Premotor Cortical Excitability after Cathodal Inhibition of the Primary Motor Cortex: A Sham-Controlled Serial Navigated TMS Study

“…We found similar, i.e. a non-significant trend to faster response latencies [21], [53], reduced M1 and enhanced PMC corticospinal excitability. This study, like previous studies, cannot exclude the possibility that the future non-spherical estimation of deep currents along the anterior wall of the central sulcus might provide evidence for higher current densities in M1.…”

“…2 cm rostral to M1) (see for example, [1], [8], [17], [41], [52]). For further validation we utilized neurophysiological parameters provided by navigated brain stimulation, as described recently [21], [53]. We found the localization of the PMC by navigated stimulation to be successful.…”

“…Building on multiple previous studies that investigate PMC stimulation and differentiate M1 effects [15]–[17], [52], [57], and as described previously [21], [53], we retrieved volts per meter estimates to add additional confirmation that PMC stimulation did not induce co-stimulation of the (on average 2.25 cm) distant M1 site. Conversely, one might contend that the eXimia system's spherical model estimates validated for a cortical surface are not adequate for the depth of e.g.…”

Functional Corticospinal Projections from Human Supplementary Motor Area Revealed by Corticomuscular Coherence during Precise Grip Force Control

“…Therefore, it might be possible that projections from SMA on hand muscle motoneurons are more developed in humans, with their higher ability to do fine manual manipulation. Interestingly, it has recently been shown that motor-evoked potentials (MEPs) can be evoked in contralateral distal hand muscles only 20 ms after transcranial magnetic stimulation (TMS) of human SMA [15], [16]. Since the latency of these MEPs was comparable to those obtained with TMS of M1, this result strongly suggests the presence of CM projections from SMA on spinal motoneurons of hand muscles.…”

“…However, until now, their existence has been mainly revealed by anatomical approaches [8]–[13] and very few studies on their functional role have been reported [14]. More recently, non-invasive transcranial magnetic stimulation (TMS) studies in humans have shown motor evoked potentials in hand muscles following SMA stimulation with a latency comparable to that found for M1 stimulation [15], [16], strongly suggesting the existence of fast CS projections from SMA. However, since these TMS studies were performed with passive subjects, the functionality of the CS projections from SMA in motor control is still a matter of debate.…”