Short interval intracortical inhibition and facilitation during the silent period in human

Abstract

Following a suprathreshold transcranial magnetic stimulation (TMS) to the primary motor cortex (M1) during voluntary muscle contraction, a motor evoked potential (MEP) occurs in the target muscle followed by a silent period (SP) in the electromyographic (EMG) activities. The present study investigated how short interval intracortical inhibition (SICI) and intracortical facilitation (ICF) change during the SP. The time course of MEP and motor threshold during the SP were examined in the right first dorsal interosseous muscle. Using a triple-pulse protocol, SICI and ICF were tested at different times during the SP. The effects of different intensities of the conditioning stimulation (CS) for SICI and ICF were also investigated during the SP and at rest. During the SP, MEP was inhibited and motor threshold was increased, whereas MEP latency and background EMG level were same as those at rest. SICI decreased during the SP over a wide range of CS intensities. ICF increased at higher CS intensity. We conclude that SICI is suppressed and ICF is facilitated during the SP and the effects are separate from the interruption of voluntary drive.

Figure 1

Experimental design Subject performed 20% of maximum EMG. Up to three stimulations were included in the experiment. S1 alone can evoke a large MEP followed by a SP of 150 ms. CS and TS refer to the conditioning and test stimulations for eliciting SICI and ICF. They were delivered at different times during the SP. The fluctuation of the second MEP (grey) was measured to evaluate the SICI and ICF during the SP.

Figure 2

Time course of MEP during the SP Mean values and standard deviations (n = 11) of MEP amplitudes at S1–TS intervals from 70 to 180 ms. Each MEP amplitude was a percentage value of that evoked by TS alone (dash line). **P < 0.01, *P < 0.05, comparing to MEP evoked by TS alone.

Figure 3

Background EMG area, MEP latency and threshold at different times during the SP Mean values and standard deviations (n = 11) of background EMG areas (A), MEP latencies (B) and MEP thresholds (C) at S1–TS intervals of 80, 110, 140 and 170 ms. Note that the intervals of 80, 110 and 140 ms were during the SP, and 170 ms was after EMG recovery. **P < 0.01, comparing to MEP threshold at rest (RMT).

Figure 4

Typical recordings of SICI and ICF during the SP (S1–TS interval of 110 ms) and at rest Every trace represents the average of 10 trials. Traces on the left side are recordings during the SP (with S1 applied 110 ms preceding the TS); those on the right side are recordings at rest. The top trace showed S1 alone was delivered. The second row shows the recordings without CS delivery (S1–TS during the SP, TS alone at rest). The traces from the third to the fifth row showed the CS–TS intervals of 1, 2.5 and 10 ms (triple-pulse during the SP, CS–TS at rest).

Figure 5

SICI and ICF during the SP when CS was 0.95 AMT Mean values and standard deviations (n = 8) of MEP amplitudes at different S1–TS intervals. Triangles show the MEPs at a CS–TS interval of 1 ms, open circles show those of 2.5 ms and filled circles show those of 10 ms. A, SICI and ICF during the SP (S1–TS intervals of 80, 110 and 140 ms) were compared to those at rest. B, SICI and ICF after EMG recovery (S1–TS interval of 170 ms) were compared to those at the active state. **P < 0.01, *P < 0.05, comparing the MEP during the SP to that at rest.

Figure 6

SICI and ICF during the SP when CS was adjusted to 0.8 MT Mean values and standard deviations (n = 6) of MEP amplitudes at different S1–TS intervals. Triangles show the MEPs at a CS–TS interval of 1 ms, open circles show those of 2.5 ms and filled circles show those of 10 ms. Note that CS intensities were adjusted to MT₈₀, MT₁₁₀ and MT₁₄₀ for different S1–TS intervals, respectively. **P < 0.01, *P < 0.05, comparing the MEP during the SP to that at rest.

Figure 7

Different SICI and ICF during the SP and at rest when same TS intensity was used Mean values and standard deviations (n = 6) of MEP amplitudes at S1–TS interval of 110 ms and at rest. Triangles show the MEPs from a CS–TS interval of 1 ms, open circles show those of 2.5 ms and filled circles show those of 10 ms. Note that same intensity of TS was used during the SP and at rest. ***P < 0.001, **P < 0.01, comparing the MEP during the SP to that at rest.

Figure 8

Effect of variations in CS intensities on SICI and ICF Open circles show mean MEP amplitudes (n = 6) at rest, and filled circles show those during the SP. Horizontal axis indicates the CS intensity. Note that the intensities used during the SP were adjusted to the ratios of MT₁₁₀, and those used at rest were the ratios of RMT. Vertical axis indicates the MEP amplitude. A, SICI at a CS–TS interval of 1 ms. B, SICI at a CS–TS interval of 2.5 ms. C, ICF at a CS–TS interval of 10 ms **P < 0.01, *P < 0.05, comparing the MEP during the SP to that at rest.

Figure 9

Proposed model The diamond (labelled I) at the centre indicates a group of common interneurons. They project to the pyramidal neurons that produce the output to the spinal motoneurons. The common interneurons receive inputs from the SICI, ICF and SP neurons. Voluntary drive is projected onto the common interneurons from other brain areas. The small filled circles show inhibitory connections, and small open circles show facilitatory connections. S1 activates the SP neuron. CS activates the SICI (1 ms and 2.5 ms) and ICF neurons. TS activates the common interneuron. SICI (1 ms and 2.5 ms) is shown as mediated by GABA_A receptor, whereas the SP neuron is mediated by GABA_B receptor. The SP neurons are shown to inhibit SICI through the presynaptic inhibition and facilitate ICF. The question marks and dash lines indicate the interactions are not proven.