Effects of volitional contraction on intracortical inhibition and facilitation in the human motor cortex

Abstract

Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF)were assessed in the cortical motor area of the first dorsal interosseous muscle (FDI) of 16 healthy subjects. Paired-pulse TMS was delivered to the left hemisphere at the following interstimulus intervals (ISIs): 2 and 3 ms for SICI, 10 and 15 ms for ICF and 1-5 ms for SICF. Motor-evoked potentials were recorded from the resting and active right FDI. The effects exerted on SICI and ICF by four intensities (60-90% of active motor threshold, AMT) of the conditioning stimulus (S1) and by three levels of muscle contraction (10%, 25%, 50% of maximal voluntary contraction, MVC) were evaluated. The effects exerted on SICF were evaluated with two intensities (90% and 70% of AMT) of the test stimulus (S2) and with the same levels of muscle contraction. Results showed that: (i) during 10% MVC, maximum SICI was observed with S1 = 70% AMT; (ii) the amount of SICI obtained with S1 = 70% AMT was the same at rest as during 10% MVC, but decreased at higher contraction levels; (iii) ICF was observed only at rest with S1 = 90% AMT; (iv) SICF was facilitated at 10% and 25% MVC, but not at 50% MVC. We conclude that during muscle activation, intracortical excitability reflects a balance between activation of SICI and SICF systems. Part of the reduction in SICI during contraction is due to superimposed recruitment of SICF. Low intensity (70% AMT) conditioning stimuli can test SICI independently of effects on SICF at low contraction levels.

Utilizing Transcranial Magnetic Stimulation to Study the Human Neuromuscular System

“…For instance, the pulse intensities used in our protocol are not being expressed relative to motor threshold. In single-and paired-pulse TMS studies in appendicular muscles it is common for motor thresholds to be defined within a relatively small range (e.g., 1-3% of stimulator output), and conditioning and test pulses being expressed relative to threshold levels (e.g., conditioning pulses equal to 70% of motor threshold) 17 . We generally choose not to perform a protocol of this nature due to the additional number of pulses that would be required to precisely determine motor threshold.…”

Comparison of transcranial magnetic stimulation measures obtained at rest and under active conditions and their reliability

“…Also, the clarification of the relationship between resting and active TMS measures is necessary to interpret and compare adequately the results of studies using different approaches to set their stimulation protocols. For example, several authors set the intensity of paired-pulse TMS or rTMS stimulations by using a percentage of the resting motor threshold (rMT) (Fisher et al, 2002;Maeda et al, 2002), while others base their calculations on a percentage of the active motor threshold (aMT) (Di Lazzaro et al, 2006;Filipovic et al, 2010;Orth et al, 2003;Ortu et al, 2008).…”

Assessment of corticospinal excitability after traumatic spinal cord injury using MEP recruitment curves: a preliminary TMS study

“…Studying SICI over a range of intensities has become increasingly important for the recruitment of neurons by the test stimulus pulse that are differentially susceptible to SICI modulation, [12][13][14] and the contribution from short-interval intracortical facilitation activated by the conditioning stimulus cannot be ruled out. 15,16 It should also be considered that the stimulus intensities chosen to assess SICI are frequently based on the motor threshold, thus damage to descending motor fibers in SCI will invariably raise these TMS intensities resulting in overactivation of the intact motor cortex.…”