Physiological processes influencing motor-evoked potential duration with voluntary contraction

Abstract

Voluntary contraction leads to facilitation of motor-evoked potentials (MEPs) producing greater amplitude, shorter onset latency, and prolonged duration of the electromyography potential. Whereas hyperexcitability of spinal motoneurons and changes in descending corticospinal volleys have been proposed as putative mechanisms for changes in MEP amplitude and onset latency, a contribution of propriospinal interneurons, exerting modulatory effects on α-motoneurons, has been proposed as a potential explanation for prolongation of MEP duration. The aim of the present study is to gain further insight into the physiological processes underlying changes in MEP duration. Transcranial magnetic stimulation (TMS) studies were undertaken on 30 healthy controls, using a 90-mm circular coil, with MEPs recorded at rest and during facilitation, produced by contraction of abductor pollicis brevis. In the same experiment, short interval-intracortical inhibition (SICI) was recorded at rest. Facilitation resulted in a significant prolongation of MEP duration, which increased with stimulus intensity and was accompanied by an increase in MEP amplitude. The main effect (TMS intensity × activation state) was correlated with MEP duration (F = 10.9, P < 0.001), whereas TMS intensity (F = 30.5, P < 0.001) and activation state (F = 125.8, P < 0.001) in isolation were correlated with MEP amplitude. There was a significant inverse relationship between SICI and MEP duration at rest (R² = 0.141, P = 0.041) and during facilitation (R² = 0.340, P = 0.001). The present findings suggest that similar physiological processes mediate changes in the facilitated MEP duration and amplitude and that both cortical and nonpropriospinal spinal mechanisms contribute to changes in MEP duration.NEW & NOTEWORTHY Muscle contraction is associated with a significant increase in motor-evoked potential (MEP) duration and amplitude. Whereas the increase in MEP duration was linear, the amplitude increase exhibited a ceiling effect. Importantly, the MEP duration increase strongly correlated with short interval-intracortical inhibition, a biomarker of motor cortical function. This suggests that whereas similar physiological processes contribute to changes in facilitated MEP duration and amplitude, cortical mechanisms appear to contribute to MEP duration changes.

Keywords: MEP duration; cortical processes; propriospinal.

Fig. 1.

The motor-evoked potential (MEP) duration at rest (A) exhibited longer onset latency (OL) and was of shorter duration when compared with the facilitated MEP response (B). Left dotted, vertical line represents the onset latency; middle dotted, vertical line depicts the end of the rest MEP response; and right dotted, vertical line depicts the end of the facilitated MEP response. The MEP tail (in milliseconds) was defined as the difference between the end of the facilitated MEP (right vertical, dotted line) and the end of rest MEP (middle vertical, dotted line) responses. MEP responses were generated by stimulus intensity set to 150% resting motor threshold (RMT).

Fig. 2.

The motor-evoked potential (MEP) duration, expressed as a percentage of the compound muscle action potential (CMAP), was significantly higher with facilitation (white bars) when compared with rest (gray bars) at each level of stimulus intensity, expressed as percentage of resting motor threshold (RMT). Horizontal, center lines show the medians; box limits indicate the 25th and 75th percentiles; and whiskers extend to minimum and maximum values. All paired comparisons between rest and facilitation were significantly different at a level of at least P < 0.05. APB, abductor pollicis brevis; TMS, transcranial magnetic stimulation.

Fig. 3.

The duration of the motor-evoked potential (MEP) tail increased with transcranial magnetic stimulation (TMS) intensity. Horizontal, center lines show the medians for each intensity, whereas individual subject results are depicted as open diamonds.

Fig. 4.

The motor-evoked potential (MEP) amplitude, expressed as a percentage of the compound muscle action potential (CMAP) amplitude, was significantly higher with facilitation (white bars) than at rest (gray bars) at each stimulus intensity [expressed as percentage of resting motor threshold (RMT)]. Horizontal, center lines show the medians; box limits indicate the 25th and 75th percentiles; and whiskers extend to minimal and maximal values. All paired comparisons between rest and facilitation were significantly different. TMS, transcranial magnetic stimulation.

Fig. 5.

Relationship between the motor-evoked potential (MEP) duration, expressed as a percentage of the compound muscle action potential (CMAP) duration, and MEP amplitude. Each data point represents an averaged MEP duration and amplitude value for a specific level of stimulus intensity and muscle activation state in each individual. The dashed line represents the optimal curvilinear model.

Fig. 6.

Relationship between mean short interval-intracortical inhibition (SICI) between interstimulus intervals 1–7 ms and the motor-evoked potential (MEP) duration, expressed as a percentage of the compound muscle action potential (CMAP) duration, at rest (A) and with facilitation (B). Each data point represents the averaged SICI and MEP duration [single pulse at 150% resting motor threshold (RMT)] for an individual subject (n = 30). The dashed lines represent the optimal linear and curvilinear models (logarithmic function).