Motor cortex excitability following repetitive electrical stimulation of the common peroneal nerve depends on the voluntary drive

Abstract

Previously, long-term changes in the motor cortex have been reported after repetitive electrical nerve stimulation (rES) as well as after motor exercise. The purpose of this study was to investigate whether the effects of voluntary motor cortical drive and of rES on the motor cortical output in healthy subjects interact with each other. A 30-min exercise session was performed during the following conditions: rES of the right common peroneal nerve (CPN) during rES at rest (A); voluntary exercise of the right ankle dorsiflexors alone (B); rES combined with voluntary dorsiflexion exercise (C); voluntary exercise of ankle plantar flexors alone (D); and plantar flexion exercise combined with rES (E). Motor evoked potentials (MEPs) were obtained before and after the exercise with a stimulation intensity of 125% of the threshold of the relaxed right tibialis anterior (TA). rES was ON for 1 s and OFF for 2 s in a cycle, and consisted of trains of five pulses, duration 1 ms and frequency 30 Hz, as applied in functional electrical stimulation (FES). MEPs of the TA muscle elicited after the training were increased in A by 38%, in B by 35%, and in C by 66%. In D and E, the MEPs of TA were decreased by 29% and 35%, respectively. The effect was maintained for at least 30 min after the nerve stimulation was completed. Consistent with previous studies (Khaslavskaia et al. (2002) Exp Brain Res 145:309–315), MEPs after the CPN rES are shown to be partly due to increased TA cortical excitability. These results suggest that the effect of FES on motor cortical excitability depends on the concurrent motor cortical drive present at the time of FES, and the combination of these factors modulates neural excitability and probably reorganization. The decrease in motor cortical excitability after plantar flexor exercise probably means that voluntary effort antagonistic to the electrical exercise is stronger and cancels out the effects of rES. Improving FES effects through an agonist voluntary drive implies an enhancement of sensorimotor reorganization through the addition of a voluntary component to a trained movement. Possible mechanisms and implications of these results on the rehabilitation of patients with paralysis and spasticity are discussed.