Facilitation of magnetic motor evoked potentials during the mixed nerve silent period

We studied motor neuron excitability during the mixed nerve silent period (MNSP) in a hand muscle using magnetic motor evoked potentials (MEPs) and F-waves. MEPs elicited between the V1 and V2 potentials of the MNSP were much larger than control MEPs elicited at rest, and were even comparable in size to control MEPs elicited during voluntary contraction. This facilitation of MEPs occurred without shortening of MEP latency, suggesting a supraspinal mechanism. MEPs were facilitated during the MNSP when elicited with a figure-8-shaped coil in a posterior-anterior orientation, but not when MEPs of the same size were elicited with the coil held in a lateral-medial orientation. F-waves elicited during the MNSP were variable between subjects, and not consistently different from control F-waves elicited at rest. Our findings may reflect increased cortical motor excitability during the MNSP, possibly related to activation of muscle afferents by mixed nerve stimulation.