Abstract
For arm movements, it is suggested that indirect corticomotoneuronal pathways, which are partly mediated by C3-C4 propriospinal neurons (PNs), have important roles as well as direct pathways. The purposes of the present study were to establish a procedure to quantify functions of PNs, and to examine left-right differences of the pathways by using the procedure. Surface electromyograms were recorded from the right or left biceps (Bi) muscle of right-handed normal human subjects (n=11), who all gave informed consent. During weak tonic voluntary contraction of the muscles, 1) electric stimulation of the ipsilateral ulnar nerve (wrist) alone, 2) transcranial magnetic stimulation (TMS) of the contralateral primary motor cortex alone, and 3) combined stimulation of both were delivered in pseudo-random order. Interstimulus intervals for the combined stimulation were set to 8.0-9.0ms (ulnar stimulation ahead). When appropriate stimulus strengths were selected, TMS-induced potentials in Bi were facilitated by combined ulnar stimulation. However, the facilitation was disappeared when strengthening ulnar stimulation and/or TMS. These were the case on both sides of all subjects, though appropriate stimulus strengths were variable among the subjects. When comparing the maximum facilitations on both sides, they tended to be stronger on the dominant (right) side. We conclude that the current procedure is valuable to quantify functions of PNs, though stimulus condition and handedness have to be considered. [J Physiol Sci. 2006;56 Suppl:S190]
