Short and Long Loop Reflexes Evoked in Thenar Muscles During Unilateral as Compared to Bilateral Hand Contraction

After electrical stimulation of the median nerve at the wrist, an initial component named V1 (short loop reflex) and a later component named V2 (long loop reflex) are recorded from contracted thenar muscles. V1 appears 26～31 msec with H reflex characteristics, and is thought to originate in the spinal circuit. V2 appears 46～61 msec after stimulation, and is thought to be transcortical origin. To find out whether contracted muscle afferents influence the excitability of cortical or spinal projections to contralateral muscles, we studied six healthy subjects. Unilateral and bilateral simultaneous isometric contraction of the thenar muscles was compared by analyzing V1 and V2 evoked from right thenar muscles. The amplitude of V1 in bilateral simultaneous contraction was significantly lower than unilateral contraction of the thenar muscles. We propose that contralateral contracted muscle afferents suppress the excitability of spinal efferents to ipsilateral flexor muscles in the hand. The inhibitory effect occurs at the short loop reflex and may assist spinal pathways by double reciprocal Ia inhibition and crossed extension reflex by II afferents. The importance of the spinal circuit for interlimb interaction during voluntary movement should be stressed