The bilateral reflex control of the trapezius muscle in humans

Electromyographic recordings were made from the trapezius muscle in 18 healthy subjects using surface electrodes placed bilaterally. A mechanical tap applied to the insertion of the lower fibres of trapezius evoked a reflex at 11.9 +/- 1.4 ms (mean +/- SD) in the ipsilateral trapezius muscle. In addition and surprisingly, short latency, facilitatory reflexes were also seen on the contralateral side at a latency of 14.3 +/- 1.7 ms. Electrical stimulation of the afferent nerve to trapezius, the cervical nerve of C3/4, also evoked short latency, facilitatory reflexes from both the ipsilateral and contralateral muscles. Since this nerve carries the la fibres from trapezius, this reflex would appear to be the equivalent of the H reflex seen in the soleus muscle. The latency of this reflex was 10.9 +/- 1.4 ms ipsilaterally and 11.9 +/- 1.5 ms contralaterally, the difference being only 1.0 +/- 0.7 ms. Voluntary activity of the ipsilateral trapezius increased the amplitude of both mechanically and electrically evoked ipsilateral and crossed reflexes, and vibration of the ipsilateral trapezius decreased the amplitude of both reflexes. These results together suggest that the earliest parts of both ipsilateral and crossed reflexes are monosynaptic in origin. If the crossed reflex is, in part, due to a common la presynaptic input, correlation of bilateral trapezius activity should produce a peak in the cross-correlogram. Indeed, cross-correlation of the bilateral trapezius activity during elevation of the arms produced a short duration peak. Moreover, this was significantly larger when compared to that constructed from activity of the upper and lower fibres of trapezius from one side. These results imply that muscle spindle afferents from the ipsilateral trapezius monosynaptically activate motoneurones contralaterally.