The source and distribution of short-term synchrony between gamma-motoneurones in the cat

Abstract

The naturally occurring background discharges of gamma-motoneurones to gastrocnemius and soleus in the decerebrated, spinal cat show a degree of short-term synchrony which is absent when the spinal cord is intact. The characteristics of this synchrony and the mechanism producing it have been examined in the preceeding paper (Ellaway & Murthy, 1985). Short-term synchrony in the background discharge is removed by cutting the ipsilateral dorsal root (S1 or L7) at the segment of origin of the gamma-efferents. It persists on cutting other ipsilateral or any contralateral dorsal roots between L6 and S3. The source is not the homonymous or synergist muscle group. Neither is the skin which lies around the insertion of gastrocnemius-soleus on the heel the afferent source of this synchrony. However, stimulation of the heel by light stroking or pressure raises the frequency of gamma-efferent discharge and increases the degree of synchrony. Such stimulation does not produce synchrony in the decerebrated cat with an intact spinal cord. Synchrony between gamma-motoneurones is as strong between different fascicles of a muscle (gastrocnemius medialis), or different heads (gastrocnemius medialis and lateralis), as it is within one fascicle of a muscle. The timing of the peak incidence of correlated discharges always indicated synchrony at the level of the spinal cord irrespective of differences in peripheral conduction time or the destination of the axons. Hamstring and pretibial flexors show a lower incidence or weaker degree of short-term synchrony between homonymous gamma-efferents compared with gastrocnemius. Short-term synchrony is either weak or absent between gamma-motoneurones to heteronymous muscles. gamma-Efferents from homologous muscles on either side of the cord lack synchrony of discharge. The results go some way to establishing the source and connectivity to the motoneurone pool of afferents capable of synchronizing the discharge of gamma-efferents.