Rubrospinal control of static and dynamic fusimotor neurones

Rubrospinal effects on about 60 extracellularyl recorded gamma-motoneurones were studied in anesthetized cats. All cells were antidromically identified from various muscle nerves. 23 cells were regarded as dynamic as they were activated from a mesencephalic region previously known to influence selectively muscle spindle dynamic sensitivity. The pattern of rubrospinal influence on static fusimotor neurones to different muscles closely followed that previously demonstrated for alpha-motoneurones with pr edominantly excitation of flexor neurones and excitation or inhibition in equal amounts of extensor cells. Dynamic fusimotor neurones were influenced in a strictly reciprocal manner with excitation of flexor cells and inhibition of extensor cells except for a few neurones which could not be reached from nucleur ruber. Evidence was also obtained indicating that the shortest path from nucleus ruber to static fusimotor neurones involves one interneurone.