An intracellular study of rubrospinal and rubro-bulbospinal control of lumbar gamma-motoneurones

The actions elicited by electrical stimulation of the rubrospinal path in NR (nucleus ruber), and by stimulation of the rubro-bulbospinal path in MesADC (mesencephalic area for dynamic control), were studied with intra-, juxta- or extracellular recordings in lumbar gamma-motoneurones of cats anaesthetized with chloralose. The results were obtained during a series of experiments in which reflex effects from muscle, skin and joint afferents were also investigated. 90 cells tested with stimulation both in NR and in MesADC were classified (cf. Appelberg 1981) as dynamic (influenced from MesADC) or static (not influenced). 84 of these gamma-cells responded to stimulation in the red nucleus. The same response pattern was found for dynamic and static gamma-cells. For flexor cells, excitation was by far predominant while equal numbers of extensor cells showed excitatory and inhibitory effects. The shortest route to both static and dynamic gamma-cells was disynaptic. With stimulation of the rubro-bulbospinal path, excitatory effects were more than twice as frequent as inhibitory effects. No clearcut difference was seen between extensor and flexor cells. The segmental latency for rubro-bulbospinally mediated excitatory effects could be measured for only one cell, and it is tentatively suggested that the pathway to this dynamic gamma-motoneurone involves one or two interneurones more than in the rubrospinal pathway. Spontaneous activity was significantly more common among the dynamic cells, but the axonal conduction velocities of dynamic and static cells did not differ. The similarities between rubro-spinal effects on alpha-motoneurones and both classes of gamma-motoneurones indicate that the rubrospinal tract has the ability to coactivate alpha- and gamma-motoneurones (Granit 1955, 1979). The comparison of the properties of the population of gamma-cells classified as dynamic or static showed marked differences strengthening the reliability of the previously introduced classification method.