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

Abstract

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.-motoneurons of cats anesthetized with chloralose. The results were obtained during a series of experiments in which reflex effects from muscle, skin and joint afferents were also investigated. Cells (90) tested with stimulation both in NR and in MesADC were classified as dynamic (influenced from MesADC) or static (not influenced). Of these .gamma.-cells 84 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 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. The segmental latency for rubro-bulbospinally mediated excitatory effects could be measured for only 1 cell, and it is tentatively suggested that the pathway to this dynamic .gamma.-motoneuron involves 1 or 2 interneurons 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 rubrospinal effects on .alpha.-motoneurons and both classes of .gamma.-motoneurons indicate that the rubrospinal tract has the ability to coactivate .alpha.- and .gamma.-motoneurons. 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.