Reciprocal Control of Spontaneous Activity and Reflex Effects in Static and Dynamic Flexor γ‐motoneurones Revealed by an Injection of DOPA

Single γ‐efferents to flexors have been studied in the spinal unanesthetized cat before and after an intravenous injection of DOPA. The γ‐efferents could be classified in two groups, one with a high level of spontaneous activity that was decreased after an injection of DOPA, and another with a low level of activity that was enhanced after DOPA. These data together with differences in reflex behaviour between the two types of γ‐motoneurones provided a basis for a comparison with the deduced properties of dynamic and static γ‐motoneurones obtained from studies of spindle afferents with primary and secondary endings. It could be concluded that the former type is identical with dynamic and the latter with static γ‐motoneurones. Both dynamic and static neurones could be reflexly activated in the long‐latency flexor reflex occurring after DOPA. Likewise both types of γ‐motoneurones were activated in the short‐latency flexor reflex, the dynamic more effectively before and the static more effectively after DOPA. The results suggest that descending noradrenergic fibres control reflex activation and the spontaneous activity of γ‐motoneurones to flexors in a reciprocal fashion.