The influence of DOPA on the static and the dynamic fusimotor activity to the triceps surae of the spinal cat.

The influence of descending “noradrenergic” fibres on the stretch reflex and the discharge of γ-motoneurones to the triceps surae has been analysed by injecting DOPA i.v. in the acute spinal unanesthetized cat. After DOPA, a tonic stretch reflex develops in the soleus muscle as revealed by an increased electromyographical activity on extension of the muscle. In parallel, static γ-motoneurones aqcuire a resting discharge, which can be inferred from the finding that secondary endings have an elevated discharge rate after DOPA. The discharge in single γ-efferents could be influenced through short latency reflex paths from afferent fibres including group II afferents; further, the γ-efferents were found to participate in the late and long-lasting reflex effects that can be evoked after DOPA. On the basis of their resting discharge and reflex effects, the γ-efferents recorded could tentatively be identified as static or dynamic. Both static and dynamic γ-motoneurones to extensors have a higher resting discharge after DOPA. Hence in contrast to previous findings on flexor γ-motoneurones no reciprocal control on the resting discharge in the two types of efferents is exerted. Only γ-motoneurones classified as static received monosynaptic activation from fast descending fibres, presumably vestihulospinal.