The effects of intravenous infusion of the direct acting muscle relaxant, dantrolene sodium (5 mg kg-1), on tension, integrated EMG, soleus muscle motor unit discharge frequency and gamma nerve fibre discharge were measured in the decerebrate rat. 2. Dantrolene sodium did not have any detectable direct effect upon the discharge of the gamma nerve fibres. 3. The soleus muscle of the decerebrate preparations exhibited spontaneous tension and reflex responses. 4. With the muscle held at constant length, dantrolene sodium caused an increase in the integrated EMG in 15 out of 18 experiments and a decrease in muscle tension in 15 out of 17. The results from these experiments showed great variability. 5. Dantrolene sodium caused a slight reduction in the tension response to tonic stretch; this was accompanied by an increase in the integrated EMG. 6. Dantrolene sodium also caused a shift in the relationship between tension and integrated EMG during the phasic component of the stretch reflex, with a greater integrated EMG being associated with a reduced tension. 7. Motor unit discharge frequencies were found to increase but not sufficiently to overcome the action of dantrolene sodium. It is concluded that motor unit recruitment must play an important role in the compensation for the muscle weakening action of dantrolene sodium.
