Several methods for the evaluation of muscle relaxants in gamma motor activity.

(1) The active tension-extension diagram of slowly (2 mm/sec) stretched triceps surae muscle of the decerebrate cat has been shown to be a sensitive indicator of spinal motor activity. For example, diazepam at doses larger than 0.01 mg/kg shifted the diagram parallel to the right. (2) The effects of central muscle relaxants on the gamma system can be observed in functionally isolated group Ia fibres but also in the "integrated" afferent discharges of the whole (or a part of) S1 dorsal root in response to stretching of the triceps surae muscle. The latter has statistically more value than the former recording. The effect of diazepam could block the gamma fibres selectively. Diazepam at doses larger than 0.1 mg/kg could block the gamma activity totally. (3) More than 3 months after degeneration with dry ice, the reinnervated muscle shows a fully recovered twitch tension but fails to show gamma activity (alpha muscle). Diazepam has no effect on the discharge from the muscle spindles in the alpha muscle.