Changes in tension-frequency relationship of motor units induced by their activity in rat muscle.

Abstract

In 57 motor units of rats medial gastrocnemius muscle the influence of activity on the course of 10 successively repeated tension-frequency curves (T-f curves) was observed. In slow motor units the steep parts of successive T-f curves usually shifted slightly towards higher frequencies. However, in some slow units of relatively short contraction these T-f curves shifted towards lower frequencies. In fast units, especially those of the FF type, more marked changes were observed in T-f curves. At first, a shift of the second to third curve towards lower frequencies was observed. Curves then shifted progressively towards higher frequencies. In all motor units greater changes in tension could be observed in unfused tetani than in fused tetani. In some motor units the tension of fused tetani showed a progressive decrease. Changes in the course of T-f curves related to changes in contraction and relaxation times. The results presented here explain how the duration of the activity can change the sensitivity of motor unit tension to the frequency of stimuli. The importance of the changes observed in the T-f relation in the process of fatigue is discussed.