Contractile properties of muscle: control by pattern of muscle activity in the rat.

[Plate IS] The denervated slow twitch rat soleus muscle was stimulated electrically for 3-6 weeks with brief trains of stimuli a t 100 HZ or long trains a t 10 Hz. I n both cases the mean stimulation frequency was 2 Hz. Muscles stimulated a t 100 Hz acquired several properties characteristic of fast muscle, whereas muscles stimulated a t 10 Hz remained slow. The results demonstrate the importance of pattern of muscle activity in determining the contractile properties of muscle. The differentiation of rat leg muscles into fast and slow muscles begins at birth and is completed within 2-3 months (Close 1964). This differentiation is probably initiated and certainly maintained by an influence of the nerve on the muscles since it may be reversed by cross connecting the nerves to the two types of muscle (Close r969 ; Barany & Close 1971). The signal responsible for these changes is not known. One possibility is that some trophic factor is released from the nerve (Buller, Eccles & Eccles 1960 b). Alternatively, the activity of the muscle itself could cause the differenhiation of fast and slow muscles (Buller et al. 1960 b ; Salmons & Vrbovh 1969 ; Brown 1973). Changes in contractile properties occur after experimental procedures which lead t o altered use of the muscle. Por example, immobilization by fixation of joints all alter contractile properties. The presence of an intact nerve in these prcparations makes it difficult to distinguish between those changes that may result directly from altered muscle activity and those that may be induced by neurotrophic factors.