1. Serial sections of flexor digitorum longus muscle (f.d.l.) of the cat were examined histochemically for four enzyme systems: adenosine triphosphatase (ATPase) with alkaline and acid pre-incubation, phosphorylase and succinic dehydrogenase (SDHase).2. The number of types into which fibres should be divided was assessed by estimating enzyme reaction intensity from measurements of light transmission through photomicrographs. It was concluded that in general the enzyme reaction intensities of fibres were distributed continuously. However, the distribution histograms showed two (phosphorylase and SDHase) or three (acid and alkaline ATPase) clear peaks. Eighteen combinations of reaction intensities (profiles) were seen of which eight were very rare. The distribution of profiles differed between individuals but were similar in right and left muscles.3. Areas of fibres were measured from muscles which had been fixed at the length at which twitch tension was maximal. The variance in fibre area with any one profile was significantly less than the variance in fibre area of all fibres within a muscle. There were significant differences between the mean areas of fibres with different profiles.4. If only three enzyme reactions are considered (acid and alkaline ATPase and phosphorylase) the majority of fibres fall into one of the three classes commonly accepted for other muscles. The remainder would fit into this classification with the minimal assumption of only one error of fibre typing resulting from the continuous distributions of enzyme reaction intensities. The SDHase reaction was not strongly correlated with the three classes and could be used to divide the fibres further into six groups. Differences between means of fibre areas were significant for all pairs out of these six groups except one.5. The grouping may be considered to reflect a dual system of enzymes, the two systems being (a) ATPases and phosphorylase, (b) SDHase. A possible role of nervous activity in determining this dual system is discussed. The hypothesis involves two partly independent characteristics of motoneuronal activity: (a) the frequency of impulses, and (b) the total number of impulses.6. The measurements are correlated with other physiological variables in the individual animals. The mean areas of fibres in all groups increased with body weight. There were changes in the proportions of light and dark SDHase fibres related to weight. The total area contributed by dark alkaline ATPase fibres decreased and that by intermediate alkaline ATPase fibres increased with increasing twitch time to peak.7. Specific tension of the group of slower muscle fibres in f.d.l. was estimated to be 0·29 N.mm-2 compared with 0·39 N.mm-2 for the faster fibres.
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