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Laboratoire de Neurophysiologie, Collège de France, Paris.

1. A study of motor units to hindlimb muscles of cat has been made, with as complete a sample as possible of the motor axons to an individual muscle. In single experiments as much as 95% of the motor supply to a muscle has been examined. 2. The following muscles have been studied: peroneus brevis, peroneus tertius, peroneus longus, plantaris, gastrocnemius medialis, soleus, tenuissimus and lumbricalis superficialis. 3. Units were identified as slow resistant (S), fast resistant (FR), fast fatigable (FF) and fast intermediate (FI). The proportion of various motor unit types differs from one muscle to another. There is also some variation in the proportions to a given muscle from one animal to another. With the exceptions of soleus, which is entirely slow resistant, and gastrocnemius, which has relatively fewer resistant units, most muscles contain 60% or more of resistant (S and FR) units. 4. The conduction velocity ranges of FF, FR and FI units overlapped. There was little overlap between the conduction velocity ranges of these F units and of S units. 5. In individual experiments there was a strong and significant positive correlation between the logarithm of maximal tetanic tension and axonal conduction velocity in S and in S+FR units. In terms of contractile response the total fatigue-resistant population appeared to be a continuum. The correlation coefficient between maximal tetanic tension and conduction velocity was also high in the totality of units of all types, although within the FF group there appeared to be little or no correlation. In pooled data there was much more scatter and these relations were less clear. This resulted largely from differences in the ranges of axonal conduction velocity for a given motor unit type from one animal to another. 6. There was a highly significant negative correlation between isometric twitch contraction time and axonal conduction velocity in individual experiments. This relationship could also be seen, but less clearly, in pooled data. 7. The possible bases for these relationships are discussed.

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