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Department of Physiology, Medical School, University of Bristol.

1. Physiological properties of motor units in the soleus muscle were studied in anaesthetized rats using ventral root splitting to isolate single units. 2. Motor unit types were classified by the same criteria used to classify cat hindlimb motor units into types FR (fast-twitch, fatigue-resistant) and type S (slow-twitch, fatigue-resistant). Type FR units were estimated to generate 10% of whole-muscle tension and type S 90%. All FR units showed sag in the unfused tetanus at frequencies with interpulse intervals greater than 175% of twitch time to peak, but not at 125% (Burke, Levine, Tsairis & Zajac, 1973). 3. The muscle fibres belonging to twelve single motor units were depleted of glycogen by prolonged stimulation, permitting analysis of their histochemical profiles. Type FR units were found to consist of type IIA muscle fibres and type S units of type I muscle fibres. 4. Direct determinations were made of fibre area, innervation ratio (number of muscle fibres supplied by one motoneurone) and hence specific tension (tetanic tension generated per unit cross-sectional area) of individually identified motor units. Motoneurons were found to innervate between 84 and 178 muscle fibres (mean 110) in type S units and between 126 and 161 fibres in type FR units (mean 142). Fibre areas were larger for type FR units and there was a significant difference in specific tension of the two unit types (type S lower). 5. Indirect estimates of innervation ratio and specific tension were obtained from counts of muscle fibre types, and relative frequencies of motor unit types in the soleus unit pool. Observations agreed well with results of direct measurements. 6. The evidence provided suggests that differences in tension generated by type FR and S units in rat soleus muscle are primarily due to differences in innervation ratio and fibre area, with a small contribution from differences in specific tension.

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