Electrical Activity and Isometric Tension in Motor Units of the Cat's Inferior Oblique Muscle

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Abstract
Single motor units of the cat's inferior oblique muscle were isolated by partial denervation of the muscle and threshold stimulation of split motor nerve filaments. With electrical tests singly innervated (SI) units, multiply innervated conducting (MIC) units and multiply innervated non‐conducting (MIXC) units could be differentiated. SI units conducted impulse activity faster (2.93 m/s) than MIC units (1.72 m/s). Singly innervated units contracted faster than multiply innervated units. Fusion frequency was found the most useful parameter for a mechanical separation; most SI units fused above 200 pps, most MIC units between 100 and 200 pps and the MIXC units below 100 pps. Tetanic tension was higher in fast (SI) than in slow (MIC and MIXC) units, but there was a considerable overlap. The range for the whole material was 25–425 mg. In fast units fusion fequency and the rate of stimulation for maximal tetanic tension coincided. In slow units maximal tetanic tension was reached at stimulus rates of 150–200 pps irrespective of the fusion frequency of the unit. Resistance to fatigue was high in the slow units and in the majority of the fast units.The contractile properties fit into the motor unit activity pattern determined through motoneuron recordings in alert animals.