Rheobase, input resistance, and motor-unit type in medial gastrocnemius motoneurons in the cat

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Abstract
Rheobase, input resistance, axonal conduction velocity and mechanical responses of medial gastrocnemius (MG) motor units were measured in barbiturate-anesthetized cats. Using a testing paradigm similar to that of Burke et al. (1973), motor units were classified as fast twitch, fast fatiguing (FF); fast twitch with intermediate fatigue resistance (FI); fast twitch, fatigue resistant (FR); or slow twitch, fatigue resistant (S). The interrelations among several neural and mechanical properties of motor units were examined. Motor-unit type was a better predictor of motoneuron and muscle unit characteristics than any other single parameter. The combination of unit-specific differences in synaptic input and excitability may result in sequential recruitment of different motor-unit classes which, although disorderly at the single-cell level, is functionally equivalent to more highly structured recruitment models.