Accommodation To Current Ramps In Motoneurons of Fast and Slow Twitch Motor Units

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Abstract
1. The response of triceps surae motoneurons to linearly rising ramps of depolarizing current injected through intracellular micropipette electrodes has been studied in cats. Most of the data were obtained in animals anesthetized with pentobarbital but some units were studied in spinal unanesthetized preparations with anemic destruction of the brain. There were no essential differences in results from the two types of preparations. 2. The speed of contraction of the muscle fibers innervated by the motoneurons studied was measured following stimulation of the cell through the intracellular micropipette. Motor units could then be classified into two groups, fast twitch or type F and slow twitch or type S, on the basis of the speed of contraction of the muscle fibers innervated. 3. The presence or absence of accommodation in each motoneuron was determined based on the shape of the current-latency curve obtained during passage of linearly rising depolarizing currents of widely varying slope. About one half of the total sample of 22 type F units studied exhibited accommodation to linearly rising current ramps according to the criteria adopted in this study, and the other one half did not. Only 2 of the 18 type S units similarly examined showed accommodation to current ramps. The difference in accommodative behavior between types F and S motoneurons was statistically significant and did not appear to be due to any systematic difference in cell injury caused by microelectrode penetration.