Properties of stimulus trains producing maximum tension-time area per pulse from single motor units in medial gastrocnemiu muscle of the cat.

Abstract

Single motor axons innervating medial gastrocnemius muscle were functionally isolated in ventral root filaments. For each motor unit, trains with 10 or less pulses were used to find the interpulse sequences of stimuli that maximize the integral of the developed tension-time curve per pulse. Computer interaction with the preparation ensured that variations in tension responses resulted from differences in interpulse sequences within the train rather than from postactivation potentiation, which may occur from train to train. For a given unit an interpulse sequence exists with intervals defined as T1*, T2*, ..., Tn*, which produces maximal tension-time area per pulse. A unit''s optimal pulse sequence does not depend on the number of pulses in the train. This property ensures that a motor unit will always develop maximum tension-time area per pulse without a prior knowledge of how long its discharge burst is, as long as its spikes are generated in time with a pattern corresponding to its optimal pulse sequence. For each unit the optimal pulse sequence always began with 1 or 2 short interpulse intervals (i.e., T1 and sometimes T2 .ltoreq. 10 ms). The optimal sequence occurring after these short, initial intervals (called transitional intervals) corresponded to that of a repetitive train (i.e., T3* = T4* = .cntdot..cntdot..cntdot. Tn* .**GRAPHIC**. T). Regression analysis showed the interpulse interval (T) associated with this repetitive train was about 1.8 .times. twitch contraction time and 1.2 .times. relaxation time of the motor unit. Two transitional intervals were found for those optimal pulse trains associated with motor units with low twitch/tetanus ratios (< 0.05). The number of transitional intervals decreased from 2-1 if a unit''s twitch/tetanus ratio was raised by post-tetanic potentiation. At times the optimal steady-state interval (T) for a given unit did not change, even though steady-state tension (0.5-0.8 of maximum tetanic tension) associated with the optimal train rose when the unit was in a state corresponding to a high twitch/tetanus ratio. Tension-time area per pulse was sensitive to lengthening of transitional interval and especially to changes in the 1st interval. Tension-time area per pulse was insensitive to deviations of the other intervals from their optimal values. Simultaneous deviations, even in the same direction, of all intervals subsequent to the transitional ones could be quite large before substantial reduction in tension area resulted. Tension-time area apparently is a useful measure of output during isometric and other contractions. The possible relationship between the optimal pulse sequence, tension-time area and Ca2+ activation of the contractile apparatus in the muscle fibers is discussed.