Sensitivity of the motor unit unfused tetanus to changes in the pattern of motoneuronal firing.

Abstract

The effects of decreasing as well as increasing the interpulse intervals on the tension produced by motor units in the rat medial gastrocnemius muscle were investigated. The increase and decrease in tension, resulting from these changes in interpulse intervals were observed and compared for each motor unit in tetani fused to a various degree. It was found that amplitudes of both changes in tension were the same when the tetanic fusion index was approximately 0.70 and the tension corresponded to 40-50% of the maximal tetanic tension. This observation concerned the all three types of motor units. We also studied the effects of introducing a short interpulse interval ("doublet") at the beginning of the stimulation. The doublet produced an increased tetanic tension with a more fused profile, however the tension was similarly sensitive to an increase as well as a decrease in the interpulse interval when the tetanic fusion index was also about 0.70. Therefore, the extent of tetanic fusion determines the sensitivity of motor unit tetani to changes in a pattern of stimulation. The tetanus of the fusion index about 0.70 can be considered as the most sensitive to changes in the pattern of motoneuronal firing.