The influence of a doublet of stimuli at the beginning of the tetanus on its time course.

The influence of a pair of stimuli generated in a short time sequence (doublet) at the beginning of stimulation on the time course of the following tetanus was investigated. Experiments were performed on single motor units in rat's medial gastrocnemius. The doublet evoked an increase in tetanic tension, tetanic fusion and the area under tension record. These effects were measured in tetani fused to varying degrees. It was found that for all types of motor units the strongest influence of the doublet was observed in half-fused tetani. Moreover, the doublet influenced the first part of tetanus significantly more as compared to the second. Slow motor units showed greater sensitivity of the tension and the tetanus area to the doublet than fast units. The results show that slow units are characterized by better summation of their tension at the beginning of a tetanus.