Synaptic strength as a function of motor unit size in the normal frog sartorius.

A wide range of motor unit sizes exists in each frog sartorius, with values of tetanus and twitch tensions extending over 62- and 400-fold ranges, respectively. These differences primarily represent differences in the number, rather than the size, of muscle fibres in each motor unit. Tetanus-to-twitch tension ratios varied markedly between different motor units, ranging from 1.5 to over 190. Because directly stimulated muscle fibres have tetanus/twitch ratios no larger than 3, it is concluded that high tetanus/twitch ratios arise from recruitment of muscle fibres during the tetanus which respond with only subthreshold depolarization during a twitch. Small motor units, as judged either by their twitch or tetanus tension, were associated with higher tetanus/twitch ratios, suggesting that the average safety margin of a motor unit increases with the motor unit's size. Indeed, when the tetanus/twitch ratio of a motor unit is used to determine the fraction of fibres in the unit with subthreshold neuromuscular junctions, it is observed that there is a direct linear relationship between the size of the motor unit and its over-all efficacy of synaptic transmission. Measurements of the effects of changes in calcium concentration on motor unit twitch tension confirmed the last conclusion. Furthermore, this analysis revealed that large motor units, although having a wide range of transmission safety margins, are largely comprised of junctions of uniformly high safety margin. In motor units of smaller size, synaptic strengths become more evenly distributed over a wide range of values. Small motor units had consistently longer twitch rise-times than did larger units. This decrease in rise-time with motor unit size paralleled the decrease in tetanus/twitch ratios, raising questions about the regulation of muscle fibre contraction kinetics.