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Instituto di Fisiologia Umana II, Milano, Italy.

1. The isometric force developed by single motor units in response to injection of ramp-and-hold currents into their motoneurones was recorded from the common tendon of the gastrocnemius muscles of the cat. The average rate of rise of the force (force-slope) produced by the ramp-evoked discharge, was found to grow almost linearly with the rate of current injection (current-slope) up to a saturation value (maximal force-slope). 2. The slope of the function which links the force slope to the current-slope is the gain (dF/dI) of the motor unit under dynamic conditions. The value of the dynamic gain, measured in the linear region of growth, displays a large variability, i.e. for each nanoampere of current injected, the force developed is as much as 40 times larger in the strongest than in the weakest motor units. Such large gain differences, however, are drastically reduced if the force is expressed as a percentage of the maximal tetanic tension, Ft: per nanoampere injected, most of the units deliver from 1.0 to 3.0% of Ft. 3. The maximal force-slope which each unit could reach exhibits a large variability, ranging from 0.06 to 4.0 g ms-1. Like the dynamic gain, the maximal force-slope is positively related to Ft. 4. It was found that the dynamic sensitivity of the motoneurone, i.e. the increase of the firing rate per unitary increase of the current-slope, governs the fractional growth of the force-slope, whereas the motor unit contraction time determines the firing rate at which maximal force-slope is reached. Together, the two factors co-operate in defining, for each motor unit, the range of input-slopes within which the force-slope is regulated. 5. The motoneurones which supply the weak motor units, those with the lowest dynamic gain, have higher dynamic sensitivity and lower rheobase than those innervating the strong motor units. This suggests that weak motor units need less synaptic current both to be recruited and to reach the maximal speed of force development when their input is supraliminal.

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