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Abstract
Isometric responses of individual gastrocnemius muscle units of both fast (F) and
slow (S) twitch type were studied during repetitive intracellular stimulation of the
innervating motoneurons in anesthetized cats. Paired stimuli usually produced maximum
responses (measured in terms of tension-time area) with interpulse intervals between
5 and 10 msec. However, longer pulse trains produced maximum tension-time area per
pulse with relatively long interpulse intervals (referred to as the optimum interval).
For each unit, stimulus intervals around the optimum produced maximum modulation of
tension during unfused tetani. Post-activation potentiation produced shifts in the
optimum toward longer intervals. The optimum intervals for type F units were shorter
(mean 49.4 msec) than those for type S units (mean 86.9 msec), but there was no difference
between the groups when the mean intervals were expressed in ratio with the twitch
contraction time, Tc (F units: 1.42 X Tc; S units: 1.31 X Tc). The range of optimum
intervals for all gastrocnemius muscle units corresponded to motoneuron firing frequencies
of 7-40 pulses/sec, well within the frequency ranges observed for gastrocnemius motoneurons
firing under natural synaptic drive. Insertion of a single short interval (5--10 msec)
at the onset of a relatively low frequency stimulus train produced a prolonged 'catch-like'
enhancement of tension output, particularly in type S units. The duration of the catch-like
effect depended on the interval between pulses in the underlying basic train, and
was maximum for intervals between 1 and 2 X Tc. The catch-like effect was also seen
in type F units but its duration was curtailed by another process called the 'sag'
property. The 'sag' property occurred in type F units over a wide range of stimulus
intervals (usually 0.5-3 X Tc) but was present in type S units only with relatively
long intervals (greater than 2 X Tc). The 'sag' property appears to represent a disenhancement
process intrinsic to muscle unit fibers activated in unfused tetani.