Effect of Frequency of Stimulation on Tension Response of Muscle

Abstract

The response of nerve, as measured by its heat production or summated action potential changes, has been shown by Bugnard and Hill 1 to decline progressively with increasing frequency of stimulation beyond that which calls forth the maximum response. A similar response-frequency relation in the case of muscle is to be expected, but an attempt to obtain it experimentally with toad's or frog's sartorius stimulated by repetitive condenser charges and discharges from a revolving commutator through a pair of silver wire electrodes lying on the tibial half of the muscle, revealed a very different relation. The curve relating the size of isometric contraction in tetani of any duration from 0.44 to 10 sec. and the frequency of stimulation, was found to be a wavy one with recurrent minima and maxima. In an experiment at 26.3°C. with 0.44 sec. tetani the contraction was at a minimum at the approximate frequencies 380, 750, and 1500 per sec. With longer tetani and lower temperatures, the positions of the minima were shifted to lower frequencies.

Further experiments then showed that this strange result was associated with the fact that the stimulating electrodes were laid on the tibial half of the muscle, and that the muscle was partly stimulated through its intramuscular nerve twigs. For this unexpected result was no longer obtained with curarized muscle or when the muscle was stimulated by electrodes on the nerve-free pelvic end. Conversely it was obtained in a much more striking form when the sartorius was excited indirectly through its nerve. The response-frequency curve of muscle when nerve participation is excluded, is a smooth one, and for short tetani generally very flat after the maximum, unless complicated by fatigue.