The Effects of Repetitive Stimulation on the Action Potential and the Twitch of Rat Muscle

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Abstract
Membrane potentials and contraction curves of different types of rat muscle fibre have been studied at body and room temperatures with special regard to the effects of repetitive stimulation. Fast fibres (from the surface of the extensor muscles of the antebrachium) have a higher resting membrane potential, a higher amplitude and shorter duration of the spike of the action potential and a larger early negative after‐potential than soleus fibres. In the fast fibres stimulation at 10 Hz for 30 s produced a decicase of the resting membrane potential, a decrease in the amplitude of the spike of the action potential and an increase in its duration, as well as an increase in the early negative after‐potential. In soleus fibres such changes were rare. These findings indicate that there are differences in the membrane properties between the two types of muscle fibres in addition to previously known differences in mechanical properties. Repetitive stimulation of fast fibres at body temperature caused a rapid increase in the twitch amplitude (staircase phenomenon). This increase occurred before any appreciable change in the action potential. Stimulation at room temperature caused only small changes in the twitch amplitude but marked increase in the twitch duration. This suggests that repetitive stimulation may alter both the amplitude and the duration of the active state curve.