The role of frequency in the effects of long‐term intermittent stimulation of denervated slow‐twitch muscle in the rat.

Abstract

SUMMARY1. Rat soleus muscle was denervated by sciatic transection and electrically stimulated for periods of between 3 and 9 weeks with intermittent 1 s bursts of pulses. Most of the bursts were either repeated every 90 s and pulses within them had frequencies between 10 and 100 Hz, or had a frequency of 50 Hz and were repeated at intervals between 60 and 600 s. Comparisons were made with continuous stimulation at 10 Hz.2. At the end of the period of stimulation, isometric twitches and tetani were measured and, in a proportion, also isotonic shortening velocity.3. Isometric twitch duration (contraction and relaxation) decreased with time of stimulation. Very similar effects were seen in all animals in which intermittent stimulation had been used. There was a significant relationship between the change in twitch duration and the frequency used within the bursts of chronic stimulation, with slightly larger effects at frequencies of 40 and 60 Hz. The lowest burst repetition rate produced the largest effects.4. It was confirmed that similar changes were found in denervated muscles that were not stimulated, although these changes were smaller and developed more slowly.5. The extreme loss of tetanic tension induced in the muscle by denervation was reduced by chronic stimulation, with no significant difference between different regimes, although there were small differences which showed the same patterns of effectiveness described for twitch durations.6. Continuous stimulation at 10 Hz maintained the twitch contraction and relaxation phases at the values found 3 weeks after denervation, that is it prevented secondary shortening of the twitch. Continuous stimulation reduced tension loss but was, perhaps, less effective than intermittent stimulation.7. Twitch-tetanus ratio increased with denervation with little spontaneous reversal later. Stimulation at all frequencies reduced the ratio, but it did not reach normal values.8. Isotonic shortening velocity was measured in many of the muscles. Maximum velocity was estimated and normalized by muscle length. Results were variable but the largest increases in velocity were seen as frequently in muscles chronically stimulated intermittently at 10 Hz as in muscles in which higher frequencies were used.W. S. AL-AMOOD AND D. M. LEWIS 9. It is concluded that the effect of intermittent stimulation is almost independent of the frequency used within the bursts of pulses. This independence of frequency reinforces a proposal made earlier that the effect of intermittent chronic stimulation is to enhance the synthesis of protein, but not to switch that synthesis from slow to fast myosin which, in rat soleus at least, is induced by denervation. Continuous stimulation (at 10 Hz) prevents the transformation from slow to fast muscle after denervation.

The denervated muscle: facts and hypotheses. A historical review

“…The slowing of the contraction is the most commonly reported change; it develops few days after denervation, is more marked in fast than in slow muscles, and is also longerlasting in fast than in slow muscles. In slow muscles, a reversal of the Wrst eVects may occur several weeks after denervation, with a shortening of contraction time (Al-Amood and Lewis 1987Lewis , 1989, which explains the speeding eVect of long-term denervation described by others in slow (Syrový et al 1971(Syrový et al , 1972 or mixed (Carraro et al 1982) muscles. There are species diVerences, e.g.…”

Influence of muscle temperature on the contractile properties of the quadriceps muscle in humans with spinal cord injury

“…Other mechanisms than a lower T m may be responsible for the paradoxical findings in muscles of SCI subjects [1,4]. For example, possible alterations in the excitation-contraction coupling mechanisms may have caused the increase in the relative twitch tension, which was also found in denervated muscles [36], and consequently may determine the relatively high forces during low-frequency stimulation in the SCI individuals ( Figure 3). …”

What does chronic electrical stimulation teach us about muscle plasticity?

“…3,4,53,56,59,100 The pattern of stimulation used to transform a denervated slow-twitch muscle into a faster muscle resembles that normally delivered by the motor nerve to a fast-twitch muscle, namely intermittent trains of high-frequency activity. The disadvantage of this model is that denervationand stimulation-induced changes overlap, 3 not to mention the technical difficulties of efficiently stimulating a denervated muscle.…”