Changes in the Compound Muscle Action Potentials (CMAPs) of Rats Following High Frequency Repetitive Stimulations : Differences in Incremental Responses in Amplitude between Two Different Muscles
Although it has been known that high frequency repetitive stimulations of the peripheral nerves of normal humans at more than 10 Hz lead to gradual increases in the amplitude of their compound muscle action potentials (CMAPs), the mechanism of this phenomenon remains unknown. Our previous studies confirmed the occurrence of this phenomenon in normal rats, and it was suggested that changes in the shape of muscle fibers were related to its appearance. In the present study, we exposed two kinds of muscles (the soleus and gastrocnemius muscles) from 70 normal rats, and stimulated the nerves electrically at two different frequencies; i.e., at 2 and 20 Hz. We compared the changes in the amplitude evoked by 8 stimuli at two different frequencies between the muscles. These two muscles were also examined histochemically. The soleus is composed primarily of type 1 muscle fibers (slow twitch fibers), whereas the gastrocnemius consists primarily of type 2 muscle fibers (fast twitch fibers). The relationships between the percentages of each muscle fiber type and increasing rates in the amplitude of CMAPs were analyzed. Following high frequency (20 Hz) stimulation, the rates of increase in amplitude were greater in the soleus than that in the gastrocnemius. In both muscles, the increasing rates in amplitude correlated well with the percentage of type 1 fibers. It is supposed that muscles rich in type 1 fibers experience greater changes in shape perpendicular to the muscle fiber arrangement following high frequency stimulations, because the speed of contraction and relaxation is slower in these muscles. Based on our findings that the rates of increase in the amplitude of CMAPs were greater in muscles with more type 1 fibers, it is suggested that the normal incremental responses in the amplitude of CMAPs following high frequency stimulations may be attributed to changes in the shape of the muscle fibers.
