Objective: Spontaneous electrical activity [SEA] can be recorded electrophysiologically from an active locus in a myofascial trigger point [MTrP] region. Recent studies on humans and rabbits suggest that the MTrP region is always in the endplate zone. The purpose of this study is to map the distribution of active loci in rat skeletal muscle in order to further confirm that SEAs are also primarily found in the endplate zone. Methods: The biceps femoris muscles of 12 Sprague-Dawley albino rats were studied. Under general anesthesia, the biceps femoris muscle was exposed and electrically stimulated with a threshold current of 0.5-1.0 mA to identify the endplate zone. A monopolar needle was inserted into one end of the muscle and slowly advanced following the direction of the muscle fiber until it reached the other end. Totally, 9-13 tracks [1 mm apart] were searched for each muscle. Each track was mapped for a total length of 14-18 mm, with the endplate sites in the middle of the fiber. Results: It was found that SEAs were recorded mainly in the endplate zone. Conclusion: The findings of this study are consistent with those of previous studies on humans and rabbits, and further suggested that SEA is dysfunctional endplate potentials. Furthermore, the rat can be a potential animal model for the study of MTrP mechanism.
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