Structural and functional abnormalities of motor endplates in rat skeletal model of myofascial trigger spots

Myofascial trigger points (MTrPs) are defined as hyperirritable spots in a palpable taut band (TB) of skeletal muscle fibers. Knowing the formation and location of MTrPs is a great help to prevent their development and inactivate existing MTrPs. This study aimed to obtain new evidence that myofascial trigger spots (MTrSs), which are similar to human MTrPs, are found in dysfunctional motor endplates by observing the morphological characteristics of muscles and changes in biochemical substances. A total of 32 male Sprague Dawley rats were randomly divided into four groups: two control groups (i.e., C1 and C2) and two model groups (i.e., M1 and M2). C1 and M1 were used for acetylcholine (ACh) content measurement, while C2 and M2 were utilized for acetylcholinesterase (AChE) staining. In the model groups, blunt striking injury was induced and eccentric exercise was applied to the left gastrocnemius for 8 weeks. After 1 month, spontaneous electrical activity(SEA), AChE optical density (OD), muscle fiber diameter, and ACh content were measured. The results showed that extensive abnormal endplate noise (aEPN), including positive neurons, fibrillation potentials, fasciculation potential, and high amplitude (endplate spikes [EPS]), is present at MTrSs in M1. Quantitative electromyography results showed that the amplitudes of aEPN and frequency of EPS in M1 were significantly higher than those of C1. The ACh content of MTrSs in M1 was significantly higher than that in C1. The AChE OD value of M2 was significantly lower than that of C2. In addition, the diameter of the muscle fibers in the AChE-stained area was longer in M2 than in C2. In conclusion, MTrSs formed at the motor endplate with a larger diameter of muscle fibers. Excessive ACh release and decreased AChE activity at MTrSs stimulated muscle action potential and muscle contraction.