Effects of denervation on neuromuscular junctions in the thyroarytenoid muscle

Objectives/Hypothesis: To evaluate the effects of denervation on muscle fibers and neuromuscular junctions (NMJ) of the rat thyroarytenoid (TA) muscle with a histochemical method to monitor the status of degenerative NMJ. Study Design: Quantitative assessment to monitor the status of degenerative muscle fibers and NMJ in the TA muscle. Methods: Wistar rats were killed at 6, 12, 18, and 24 hours and at 2, 4, and 10 weeks after left recurrent laryngeal nerve (RLN) transection. Hematoxylin-eosin stain was used to evaluate the atrophic changes of the TA muscle. The pre- and postsynaptic structures of the NMJ were detected histochemically. These changes were evaluated by comparing the results between the treated (T) and untreated (U) sides (T/U ratio) in the same section. Results: The atrophic changes in the TA muscle progressed gradually, and at 10 weeks, the T/U ratios of the entire muscle area and of the muscle fiber size decreased to 53.2 ± 10.7% and 55.5 ± 6.8%, respectively (P < .01). The number of nerve terminals decreased significantly at 18 hours (P < .01), and they disappeared completely by 24 hours. In contrast, at 10 weeks, 70.5 ± 12.4% (P < .01) of acetylcholine receptors (AchRs) were preserved. Conclusions: In the rat TA muscle, denervation influences the presynaptic nerve terminals more than the postsynaptic AchRs and the muscle fibers. The results could be a basis for understanding the mechanism of laryngeal denervation and reinnervation processes in animal models.