Miss

Abstract

Abstract Background The neuromuscular junction (NMJ) is a specialised connection through which a motor nerve initiates skeletal muscle contraction. Although NMJs have been well-characterised in rodents, morphometric data from human muscles remain limited. To address this gap and enable direct comparison, this study analysed NMJs from the mouse forelimb lumbrical and peroneus longus (PL) muscles as representative models of fine and gross motor function. In addition to this intra-species comparison, mouse forelimb lumbricals were compared with functionally similar human dorsal interosseous (DI) muscles. By combining confocal and transmission electron microscopy (TEM), the study evaluated whether the small-calibre axons observed in confocal datasets represented genuine anatomical features or artefacts caused by incomplete antibody labelling. Aims This study aimed to (1) compare NMJ morphology between two mouse muscles with distinct motor functions (forelimb lumbrical vs. PL), (2) contrast mouse forelimb lumbrical NMJs with those in the functionally similar human DI muscles, and (3) examine whether the human DI small-calibre axons observed in confocal microscopy reflect true anatomical features by validating them through EM of the same muscle. Methods Confocal and EM were used to evaluate NMJ and motor axon morphology in adult mice and human DI muscles. Quantitative morphometric analysis was performed using NMJ-morph and ImageJ. Results Statistical comparisons between mouse forelimb lumbrical and peroneus longus revealed significantly larger endplate areas in the peroneus longus (p = 0.0458). In cross-species comparisons, human DI axons were significantly smaller in diameter than mouse lumbrical axons (p = 0.0179). EM of the DI showed that axon diameters differed from confocal estimates by less than 0.5 μm. Conclusion These findings highlight distinct structural features in mouse and human NMJs, support the anatomical plausibility of small-calibre axons in human DI muscles, and offer preliminary morphometric data to guide future studies.