Spatial Distribution of Motor Endplates and its Adaptive Change in Skeletal Muscle

Motor endplates (MEPs) are the important interfaces between peripheral nerves and muscle fibers. Investigation of the spatial distribution of MEPs could help us better understand neuromuscular functional activities and improve the diagnosis and therapy of related diseases. Methods: Fluorescent α-bungarotoxin was injected to label the motor endplates in whole-mount skeletal muscles, and tissue optical clearing combined with light-sheet microscopy was used to investigate the spatial distribution of MEPs and in-muscle nerve branches in different skeletal muscles in wild-type and transgenic fluorescent mice. Electrophysiology was used to determine the relationship between the spatial distribution of MEPs and muscle function. Results: The exact three-dimensional distribution of MEPs in whole skeletal muscles was first obtained. We found that the MEPs in the muscle were distributed in an organized pattern of lamella clusters, with no MEPs outside the lamella zone. Each MEP lamella was innervated by one independent in-muscle nerve branch and mediated an independent muscle subgroup contraction. Additionally, the MEPs changed along the lamella clusters after denervation and regained the initial pattern after reinnervation. The integrity and spatial distribution of MEPs could reflect the functional state of muscles. The signal absence of a certain MEP lamella could suggest a problem in certain part of the muscle. Conclusions: The MEP lamella clusters might be the basis of neuromuscular function, and the spatial distribution of MEPs could serve as a testbed for evaluating the functional status of muscle and the therapeutic targeting map related to MEPs.