Human neuromuscular junction: anatomical features and the effects of increasing age

Abstract

The neuromuscular junction (NMJ) is a specialised chemical synapse that mediates communication between motor neurons and skeletal muscle fibres, playing a critical role in voluntary motor control. Despite its functional importance, our understanding of human NMJ morphology remains limited, particularly in terms of ageing and nanostructured details. This thesis addresses key gaps in human NMJ research through investigating anatomical characterisation, age-associated structural changes in human upper limb muscles, and applying a new super-resolution (SR) technique to reveal fine structures.

In the first part of this study, NMJs were systematically analysed in upper limb muscles obtained from healthy adult donors. Using immunohistochemistry (IHC) staining and confocal microscopy, a baseline morphological profile was established.

The second part explored age-related changes of the NMJ, focusing on terminal Schwann cells (TSCs) and voltage-gated sodium channels (NaV1.4) besides the basic presynaptic and postsynaptic elements. Aged NMJs exhibited subtle structural alterations, including changes in TSCs architecture and disruption in NaV1.4 expression, suggesting progressive synaptic decline with preserved overall integrity. These findings indicate that human NMJ ageing changes diverge from the rodent scenario, where the latter reveals a progressive structural denervation in response to ageing.

The final part of the thesis introduced and validated Expansion Microscopy (ExM) as a novel technique for NMJs imaging at nanoscale resolution. ExM results are enabled by isotropic tissue expansion, capturing synaptic details in three-dimensional visualisation. As a recent SR technique, the ExM is cost-effective, accessible and applicable.

Collectively, these three studies represent the aims of the thesis, provide a comprehensive and novel anatomical account of the human NMJ. They offer critical reference data, reveal patterns of structural ageing, and establish a powerful imaging platform for future research.