Cellular and Subcellular Characteristics of Neuromuscular Junctions in Muscles with Disparate Duty Cycles and Myofiber Profiles

Abstract

The neuromuscular system accounts for a large portion (~40%) of whole body mass while enabling body movement, including physical work and exercise. At the core of this system is the neuromuscular junction (NMJ) which is the vital synapse transducing electrical impulses from the motor neurons to their post-synaptic myofibers. Recent findings suggest that subcellular features (active zones) of the NMJ are distinctly sensitive to changes in activity relative to cellular features (nerve terminal branches, vesicles, receptors) of the NMJ. In the present investigation, muscles with different recruitment patterns, functions, and myofiber type profiles (soleus, plantaris, extensor digitorum longus [EDL]) were studied to quantify both cellular and subcellular NMJ characteristics along with myofiber type profiles. Results indicated that, in general, dimensions of subcellular components of NMJs mirrored cellular NMJ features when examining inter-muscle NMJ architecture. Typically, it was noted that the NMJs of the soleus, with its most pronounced recruitment pattern, were larger (p < 0.05) than NMJs of less recruited muscles. Moreover, it was revealed that myofiber size did not dictate NMJ size as soleus muscles displayed the smallest fibers (p < 0.05) while the plantaris muscles exhibited the largest fibers. In total, these data show that activity determines the size of NMJs and that generally, size dimensions of cellular and subcellular components of the NMJ are matched, and that the size of NMJs and their underlying myofibers are uncoupled.

Ladder-based resistance training with the progression of training load altered the tibial nerve ultrastructure and muscle fiber area without altering the morphology of the postsynaptic compartment

“…Therefore, it seems that the duration of the training might be an important issue in assessing postsynaptic effects. Also, according to Deschenes et al (2020) and Deschenes et al (2023) , it is more likely that changes in the structure and size of the NMJ are associated with changes in the active zones and cellular and subcellular components of the NMJ. Therefore, we suggest further studies be conducted to assess whether LRT’s longer duration can induce changes in these parameters.…”

Slow or fast: Implications of myofibre type and associated differences for manifestation of neuromuscular disorders

“…261 There are conflicting reports about the size of NMJ axon terminal area on slow and fast myofibres in soleus and EDL muscle from rats, [261][262][263] with a recent paper concluding that muscle activity determines the size of NMJs. 264 NMJ synaptic transmission is efficient due to excess neurotransmitter release (for triggering an action potential) and receptor activation (for reaching threshold), with slow and fast NMJs specialised to deal with the different discharge patterns of slow and fast axons (reviewed in Schiaffino and Reggiani 19 ). Slow NMJs enable a sustained response to motoneurons during repetitive discharge through reduced synaptic depression (i.e., the reduction in neurotransmitter-containing vesicles) compared with fast NMJs.…”

“…For NMJs, the synapse between a motoneuron and myofibre, it is proposed that their slow or fast character is determined by motoneuron‐type rather than myofibre‐type 261 . There are conflicting reports about the size of NMJ axon terminal area on slow and fast myofibres in soleus and EDL muscle from rats, 261‐263 with a recent paper concluding that muscle activity determines the size of NMJs 264 . NMJ synaptic transmission is efficient due to excess neurotransmitter release (for triggering an action potential) and receptor activation (for reaching threshold), with slow and fast NMJs specialised to deal with the different discharge patterns of slow and fast axons (reviewed in Schiaffino and Reggiani 19 ).…”