Structure of neuromuscular junctions during muscle fiber degeneration and regeneration induced by bupivacaine hydrochloride.

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We observed the morphological features of the neuromuscular junctions following a single direct injection of 0.5% bupivacaine hydrochloride(Marcaine)into extensor digitorum longus of young adult female Fischer344 rats induced muscle fiber degeneration and regeneration. Four obvious structural degeneration and regeneration pattern were observed after treatment with bupivacaine : (1)Z-line loss fiber, (2)opaque(hyper-contracted)fiber, (3)empty sarcoplasm(myofibrils were totally disrupted)fiber, (4)necrotic fiber. Until 4 days after BPVC injection the degenerated fibers increased significantly. After 6 days, regenerating fibers appeared whereas the endplate spread of cholinesterase and planar neuromuscular junction area reached the smallest level at 4 days and 6 days, respectively. On electron microscopy the most striking findings in the early phase were the focal disappearance of the secondary synaptic cleft and Z-line loss with no destruction on the peripheral nerves(terminal branch axons). 14 days after muscle injury, regenerating fibers characterized by a redeveloped the secondary synaptic cleft and Z-line appeared. These observations suggest that the neuromuscular junctions regeneration depends on the remodeling of synaptic membrane systems.
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