ALTERATIONS IN NEUROMUSCULAR-JUNCTION MORPHOLOGY DURING FAST-TO-SLOW TRANSFORMATION OF RABBIT SKELETAL-MUSCLES

Chronic low frequency stimulation of motor nerves results in transform ation of muscle fibre phenotype from fast- to slow-twitch. We examined the light and electron microscopic structure of neuromuscular junctio ns in normally fast twitch muscles, tibialis anterior and extensor dig itorum longus of rabbit after 3 weeks of stimulation to determine whet her synaptic structure is also modified during fibre type transformati on. Neuromuscular junctions of stimulated and unstimulated (control) t ibialis anterior and extensor digitorum longus muscles and unstimulate d slow twitch soleus muscle were visualized with rhodamine-conjugated alpha-bungarotoxin. Video Light microscopic images of neuromuscular ju nctions were digitized to allow quantification of their surface areas, perimeters, lengths and widths. Three weeks of stimulation resulted i n a decrease in the maximal velocity of muscle fibre shortening and au gmentation of mitochondrial volume in fast muscles, demonstrating the efficacy of the stimulation protocol employed in altering muscle fibre phenotype. Neuromuscular junctions of control tibialis anterior and e xtensor digitorum longus are thin, compact, and continuous, with compl ex branching patterns. In contrast, those of slow-twitch soleus are th icker and discontinuous. Neuromuscular junctions in control tibialis a nterior and extensor digitorum longus are larger than those in soleus. Three weeks of stimulation causes a marked decrease in the size of ne uromuscular junctions in tibialis anterior and extensor digitorum long us, as reflected in the significant reduction in neuromuscular junctio n surface area, length and width. Electron microscopy of these junctio ns suggests that secondary postsynaptic folds in stimulated muscles ar e more closely spaced. Also, axon terminals of stimulated muscles appe ar to contain more densely packed synaptic vesicles and mitochondria t han controls. Decreases in neuromuscular junction dimensions can be pa rtly explained by muscle fibre atrophy. However, the decrease in neuro muscular junction size is proportionately greater than that of muscle fibre diameter in both muscles, indicating that factors other than fib re atrophy may contribute to the reduced neuromuscular junction size i n stimulated muscles. Neuromuscular junctions of stimulated tibialis a nterior and extensor digitorum longus muscles exhibit some features ch aracteristic of normal soleus neuromuscular junctions, indicating stru ctural adaptations consistent with the altered muscle fibre phenotype. On the other hand, neuromuscular junctions of 3 week stimulated tibia lis anterior and extensor digitorum longus and their synaptic branches remain as thin and continuous as those of unstimulated controls, sugg esting that the transformation of neuromuscular junctions towards a mo rphology characteristic of slow muscle, is only partial. These results demonstrate that an altered pattern of impulse activity causes signif icant synaptic remodelling in adult rabbit skeletal muscles.