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Myonuclei of mamalian skeletar muscle fibers are packed at much higher density in the neu­romuscular junction region than in the extrajunctional parts of the fiber. Slow muscle fibers are electromechanically active for longer time periods than fast fibers and the frequency pat­terns of action potentials are different in both fiber types. This difference might reflect in the distribution of synaptic myonclei and the arrangement of synaptic Schwann cells. Our hypotheses were: 1. The number of synaptic myonuclei is significantly higher in slow fibers than in fast fibers. 2. The number of synaptic Schwann cells determined by counting their nuclei is proportional to the neuromuscular junction area. 3. The reach of the influences of neural trophic factors in the skeletal muscle fibers does not differ in regard to the fiber type.

Nuclei in single muscle fibers of m. extensor digitorum longus (fast) and m. soleus (slow) were labelled with SYTOX green nuclear acid stain and the endplates with rhodamine-tagged a-bungarotoxin. Images obtained by confocal microscopy were morphometircally analysed.

There are 7,15 ± 2,37 (n = 20) synaptic myonuclei in slow muscle fiber and 4,60 ± 0,75 (n = 20) synaptic myonuclei in fast muscle fiber. The difference is significantly different (p < 0,05; Student’s T-test), The correlation coefficient between the area of the neuromuscular junc­tion and the number of Schwann cell nuclei was higher in fast fibers (KK= 0,4) than in slow fibers (KK = 0,07), but to our opinion still too low to confirm our second hypothesis; 3. Ma­ximal distance of the action of neuronal trophic factors was estimated by measuring the distance between the border of the neuromuscular junction and the first extrasynaptic nucleus. It was found to be 25,5 ±9,3 pm, (n = 20) in fast fiber type and 26,9 ±7,4 pm (n = 20) in slow fiber type and the difference was not statistically significant (p > 0,05; Student’s T-test).