[A scanning electron microscopical and morphometrical analysis of neuromuscular junctions in different muscle fiber types in the zebra finch and rat].

The differences of neuromuscular junctions (NMJs) between different muscle fibers had been examined with scanning electron microscopy and analyzed morphometrically. The anterior and posterior latissimus dorsi muscles of the zebra finch were compared. The former consisted exclusively of slow tonic fibers and the latter of fast twitch fibers. The former had numerous, small NMJs. The synaptic depressions were small in number. The latter had a large NMJ. The synaptic depressions were large in number, and subsynaptic folds were found. The extensor digitorum longus and soleus muscles in the rat were also compared. The former consisted mostly of fast twitch fibers whereas the latter consisted of slow twitch fibers (75%) and fast twitch fibers (25%). NMJ of slow twitch fiber was small and the subsynaptic folds had sparse, narrow slit-like and pit-like openings. NMJ of fast twitch fiber was large and the subsynaptic folds had numerous, wide and slit-like openings.