Three-dimensional structure of the synaptic contact of the neuromuscular junction in the rat lumbrical muscle

This study examined the three-dimensional structures of the synaptic contac t in rat lumbrical muscles by scanning electron microscopy using three diff erent methods: the aldehyde prefix-osmium-dimethyl sulfoxide-osmium method (A-O-D-O method), the cell-extraction method, and the NaOH-digestion method . These three methods visualized the motor nerve endings, subneural basal l amina and postsynaptic sarcolemma, respectively, The motor nerve endings we re composed of a cluster of spherical and cylindrical terminals. Pores on t he presynaptic membrane were considered openings of exocytotic vesicles. Th e postsynaptic side of the subneural basal lamina showed numerous ridges, c orresponding to junctional folds. Most of the ridges rose vertically from t heir base. The ridges showed widening, narrowing, and branching, The subneu ral basal lamina appeared to be composed of small granular substances. The basal lamina of the primary synaptic clefts had pores 25-30 nm in diameter, which may facilitate the transport of acetylcholine (ACh) without being hy drolyzed by ACh esterase in the lamina, On the outer surface of the postsyn aptic sarcolemma in a sole plate, the primary synaptic clefts were composed of a mixture of depressions and gutters; so far as we know, this represent s the only example of such a phenomenon. These depressions and gutters seem to at respectively into the spherical and cylindrical terminals of the mot or nerve endings. The openings of the junctional folds consisted of a mixtu re of many slits and a few pits in the primary synaptic clefts.