Three-dimensional structure of the sarcoplasmic reticulum at the neuromuscular junctions of the rat diaphragm.

The three-dimensional structure of the sarcoplasmic reticulum at the neuromuscular junctions of the rat diaphragm was investigated by means of a selective staining technique allowing studies of thick sections combined with stereoscopic observations. The sarcoplasmic reticulum (SR) has different organizations related to the various regions of the postsynaptic cytoplasm. At the base of the membranous folds or deep region, flattened cisternae are smaller and connected by numerous anastomosed tubules. In the mitochondria-rich region, the SR is composed of large flattened cisternae. Between the postsynaptic folds, the SR is formed of poorly anastomosed thin tubes which give rise to single tubules running close to the plasmalemma. The SR is also associated with the Golgi apparatus present in the deeper mitochondria-rich region. This specific organization of the SR in the postsynaptic cytoplasm of the neuromuscular junctions might, like the Golgi apparatus, be involved in the synthesis of some specific proteins, enzymes and receptors.