Postsynaptic arch in the frog neuromuscular junction: paramembranous protuberances coating the inner surface of the postjunctional membrane.

The frog neuromuscular junction was fixed and processed for electron microscopy according to the method of rapid freezing followed by freeze-substitution. The synaptic structures, including cleft material, paramembranous cytoplasmic coating on the postsynaptic membrane, and subsynaptic cytoplasmic elements, were examined in thin sections. The basal lamina, about 50 nm thick, was seen to bisect a synaptic cleft approximately 100 nm wide. The lamina consists of two parts: the central dense line and the fine filaments protruding from it in the direction of the apposing postjunctional membrane. Present on the cytoplasmic surface of the postjunctional membrane are electron-dense protuberances, 41 +/- 5 nm in width and 27 +/- 5 nm in height (top to membrane centre). They are arranged in regular parallel rows, at 54 +/- 4 nm intervals (centre to centre). The paramembranous protuberance coats the inner surface of the postjunctional membrane at its apex as well as at the middle portion of the junctional process, pointing to its probable hairpin-like course in a transverse plane of the process. From its location and three-dimensional arrangement, this protuberance was termed the 'postsynaptic arch'. A filamentous meshwork is present just beneath the postjunctional membrane and extends into the cell interior. The submembranous meshwork appears to connect to the underlying bundles of cytoskeletal filaments. The possibility is discussed that the postsynaptic, electron-dense arch corresponds to the 43-kDa protein, a major alkaline-extractable protein thought to be associated with the cholinergic receptor molecules in the postsynaptic membrane.