Synaptic membrane structure in Torpedo electric organ

The innervated and noninnervated membranes of Torpedo electrocytes have been examined by electron microscopy of thin-sectioned and freeze-fractured specimens. The ventral innervated membrane is approximately 120 A thick and is characterized by an unusually broad outer dense lamina (approximately 60 A) in which a granular substructure can be resolved. The granules are approximately 70 A in diameter and are spaced irregularly. The same membrane specialization was noted in a previous study of amphibian myoneural junctions, and it was proposed then that the granular elements represent ACh receptor molecules. The morphologically equivalent structures presumably have the same significance in the Torpedo electric organ. However, in this case the specialized membrane covers the entire innervated surface, leading to the conclusion that high concentrations of receptors occur normally in extrajunctional as well as post-junctional regions of the innervated membrane of the electrocyte. In replicas of freeze-fractured specimens, the A face of this membrane is covered with large particles having the same distribution and approximate concentration as the granules visible in thin sections, indicating that the granules visible at the outer surface of the membrane extend at least into the hydrophobic middle layer of the membrane. The cytoplasmic surface of this membrane has an amorphous coating into which 'decorated' cytoplasmic filaments insert. Synaptic vesicle and axon terminal membranes also contain granules visible in thin sections but with a much sparser distribution. These probably correspond to the intramembranous particles seen in freeze-fractured specimens. Vesicles are occasionally attached to the axolemma by thin linear strands.