[The ultrastructure of the individual nerve endings of the tonic fibers in the frog].

A regular diminution of specific areas of synaptic contacts and active zones (AZs) along the terminals towards the distal parts has been first shown. The discontinuity was found in the terminal nerve branch apposition to the muscle fiber as a result of periodic intercalation of the Schwann cell and its processes into the synaptic gap. A significant variability in shapes of active zones lengthways the terminal is revealed. It is suggested that all these structural features may reflect the functional properties of synapses on the tonic muscle fibres.