[Nonuniformity of the fine structure of various portions of the nerve terminal in frog somatic muscles as the basis for the variation in its efficiency].

The ultrastructure of serially sectioned single terminals of two neuromuscular synapses differing in efficacy in frog sartorius and cutaneous pectoris muscles was investigated. Their central (CP) and distal (DP) parts were studied morphometrically, and the data on the terminal ultrastructure in different muscles were compared. The areas of active zones (AZ) in CP of terminals in both the muscles were significantly larger than in DP. At the same time the number of AZ in CP specific terminal length of sartorius muscle exceeded that in DP, whereas at terminal of cutaneous pectoris muscle the AZ were arranged more uniformly along the whole terminal length. Some other structural features of these terminals were revealed exerting their influence on the efficacy of transmitter release. We suggest that changes of AZ dimensions may closely correspond to the efficacy both in different parts of the same terminal and in two different terminals.