On the connection between the T-system and the subsynaptic folds in the motor end plate of amphibians

Using lanthanum as an extracellular marker, the transition between the subs ynaptic folds of the motor end plate and the T-system of frog muscle fibres is portrayed for the first time. On the lower segment of the subsynaptic f olds of frogs, there are numerous caveolae which can connect with one anoth er to form meandering, branching tubes. The T-system is in contact with the se tubes (which run through the sarcoplasm) beneath the motor end plate. In those segments of the end plate with massed sarcoplasm and a cell nucleus, these tubes form networks in close proximity to the cellular organelles. T he morphological findings obtained here are compared with findings from mam mals. The physiological significance of the transition between the subsynap tic fold and the T-system is discussed.