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The transverse tubular system (T-system) of skeletal muscle links surface membrane action potential and release of activator calcium from sarcoplasmic reticulum (SR). The spread of depolarization along this system has been studied in voltage-clamped frog muscle fibers by using the spread of contractile activation in a thin optical cross section through the fiber center as an index. In tetrodotoxin treated fibers as depolarization of the fiber is increased contraction spreads from superficial to axial myofibrils. In tetrodotoxin free fibers the radial gradient of activation is reversed indicating that normally the activating signal is propagated along the T-system. Activation across the SR-T tubule junction does not appear to trigger an all-or-none response from the SR,-Costantin, L.L. Electrical properties of the transverse tubular system.