the conversion of an AP to a contraction in a muscle fiber. An AP passes through the entire sarcolemma and is conducted into the interior by transverse tubules (t-tubules) that are regularly spaced and highly branched. At numerous junctions, the t-tubules make contact with the SR (sarcoplasmic reticulum). SR form terminal cisternae. One t-tubule sandwiched between two terminal cisternae compose a triad. Their membranes are linked by voltage-gated calcium channels that open, through conformational change, when AP passes down the t-tubule. Rapid influx of calcium into the sarcoplasm from the SR triggers a contraction of skeletal muscle fiber.