However, for the action potential to affect the sarcoplasmic reticulum deep within the muscle fiber, it must be propagated inward as well as along the surface. This is accomplished through invaginations of the sarcolemma called the transverse tubules, or T tubules. In mammalian skeletal muscle, T tubules occur in register with the junction between the A bands and the I bands. Thus, each sarcomere is associated with two systems of T tubules, one at each end of the A band. (This is not true throughout the animal kingdom. In frogs, from which we have derived much of our knowledge about the structure and function of skeletal muscle, the T tubules occur in register with the Z band; thus, there is only one system of T tubules per sarcomere.)

Flanking the T tubules are paired dilatations of the sarcoplasmic reticulum called cisternae, or cisterns. On electron microscopy, the characteristic grouping of one T tubule and two cisterns seen in cross sections is called the triad. Thus, the action potential is propagated into the depths of the muscle fiber very close to elements of the sarcoplasmic reticulum. A protein in the T-tubule membrane called the dihydropyridine receptor functions as a voltage sensor and undergoes a conformational change as a result of the action potential. This change is transmitted to another adjacent large protein, the ryanodine receptor, which is located between the sarcoplasmic reticulum and the T tubules. The ryanodine receptor functions as a calcium channel and leads to a release of calcium from the sarcoplasmic reticulum, where calcium is bound to calsequestrin. Note that in skeletal muscle, influx of extracellular calcium is not required for muscle contraction, unlike the case for motor nerve terminals.