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How does an action potential travel from the sarcolemma into a muscle fiber?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

When an action potential reaches the sarcolemma, it is transmitted into the interior of the muscle fiber through the transverse tubules (T-tubules).12

These tubules are channels that open through the sarcolemma and are continuous with the surface membrane, allowing the electrical signal to travel deep into the fiber to reach the functional contractile units. As the action potential moves along the tubular membranes, it reaches the triads—complexes where the transverse tubules are positioned in close proximity to the terminal cisternae of the sarcoplasmic reticulum.1

The change in electrical properties within the transverse tubules triggers the terminal cisternae to release calcium ions into the sarcoplasm. This release of calcium is the critical step that initiates the interaction between actin and myosin filaments, ultimately leading to muscle contraction.12