The sarcolemma of muscle fibers fold inwards and form system of tubes which runs through the sarcoplasm called:

Explanation

Transverse tubules, also known as T-tubules, are invaginations of the sarcolemma that penetrate deep into the sarcoplasm of muscle fibers. These tubular structures run perpendicular to the myofibrils and are closely associated with the sarcoplasmic reticulum. T-tubules allow the action potential (electrical signal) to rapidly penetrate into the interior of the muscle fiber, ensuring synchronized contraction of the myofibrils. They are essential for the excitation-contraction coupling process, which links the electrical excitation of the muscle fiber to the contraction of the myofilaments.