T-tubules are continuous with sarcolemma T tubules are open to the extracellular space. They conduct the membrane depolarization down into the muscle cell. Longitudinal tubules thin tubules , oriented in the direction of myofibrillar axis. Fenestrated collar a perforated sheet formed by convergence of longitudinal tubules in the H zone region. It is a structure formed collectively by the central T tubule and two tubular elements has two triads.
SR is extensive and runs around the myofibrils.
SR and T- tubules control contraction of myofilaments. Neither SR or T tubules are visible under a microscope.