Ttubuli

Ttubuli, often referred to as transverse tubules or T-tubules, are deep invaginations of the sarcolemma (muscle cell membrane) that extend into the interior of striated muscle fibers. They form an extensive network that carries action potentials from the cell surface to the cell’s interior, enabling rapid and coordinated muscle contraction through excitation-contraction coupling.

In skeletal muscle, Ttubuli typically run perpendicularly to the myofibrils and align with the junction between the A and I bands to form triads, in which each T-tubule is flanked by two terminal cisternae of the sarcoplasmic reticulum. In cardiac muscle, the analogous structures are dyads, where a T-tubule sits near a single terminal cisterna on each side. The biogenesis of Ttubuli involves membrane remodeling during development, with proteins such as BIN1 (amphiphysin II) and junctophilins playing important roles in shaping and linking the T-tubule system to the sarcoplasmic reticulum.

Functionally, Ttubuli enable the rapid propagation of action potentials into the cell interior, which triggers voltage-gated L-type calcium channels (DHPRs) in the T-tubule membrane. This activates ryanodine receptors (RYRs) on the sarcoplasmic reticulum, causing calcium release and initiating contraction. They also participate in ongoing calcium homeostasis and modulation of calcium transients in response to physiological demand.

Clinical relevance includes Ttubule remodeling or disorganization observed in heart failure and various myopathies, where disrupted coupling between the membrane and the sarcoplasmic reticulum can impair excitation-contraction efficiency.