T-Tubules: The Conduit for Action Potential Penetration in Muscle Fibers

Transverse tubules (T-tubules) are deep invaginations of the sarcolemma that penetrate the muscle fiber perpendicular to its long axis, forming a grid-like network at the Z-disc level (mammals) or A-I junction (amphibians).

Function: The action potential traveling along the sarcolemma enters the T-tubules, rapidly depolarizing the interior. This triggers voltage-sensitive dihydropyridine receptors (DHPR), which mechanically couple to ryanodine receptors (RyR1) on the sarcoplasmic reticulum, releasing Ca²⁺ and initiating contraction.

Without T-tubules, central myofibrils would experience delayed depolarization, causing asynchronous contraction. T-tubule disruption (e.g., in muscular dystrophies, aging) contributes to excitation-contraction uncoupling and weakness.