What key event does the action potential trigger when propagating down the T tubules?

The propagation of an action potential down the T tubules is a critical step in muscle contraction. When the action potential travels along the T tubules, it causes a change in the voltage-sensitive proteins embedded in the T tubule membrane. These proteins, known as dihydropyridine receptors, undergo a conformational change due to the alteration in voltage. This change is crucial as it allows these receptors to interact with the ryanodine receptors located on the sarcoplasmic reticulum (SR).

The interaction between these voltage-sensitive proteins and the ryanodine receptors is essential for muscle contraction. As the action potential triggers the change in voltage-sensitive tubule proteins, it results in the opening of the ryanodine receptors, leading to the release of calcium ions from the sarcoplasmic reticulum into the cytoplasm. This increase in intracellular calcium is what ultimately initiates the sliding filament mechanism, allowing muscle fibers to contract.

Understanding this mechanism highlights the importance of the change in voltage-sensitive proteins in the process of muscle excitation and contraction, illustrating how electrical signals result in mechanical action.