Which structure is responsible for the spread of action potential in excitation-contraction coupling?

The structure responsible for the spread of action potentials in excitation-contraction coupling is the T tubules. They are invaginations of the sarcolemma (the muscle cell membrane) that penetrate into the muscle fiber, allowing action potentials that travel along the sarcolemma to quickly spread into the interior of the muscle cell. This rapid propagation of the action potential is crucial for ensuring that the signal for muscle contraction reaches all areas of the muscle fiber simultaneously, facilitating a coordinated contraction.

T tubules are closely associated with the sarcoplasmic reticulum, which stores calcium ions. When the action potential is transmitted along the T tubules, it triggers voltage-sensitive receptors that cause the release of calcium from the sarcoplasmic reticulum, leading to muscle contraction. This coupling between the action potential and calcium release through T tubules is essential for the effective functioning of skeletal and cardiac muscles.

While the sarcoplasmic reticulum plays a significant role in storing and releasing calcium, it does not directly spread the action potential itself. Intercalated discs are specialized structures found in cardiac muscle that facilitate communication between adjacent cells but do not propagate the action potential as T tubules do. Myofibrils are the contractile elements of muscle fibers and do