Final answer:
The statement about T-tubules is true; they are essential for carrying the action potential into the muscle cell interior and initiating the release of calcium ions from the sarcoplasmic reticulum, which then triggers muscle contraction.
Step-by-step explanation:
The statement that T-tubules are inward extensions of the sarcolemma that trigger the release of Ca++ from the SR during an Action Potential is true. T-tubules, also known as transverse tubules, are structures that carry the action potential deep into the muscle cell, ensuring that it reaches the sarcoplasmic reticulum (SR). This contact triggers the release of calcium ions (Ca++) from the SR into the sarcoplasm, initiating the muscle contraction process by interacting with the contractile units, or sarcomeres.
During excitation-contraction coupling, an action potential propagating along the sarcolemma is carried into the T-tubules, where it causes the opening of calcium channels in the adjacent SR, leading to the diffusion of Ca++ into the sarcoplasm. The presence of Ca++ is pivotal for muscle fiber contraction as it binds to troponin, thereby allowing myosin to bind to actin and the muscle contraction cycle to begin.