Final answer:
An action potential is transduced to the sarcoplasmic reticulum by the T-tubule, which triggers the release of Ca2+ ions necessary for muscle contraction.
Step-by-step explanation:
When an action potential arrives, it is transduced to the sarcoplasmic reticulum (SR) by a special structure called the T-tubule. During the excitation portion of excitation-contraction coupling, the action potential propagates along the sarcolemma and enters these T-tubules, which are periodic invaginations in the sarcolemma. T-tubules serve to carry the action potential into the interior of the muscle cell. This in turn triggers the release of calcium ions (Ca2+) from storage within the adjacent SR.
The complex formed by the membrane of the T-tubule and the membranes of the SR on either side is termed a triad. These calcium ions then diffuse into the sarcoplasm and initiate the contraction of muscle fibers by binding to troponin on sarcomeres, the contractile units within myofibrils. These myofibrils contain the actin and myosin necessary for muscle contraction.