Final answer:
The structure that facilities Ca++ uptake in the sarcoplasmic reticulum is the triad, which includes T-tubules and terminal cisternae of the SR.
Step-by-step explanation:
The transport structure that facilitates Ca++ uptake in the sarcoplasmic reticulum (SR) is the triad. The triad is composed of two terminal cisternae (enlarged areas of the SR) and one T-tubule, with the T-tubule sandwiched between the SR membranes. During muscle contraction, action potentials propagate along the sarcolemma and enter the T-tubules, which are narrow invaginations that allow electrical impulses to reach deep inside the muscle fiber near the SR. This triggers the opening of calcium channels in the SR, causing Ca++ to diffuse out of the SR and into the sarcoplasm, initiating the contraction of the muscle by its contractile units, or sarcomeres.