Final answer:
The terminal cisternae of the sarcoplasmic reticulum is where calcium pumps work to lower sarcoplasmic calcium levels, leading to muscle relaxation.
Step-by-step explanation:
The terminal cisternae element of the sarcoplasmic reticulum is where calcium pumps function to decrease sarcoplasmic Ca²⁺ concentration for muscle relaxation.
In muscle cells, the sarcoplasmic reticulum plays a crucial role in the contraction and relaxation of muscles. It stores calcium ions (Ca²⁺), which are essential for muscle contraction. When a muscle relaxes, the ATP-driven pumps in the sarcoplasmic reticulum actively transport calcium ions out of the sarcoplasm and back into the sarcoplasmic reticulum. This process is important for 'reshielding' the actin-binding sites on the thin filaments, leading to muscle relaxation as it prevents further cross-bridging between the thin and thick filaments.