Final answer:
The sarcoplasmic reticulum is the structure within skeletal muscle cells responsible for storing calcium ions which are essential for muscle contraction and relaxation.
Step-by-step explanation:
The structures within skeletal muscle cells that serve as reservoirs of calcium ions are called the sarcoplasmic reticulum. This specialized form of the smooth endoplasmic reticulum stores, releases, and retrieves calcium ions (Ca++) that are essential for the coordinated contractions of muscle fibers. The sarcoplasmic reticulum is crucial for muscle fiber contraction and relaxation, dispatching the calcium ions to initiate the interaction between actin and myosin, the contractile proteins, leading to muscle movement.