Final answer:
High levels of Ca2+ in the sarcoplasm indicate muscle contraction, with calcium ions binding to troponin and initiating myosin and actin interaction. Following contraction, calcium ions are actively pumped back into the sarcoplasmic reticulum for muscle relaxation, while a small amount remains to maintain muscle tone.
Step-by-step explanation:
When Ca2+ is present at high levels in the sarcoplasm of striated muscles, this indicates that muscle contraction is occurring. These calcium ions are released from the sarcoplasmic reticulum (SR), a specialized form of the smooth endoplasmic reticulum in muscle cells, and they bind to troponin on the actin filaments. This binding removes the shielding effect of tropomyosin, thus exposing the binding sites for myosin on the actin filaments. The myosin heads can then form cross-bridges with actin, which, powered by ATP, pull the actin filaments past the myosin, resulting in muscle contraction.
After the contraction, calcium ions are actively transported back into the SR, thus decreasing their concentration in the sarcoplasm. This active transport requires ATP and is necessary for muscle relaxation. A small amount of Ca2+ however remains in the sarcoplasm to maintain muscle tone, ensuring that the muscle remains ready to contract when needed again.