Final answer:
High levels of calcium in the sarcoplasm of striated muscles initiate muscle contraction by causing structural changes in actin that enable the power stroke of contraction. This process is regulated by calcium's release from and reuptake into the sarcoplasmic reticulum.
Step-by-step explanation:
When calcium is present at high levels in the sarcoplasm of striated muscles, it triggers the muscles to contract. This process begins when an action potential from a motor neuron excites the sarcoplasmic reticulum, leading to the release of calcium ions. These ions bind to troponin on the thin filaments within the muscle cells, causing a change in the structure of the actin filaments. This alteration exposes the active sites on actin, which allows for the power stroke of contraction to occur. Muscle contraction will continue until ATP-dependent pumps transport calcium ions back into the sarcoplasmic reticulum and out of the cell, although a lower concentration of calcium remains to maintain muscle tone.