Final answer:
The release of calcium ions from the sarcoplasmic reticulum within muscle cells is called calcium triggered because it occurs in response to an action potential and is necessary for initiating muscle contraction. The sarcoplasmic reticulum releases Ca++ which then binds to troponin, allowing muscle contraction to occur through the interaction of sarcomere filaments. This process is sustained as long as Ca++ and ATP are available.
Step-by-step explanation:
Calcium-Triggered Release from the Sarcoplasmic Reticulum
The release of Ca++ from the sarcoplasmic reticulum is termed calcium triggered because it is part of a process called excitation-contraction coupling, where an action potential in the muscle fiber's sarcolemma leads to muscle contraction. This process initiates when acetylcholine (ACh) is released from a motor neuron at the neuromuscular junction and binds to receptors on the sarcolemma, setting off an action potential. This travels down the membrane, including T-tubules, and alters the voltage within the muscle cell.
Such electrical signals cause the specialized sarcoplasmic reticulum (SR) within muscle cells, which serves as a storage site for calcium ions (Ca++), to release these ions into the sarcoplasm. The released Ca++ then initiates the contraction of the muscle by binding to troponin, unshielding the actin-binding sites, and facilitating the interaction between thick and thin filaments within the sarcomere. The presence of ATP ensures that actin strands can be pulled by myosin, leading to the shortening of the muscle fiber.
The continuous availability of Ca++ ions in the sarcoplasm, which is actively transported out or diffuses from the SR in response to neuronal signals, enables the muscle to maintain contraction as long as necessary signals and ATP are present.