Final answer:
Muscle contractions are initiated and halted by the Excitation-Contraction Coupling process, which relies on the action potential from a motor neuron and subsequent calcium ion release and reabsorption to start and stop muscle fiber contractions.
Step-by-step explanation:
How muscle contractions are turned on and off is best explained by A) Excitation-Contraction Coupling. This physiological process begins with a neural signal from a motor neuron, which generates an action potential on the sarcolemma of a skeletal muscle fiber. When the action potential travels along the sarcolemma, it triggers the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). These calcium ions then bind to and alter the structure of the troponin-trypomyosin complex, changing its position and exposing the binding sites on the actin filaments for the myosin heads. This results in myosin heads forming cross-bridges with actin, pulling the actin filaments towards the center of the sarcomere, and thereby shortening the muscle fiber to produce a contraction.
Turning off muscle contraction occurs when the action potential ends, calcium ions are reabsorbed by the SR, and troponin and tropomyosin return to their original position, covering the actin binding sites, thus preventing further cross-bridge formation. Without the cross-bridges, the muscle fiber relaxes. This cycle of on and off in muscle contraction is a key concept in understanding how movement is controlled at the cellular level.