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Skeletal muscle cells release and resequester stored calcium ions to contract and subsequently relax. CalDyx (short for “Calcium Dynamics”) is a self-contained software program that teaches the student about the major events that underlie the excitation-contraction (EC) coupling process in skeletal muscle. The program simulates changes in the concentrations of myoplasmic calcium (Ca2+) in three types of adult vertebrate muscle fibers (slow-twitch, fast-twitch, and superfast-twitch) in response to action potentials. The simulations are closely tied to experimental measurements of changes in the myoplasmic free Ca2+ concentration performed with calcium indicator dyes introduced by microinjection into these fiber types. The program includes 10 exercises that guide the student through the major signaling events of EC coupling, including the release of Ca2+ from the sarcoplasmic reticulum (SR), the binding of Ca2+ to the major myoplasmic Ca2+ binding sites, and the return of Ca2+ to the SR by the Ca “pump” proteins of the SR. The program is quantitatively based and should increase the student’s understanding of kinetic processes within cells.