Final answer:
The calcium involved in muscle contraction comes from the extracellular fluid, which is essential for the activation of the myosin heads and initiation of muscle contraction.
Step-by-step explanation:
The calcium involved in muscle contraction ultimately comes from the extracellular fluid. In skeletal muscle fibers, the action potential stimulates the release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR) in response to the interaction between Ca++ and proteins (troponin and tropomyosin) that expose specific binding sites on actin. This allows the myosin heads to attach to actin and initiate muscle contraction. Smooth muscle cells, on the other hand, rely on external Ca ions passing through calcium channels in the sarcolemma, as well as Ca released from the SR, to trigger the activation of the myosin heads and subsequent contraction.