Final answer:
The calcium needed for smooth muscle cross-bridging activation primarily comes from extracellular calcium ions that enter through sarcolemma calcium channels and are supplemented by calcium from the sarcoplasmic reticulum.
Step-by-step explanation:
The calcium that is needed to activate cross-bridging in smooth muscle mainly comes from extracellular sources through the calcium channels in the sarcolemma and the limited calcium-storing sarcoplasmic reticulum (SR). Unlike skeletal muscle, smooth muscle does not have troponin, and the regulatory protein calmodulin plays a crucial role in this process.
When external Ca2+ ions pass through the opened calcium channels and additional Ca2+ released from SR, they bind to calmodulin. The Ca2+-calmodulin complex then activates an enzyme called myosin light chain kinase, which phosphorylates the myosin heads, allowing them to attach to actin and facilitate muscle contraction.
Smooth muscle contraction is regulated by the protein calmodulin. The required calcium ions for activating cross-bridging in smooth muscle mainly come from extracellular sources. External calcium ions passing through opened calcium channels in the sarcolemma, as well as additional calcium released from the sarcoplasmic reticulum, bind to calmodulin. The calcium-calmodulin complex then activates an enzyme called myosin (light chain) kinase, which in turn activates the myosin heads for cross-bridging and muscle contraction.