In smooth muscle the calcium needed for contraction comes mainly from?

In smooth muscle, the calcium needed for contraction primarily comes from the extracellular space. When smooth muscle cells are stimulated, calcium channels in the cell membrane open, allowing calcium ions to flow into the cell. Additionally, some calcium can also be released from the sarcoplasmic reticulum, but the extracellular influx is the main source for initiating contraction. This influx of calcium activates the contraction process by interacting with calmodulin and subsequently activating myosin light chain kinase.