where are actin filaments located in the cell
Actin An actin protein is the monomeric subunit of two types of filaments in cells: microfilaments, one of the three major components of the cytoskeleton, and thin filaments, part of the contractile apparatus in muscle cells. It can be present as either a free monomer called G-actin (globular) or as part of a linear polymer microfilament called F-actin (filamentous), both of which are essential for such important cellular functions as the mobility and contraction of cells during cell division.
Terminal cisternae Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. These discrete regions within the muscle cell store calcium (increasing the capacity of the sarcoplasmic reticulum to release calcium) and release it when an action potential courses down the transverse tubules, eliciting muscle contraction. Because terminal cisternae ensure rapid calcium delivery, they are well developed in muscles that contract quickly, such as fast twitch skeletal muscle. Terminal cisternae then go on to release calcium, which binds to troponin. This releases tropomyosin, exposing active sites of the thin filament, actin.