Research Paper On Vascular Anomalies
al 2016). In a single unit smooth muscle cell, gap junctions are used to send signals between the cells (Hill et. al 2016). However, in multiunit smooth muscles, the muscle cells function as independent units because there are few, if any, gap junctions (Hill et. al 2016). This would be used to describe a large artery or vein (Hill et. al 2016). Like skeletal muscles, smooth muscles use the contractile proteins actin (thin filament) and myosin (thick filament); however, these proteins are not arranged into sarcomeres, so the cells do not appear striated. These thin and thick filaments interact with each other in the presence of calcium (Ca²⁺) in order to form cross bridges. The cross bridges allow for these filaments to slide by one another, thereby undergoing the sliding filament model, to accomplish contraction. In order for contraction to occur, a signaling molecule must bind and activate a G-coupled protein receptor, containing alpha, beta, and gamma subunits, on the plasma membrane of a smooth muscle cell. The alpha subunit then dissociates from the rest and activates phospholipase C (PLC). PLC then yields diacylglycerol (DAG) and IP3. IP3 then binds to a ligand-gated channel thereby releasing Ca²⁺ from the sarcoplasmic reticulum (SR) into the cytosol. Ca²⁺ then binds to calmodulin and forms the Ca²⁺-calmodulin complex. This complex activates myosin light chain kinase (MLCK) which then phosphorylates myosin light chain. The phosphorylation induces a conformation change that allows actin and myosin binding in order for contraction to occur. As long as there is Ca²⁺ present, the myosin light chain remains phosphorylated and repeat cross-bridges can form. Therefore, the number of cross-bridges increases when Ca²⁺ increases in the cytoplasm. On the other hand, relaxation occurs by pumping Ca²⁺ out of the cytoplasm and back into the SR or out of the cell.