I don't understand how muscle contraction starts, can you explain it?

So, if we looks at the sarcomere of the muscle (which is from one Z line to another) you'll see a thicker looking strand/filament called myosin and thinner strands called actin. Myosin has bulbous structures along it called myosin heads. Basically what myosin wants to do is attach its myosin head to the actin and keep repeating this moving further along the actin towards the Z-lines to pull the actin strands closer together. The problem is there's a structure called tropomyosin twisted around the actin, blocking the myosin head binding sites. This is where action potentials come in. Once the action potential reaches the muscle and depolarises it through a T-tubule, the voltage-gated calcium channels on the Smooth Endoplasmic Reticulum opens allowing Ca to diffuse down it's concentration gradient by facilitated diffusion. The Ca will bind to tropomyosin to move it and expose the binding sites on actin. This allows the myosin head, which is bound to ADP to form a cross bridge with actin. As long as there is a high concentration of Ca in the sarcoplasm (muscle cytoplasm), it will be bound to tropomyosin freeing up the actin binding sites.