Each skeletal muscle is composed of bundles of myofibrils. Muscle fibers are made up of units called Sarcomeres. A sarcomere is a series of thick and thin filaments that overlap longitudinally. Where one sacromere meets its neighboring sacromere, it is called the “z-line”. Sarcomere repeating units explain the unique banding pattern observed in striated muscles. The thick filament of the sarcomere constitutes the “A band”. It is in the center of the sarcomere. These thick filaments are made of myosin. These myosin molecules have two heads attached to a tail. Think of them as looking like a hammer placed with the head pointing upwards. These heads bind ATP (the fiber's energy source) and create a cross-bridge with the thin filament. The thin filaments are anchored to the sarcomere at the Z line. When you see a diagram of the sarcomere, they make up the I band. The thick filaments are intertwined in part of the A band. These thin filaments are made of actin, tropomyosin and troponin. Imagine these thin filaments as spirals of thread with little dots of troponin along them. I've included an image that I imaged from Google to help you imagine a sarcomere: In this image, you can see the heads of the myosin fiber scattered around. Here's a picture of the interaction between the two fibers: These diagrams are a little complicated but just pay attention to what we're talking about. Troponomysin and troponin are attached to each other. Tropomyson acts as a block to the myosin head preventing it from attaching to actin, while troponin acts as a regulatory protein. When troponin is exposed to calcium, tropomyosin moves and allows access for myosin heads to bind to actin. Around these sarcomeres is a structure of channels called the sarcoplasmic reticulum and they are connected to the extracellular space. (around muscle fibers). The T-tubules constitute a vast tubular network that opens onto the sarcomere. These tubules are located at the junction between the A bands and the I bands. Action potentials travel along these T tubules to the interior of cells at the sarcoplasmic reticulum...