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. The point where a sacromere meets its neighboring sarcomere is called the “z-line”. The repeating units of sarcomeres represent the unique banding pattern seen in striated muscles. The thick filament of the sarcomere constitutes the “A band”. This is in the center of the sarcomere. These thick filaments are made of myosin. These myosin molecules have two heads attached to a tail. Imagine them as resembling a hammer lying with its head pointing upwards. These heads are what bind the ATP (the energy source for the fiber) and create a cross-bridge with the thin filament. The thin filaments are anchored to the sarcomere on the Z line. When you see a diagram of the sarcomere, they make up the I band. There are intertwined between the thick filaments within part of the A band. These thin filaments are made up 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 I Googled to help you imagine a sarcomere: In this image you can see the myosin fiber heads 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. Tropomisone 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 is displaced and allows the myosin heads to have access to bind to actin. Surrounding these sarcomeres is a structure of channels called the sarcoplasmic reticulum and they are connected to the extracellular space. (around the muscle fibers). T tubules are an extensive tubular network that opens to the sarcomere. These tubules are found at the junction between the A bands and the I bands. Action potentials travel along these T tubules into the interior of the cell in the sarcoplasmic reticulum.