Each sarcomere is bounded by Z lines - they anchor the thin actin filaments. In the center of the sarcomere is the A band - the myosin filaments. The dark stripe within the H zone is called the M band - proteins that help hold the myosin filaments. When the muscle contracts, the sarcomere shortens. The H zone and the I band become much narrower, and the Z lines move towards the A band.
Contraction occurs after an action potential.
Muscles can only contract and relax. Hydrostatic skeletons 2. When muscles oriented in a certain direction contract, the fluid-filled body cavity bulges out in the opposite direction. Exoskeleton - a hardened outer surface to which internal muscles can be attached. Contractions of these muscles cause jointed segments of the exoskeleton to move relative to each other.