Final answer:
In muscle tissue, the largest unit is the myofiber, which contains many myofibrils made up of myofilaments. I bands and H zones contain only thin or thick myofilaments, respectively, while the A band consists of both. Contraction involves the shortening of I bands and H zones, increasing the overlap between filaments, with the A band remaining constant in length.
Step-by-step explanation:
Structure and Function of Muscle Fibers
The myofilaments, myofibril, and myofiber are parts of muscle tissue organized in a hierarchical structure. A myofiber or muscle fiber is the largest among the three and is composed of many myofibrils, which in turn are made up of myofilaments. Myofilaments are the smallest and most fundamental component, consisting of two types: thick filaments and thin filaments, also known as myosin and actin filaments respectively.
Within the sarcomere, which is the functional unit of a myofiber, there are specific regions characterized by the presence of these filaments. The I band is the area containing just thin filaments, while the A band consists of overlapping thick and thin filaments. The H zone is found within the A band and contains only thick filaments. At the midpoint of the H zone resides the M line, where accessory proteins maintain the structural integrity of the thick filaments.
During muscle contraction, the A band remains the same length while the I band and H zone shorten, increasing the zone of overlap between thick and thin filaments. This contractile activity is powered by ATP, which fuels the movement of myosin heads that bind and pull on the actin filaments.