Final answer:
The sarcomere is the functional unit of muscle fibers, composed of Z discs, I and A bands, the H zone, and the M line. Under electron microscopy, these components show distinct patterns that illustrate the sliding filament theory during muscle contraction.
Step-by-step explanation:
The sarcomere, visible under electron microscopy, includes various components that contribute to muscle contraction. At each end of the sarcomere is a Z disc (Z line), which marks its boundaries. The I band consists of thin (actin) myofilaments and appears lighter. The A band, at the sarcomere's center, includes both thick (myosin) and thin filaments overlapping.
Within the A band is the lighter H zone, where only thick filaments are present, and in its center, the M line helps hold these thick filaments together. During muscle contraction, the distance between the Z discs reduces, the I and H zones shorten, but the A band remains the same length, showing the sliding filament theory of muscle contraction.