Final answer:
The thick and thin filaments in muscle cells are arranged alternately within the sarcomere and slide past each other during muscle contraction. Cross-sectional electron micrographs show a dense overlapping region essential for forming cross-bridges that enable contraction. Thick filaments are within the A band, while thin filaments extend from the Z disc into the I band and into the A band, partially.
Step-by-step explanation:
In muscle cells, the arrangement of thick and thin filaments can be described as alternating with each other to form a structure known as a sarcomere, which is the functional unit of muscle contraction. When viewed in a cross-sectional electron micrograph, the region where these filaments overlap appears densely packed due to the little space between them. The thick filaments are made up of myosin and are found in the A band of the myofibril, whereas the thin filaments, composed of actin, are attached to the Z disc at one end and extend across the I band and partially into the A band. The central region of the A band, which contains only thick filaments and appears slightly lighter, is termed the H zone, with a vertical line called the M line where accessory proteins bind thick filaments together.
During contraction, these filaments do not shorten; instead, they slide past one another. The myosin heads bind to the actin filaments and, using energy from ATP, pull them inwards, shortening the sarcomere and causing the muscle to contract. This sliding filament model is supported by the formation of cross-bridges between actin and myosin that determine the muscle fiber's amount of tension. The number of cross-bridges can only increase in regions where the thick and thin filaments overlap, highlighting the importance of this structure.