Skeletal muscle is visually distinctive (viewed in longitudinal section) in that it appears as long, parallel fibers with striations. These striations result from the repeated overlapping nature of the proteins in the muscle fiber (cell). Describe the appearance of the striations at high power (400X total magnification): None
The correct answer and explanation is:
At high power (400X total magnification), the striations in skeletal muscle fibers appear as alternating dark and light bands that run perpendicular to the length of the muscle fiber. These striations are highly visible due to the specific organization of contractile proteins within the muscle fiber. The striations are a result of the structural arrangement of myofilaments—actin (thin filaments) and myosin (thick filaments)—within the muscle cells. The repeating pattern of these filaments within the sarcomeres (the functional units of muscle contraction) produces the characteristic striations seen under the microscope.
The light bands, known as I-bands, represent areas where the actin filaments are present without overlapping myosin filaments. The A-bands, which are the dark bands, show regions where both actin and myosin filaments overlap. Within the A-band, there is a lighter zone called the H-zone, which represents the area where only myosin filaments are present when the muscle is relaxed. The center of the A-band is also where the M-line is located, a structure that helps anchor the myosin filaments in place.
The repeating sequence of I-bands, A-bands, H-zones, and M-lines along the muscle fiber is visible as striations at high magnification. These striations are essential for understanding muscle contraction because they reflect the alignment of the contractile proteins that work together to generate force. As the muscle contracts, the actin filaments slide over the myosin filaments, causing the sarcomere to shorten and the striations to become more closely spaced.
Thus, the striations in skeletal muscle fibers at 400X magnification provide a detailed view of the highly organized, repeating pattern of protein filaments essential for muscle contraction. This structural organization is crucial for the muscle’s function in generating movement and force.