When looking at skeletal muscle sectioned longitudinally, you can see the bands of alternating dark and pale stain. What accounts for these bands?
The bands of alternating dark and pale stain seen in longitudinally sectioned skeletal muscle are called striations. These striations are caused by the alignment of the myofibrils within the muscle fibers. Myofibrils are composed of repeating units called sarcomeres, which are the functional units of skeletal muscle contraction.
The dark bands, called A bands, correspond to areas where thick myosin filaments and overlapping thin actin filaments are present. The pale bands, called I bands, correspond to regions where only thin actin filaments are present. The Z lines mark the boundaries of each sarcomere and appear as dark lines in the I bands.
The alternating pattern of dark A bands and pale I bands gives skeletal muscle its striated appearance. This striated pattern is essential for the proper functioning of skeletal muscle contraction, as it allows for the coordinated sliding of actin and myosin filaments during muscle contraction.