ECPI Anatomy and Physiology (A&P) Exam 1 Practice Test

In skeletal muscle, the light bands are the regions with only thin actin filaments. The band that contains actin only is the I band. It lies between the Z discs and lacks any overlap with myosin, which is why it appears light under the microscope. The darker A band contains the full length of thick myosin filaments and any overlap with thin filaments. Within the A band, the central H zone has only myosin and no actin. During contraction, actin filaments slide inward, shortening the I band, while the A band stays essentially the same length.