Final answer:
The sliding filament assay involves observing the contraction of an actomyosin film when ATP is added, enabling the study of myosin movement along actin filaments. The direction of myosin's power stroke toward the M line is generally used to determine the direction of movement within the sarcomere.
Step-by-step explanation:
The ability of myosin to walk along an actin filament can be observed with a "sliding filament assay". This assay involves forming a film of actinomyosin on an aqueous surface. When ATP is added, this film visibly contracts, similar to muscle fiber contractions. This contraction is indicative of myosin heads, powered by ATP hydrolysis, pulling the actin filaments and thereby shortening the sarcomere-like structures within the film. The visible contraction and the resulting surface area reduction of the film are akin to the shortening of muscle fibers.
Determining the direction of myosin movement is possible by closely studying the changes in sarcomere structure and measuring the distances moved toward the center of the sarcomere, known as the M line, during the assay. This movement is called the power stroke and results in the contraction of the sarcomere. The use of specific markers or fluorescent tags on actin or myosin can elucidate the directional movement of these filaments during the contraction.