To determine whether a structure contains actin or myosin, one must consider the roles and characteristics of these proteins within muscle cells. Actin is a thin filament, while myosin forms the thick filament in muscle tissue.
If the description mentions a structure involved in muscle contraction and refers to filaments that slide past each other, it is likely referencing both actin and myosin as they work together during the contraction process. However, if the focus is specifically on the filament itself and it is described as being thin, then it is actin. Conversely, if the description indicates thick filaments or myosin heads, then it pertains to myosin.
In summary, recognizing the functional roles and structural differences between actin and myosin helps in identifying the structure described. For structures primarily known for contraction, you should consider the combined actions of both proteins rather than isolating one over the other.