Actin, Myosin and Cell Movement

Actin filaments, usually in association with myosin, are responsible for many types of cell movements. Myosin is the prototype of a molecular motors—a protein that converts chemical energy in the form of ATP to mechanical energy, thus generating force and movement. The most striking variety of such movement is muscle contraction, which has provided the model for understanding actin-myosin interactions and the motor activity of  myosin molecules. However, interactions of actin and moysin are responsible not only for muscle contraction but also for a variety of movements of nonmuscle cells, including cell division, so these interactions play a central role in cell biology. Moreover, the actin cytoskeleton is responsible for the crawling movements of cells across a surface, which appear to be driven directly by actin polymerization as well as actin-myosin interactions.
The structure of actin-myosin:

As shown in the figure. The myosin filaments lie next to the actin filaments and have the capability to temporarily bind to the actin, causing the muscle to move. This binding capability provided the basis of the sliding filament model of muscle contraction. This model proposes the myosin head binds to the actin filament and then rotates to a different position, possibly as much as a forty five degree change, which can be accomplished by a change in structure. The myosin head is comparable to a stretched spring held in place by free energy generated by ATP hydrolysis.

The following video shows the sliding-filament model composed by actin and myosin. Myosin is thick, actin is thin.