Describe the sliding filament theory

according to the sliding filament theory, during muscle contraction, actin filaments slide against myosin filaments to produce movement.

How is movement produced? Myofibrils, which is the contractile element of the muscle cells, is consisted by sarcomeres. Sarcomeres are consisted by the two types of filaments. Myosin is centrally positioned, and actin is placed to each end of the sarcomere (see picture). The filaments are parallel to each other, so that they overlap (see picture).
When actin filaments start sliding against myosin filaments, the sarcomere is shortening (see picture again) and that causes the contraction.

How are filaments sliding against each other?
Before we explain it, I'll remind you that myosin filaments are consisted of myosin heads with ATPase activity. At the region where actin and myosin overlaps, myosin is full of myosin heads. These heads are "dragging" against actin to produce the movement. The mechanism involves hydrolysis of ATP and goes as follow:

See the second picture for a better understanding

1) A myosin head is attached to an actin filament
2) An ATP binds on the myosin head, and the myosin head releases actin
3) The myosin head moves along the actin filament and the ATP is hydrolised.
4) Myosin head is then reattached to actin and the Pi is released. The release of Pi causes the myosin head to move to it's initial place, dragging the actin filament with it
5) The myosin head releases the actin and is ready to accept another ATP.



Pictures from: Essential cell biology (alberts etc) chapter 17.