Question

muscle contraction interactive and video
in the sliding filament model of muscle contraction, what do myosin heads do?
myosin heads create calcium that is used in muscle contraction.
myosin heads break actin down.
myosin heads pull on actin filaments to shorten the sarcomere.
myosin heads push on tropomyosin to cause muscle relaxation.
all of these.

Explanation:

In the sliding filament model, myosin heads form cross - bridges with actin filaments. They undergo a power stroke, pulling on actin filaments. This action shortens the sarcomere (the basic unit of muscle contraction). Myosin heads do not create calcium (calcium is released from the sarcoplasmic reticulum), they do not break actin down (actin is a structural protein in the thin filament), and they do not push on tropomyosin to cause relaxation (tropomyosin movement is regulated by calcium and other proteins for contraction/relaxation, and myosin's main role in contraction is pulling actin).

Answer:

Myosin heads pull on actin filaments to shorten the sarcomere.