Biology, Eighth Edition (Raven)

Chapter 43: The Animal Body and Principles of Regulation

Myofilament Contraction

Why are muscles organized into opposing groups? Skeletal muscles, particularly those on the long bones are often organized into opposing groups of muscle fibers. For example the biceps and triceps muscles are on opposite sides of the humerus and cause opposite movements of the forearm. Opposing groups allow a wide range of movements to be powered by muscle contraction. The active part of muscle movement is contraction, when the actin and myosin filaments slide over one another. Extension of muscles is passive – the action of extension does not exert any force. To exert force in both directions of movement, such as flexing or extending your arm, it is necessary to have muscles acting in opposition to one another.

View the animation below, then complete the quiz to test your knowledge of the concept.


Each actin filament is composed of
A)a single strand of actin molecules.
B)two strands of actin molecules wrapped together.
C)three strands of actin molecules wrapped together.
D)four strands of actin molecules wrapped together.
E)hundreds of strands of actin molecules wrapped together.

As actin and myosin filaments slide past each other during muscle contraction,
A)actin filaments shorten, while myosin filaments do not.
B)myosin filaments shorten, while actin filaments do not.
C)either actin or myosin filaments shorten, but not both at the same time.
D)both actin and myosin filaments shorten.
E)neither actin nor myosin filaments shorten.

Which of the following statements about the "heads" of the myosin molecules is TRUE?
A)they form permanent links with actin filaments
B)they can attach to different sites on the actin filament
C)they straighten the myosin filaments
D)they allow the myosin filaments to wrap around each other
E)none of the above

Each myosin protein has a globular head that extends outward from the myosin filament.

Unflexing of the myosin head requires ATP.
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