Biology, Eighth Edition (Raven)

Chapter 47: The Musculoskeletal System

Sarcomere Contraction

What limits the extent to which muscles can contract? Muscle contractions occur through the sliding of actin and myosin filaments past one another. The filaments themselves do not contract and the other elements of an individual sarcomere are not compressed. The sarcomere is able to contract without seriously bending or distorting itself. Contraction ends when the actin filaments on opposite sides of the sarcomere come into contact and the myosin comes into contact with the Z-lines forming the boundaries of the sarcomere.

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


1.	Muscle contraction is caused by

		A)	actin filaments sliding past each other.
		B)	myelin filaments sliding past each other.
		C)	myosin filaments sliding past each other.
		D)	actin filaments sliding past myelin filaments.
		E)	actin filaments sliding past myosin filaments.

2.	Which of the following statements about H zones and I bands is TRUE?

		A)	the H zones contain only myosin, while the I bands contain only actin
		B)	the H zones contain only actin, while the I bands contain only myosin
		C)	the H zones contain only myosin, while the I bands contain both actin and myosin
		D)	the H zones contain both actin and myosin, while the I bands contain only actin
		E)	the H zones and the I bands each contain both actin and myosin

3.	During muscle contraction

		A)	the A band remains the same and the I band narrows.
		B)	the I band remains the same and the A band narrows.
		C)	both the A band and I band remain the same.
		D)	both the A band and I band narrow.
		E)	both the A band and I band become wider.

4.	In a contracting muscle the Z lines come closer together.

		A)	True
		B)	False

5.	In a relaxed muscle, the H zone is at its narrowest width.

		A)	True
		B)	False