Video Transcript
The diagram provided shows how a
sarcomere changes when a muscle fiber contracts. Which of the following best
describes how the sarcomere changes? (A) The sarcomere shortens as the
myosin pulls the actin filaments toward the M line. Or (B) the sarcomere extends as the
actin filaments pull the myosin away from the M line.
To answer this question, let’s
first look at what a sarcomere is. A sarcomere is the functional unit
of a myofibril, which is a specialized organelle found in muscle fibers. Myofibrils are formed of many
sarcomeres, each of which is defined by two Z lines which mark the sarcomere’s
boundary.
In this diagram, we can see two of
the protein filaments that are present in a sarcomere: thick myosin filaments and
thinner actin filaments. The diagram shows a sarcomere in a
relaxed muscle at the top and in a contracted muscle below. When the muscle contracts, the
sarcomere shortens as the distance between the two successive Z lines shortens. This is because the H zone, a
region in the center of the sarcomere that only contains thick myosin filaments,
shortens when the muscle contracts.
The contraction occurs as myosin
pulls the actin filaments toward the M line in the center of the sarcomere. When the muscle relaxes, the actin
moves back to its original position, away from the M line, and the sarcomere extends
again.
Using this information, we can
deduce that the correct answer to this question asking about the changes to a
sarcomere when a muscle fiber contracts is (A). When a muscle fiber contracts, the
sarcomere shortens as the myosin pulls the actin filaments toward the M line.
