What happens to the length of the sarcomeres in the contraction
The sarcomere shortens as a result of the contraction shown in
Have the thin and thick filaments shortened because of the
Thin (actin) and thick (myosin) filaments have NOT shortened
What has happened to the I band?
The I band shortens.
What has happened to the H zone?
The H zone disappears (because the actin filaments slide toward
the center of the sarcomere).
What has happened to the A band?
There is no change in the appearance of the A band
Explain the change or lack of change in the A band
The A band does not change because the myosin filament does not
slide (to the left or right) or shorten during the contraction.
(The sliding of the actin filaments is what causes the sarcomere
to shorten, the I band to shorten, and the H zone to disappear.)
Which filament slides toward the center of the sarcomere
during this contraction?
The actin filaments (on the left and right sides of the
sarcomere) slide toward the center of the sarcomere in the
What causes this sliding to occur?
Myosin is responsible for the sliding of the actin filaments.
Myosin heads attach to the actin “beads” and swing, pulling the
Notice that in this contraction the sliding of filaments causes the
sarcomeres to shorten, which shortens the muscle. (For example,
when standing, muscles on top of the thigh shorten to lift your foot
off the ground). What is the more specific name of this contraction?
The general name of this contraction--when sliding of the actin
filaments occurs---is the isotonic contraction. However, the
more specific name for a contraction where sliding of filaments
causes the sarcomeres to shorten, is the concentric contraction.
The sliding of filaments can also cause the sarcomeres to lengthen,
which lengthens the muscle. (For example, muscles on top of the thigh
lengthen when you put your foot back on the ground.) What is the
more specific name of this contraction?
An isotonic contraction where the sarcomeres lengthen is called
an eccentric contraction. (The actin filaments are sliding AWAY from
the center of the sarcomere instead of toward it).
The sliding of filaments inside the sarcomere does NOT occur in all
types of contractions. What is the contraction called if there is no
sliding of actin at all?
A contraction where sliding does not occur is called an isometric
contraction. Isometric means “Same Length” since the sarcomere
doesn’t shorten (or lengthen) in this contraction
In this second type of contraction, where no sliding occurs, what
DOES happen during the contraction?
The myosin heads still attach to actin, which creates tension (a
tightness or tauntness) in the muscle. For this reason an
isometric contraction is also known as a Muscle Tone contraction.
(tone= tension) There is no swinging of the myosin heads or
sliding of actin, just the attachment of myosin to actin. See
#57 for more on Muscle Tone.