Covering, Support, and Movement of the Body
the thick ﬁlaments in place, thus maintaining the organiza-
tion of the A band, and helps the muscle cell spring back into
shape aFer stretching. (Te part of the titin that spans the I
bands is extensible, unfolding when the muscle stretches and
recoiling when the tension is released.) ±itin does not resist
stretching in the ordinary range of extension, but it stiﬀens
as it uncoils, helping the muscle resist excessive stretching,
which might pull the sarcomeres apart.
Another important structural protein is
links the thin ﬁlaments to the integral proteins of the
Flexible hinge region
Each thick filament consists of many myosin molecules
whose heads protrude at opposite ends of the filament.
A thin filament consists of two strands of actin subunits
twisted into a helix plus two types of regulatory proteins
(troponin and tropomyosin).
In the center of the sarcomere, the thick filaments
lack myosin heads. Myosin heads are present only
in areas of myosin-actin overlap.
Longitudinal section of filaments within one
sarcomere of a myofibril
Portion of a thick filament
Portion of a thin filament
Composition of thick and thin ﬁlaments.
sarcolemma (which in turn are anchored to the extracellular
Other proteins that bind ﬁlaments or sarcomeres together
and maintain their alignment include
Sarcoplasmic Reticulum and T Tubules
Skeletal muscle ﬁbers contain two sets of intracellular tubules
that help regulate muscle contraction: (1) the sarcoplasmic re-
ticulum and (2) ± tubules.