Muscles and Muscle Tissue
Follow the events of excitation-contraction coupling that
lead to cross bridge activity.
Te sliding ﬁlament model tells us how a muscle ﬁber contracts,
but what induces it to contract in the ﬁrst place? For a skeletal
muscle ﬁber to contract:
1. Te ﬁber must be activated, that is, stimulated by a nerve
ending so that a change in membrane potential occurs.
2. Next, it must generate an electrical current, called an
, in its sarcolemma.
Te roles of the ± tubules and SR in pro-
viding signals for contraction are tightly linked. At the triads,
where these organelles come into closest contact, integral pro-
teins (some from the ± tubule and others from the SR) pro-
trude into the intermembrane spaces. Te protruding integral
proteins of the ± tubule act as voltage sensors. Tose of the SR
form gated channels through which the terminal cisterns re-
. We will return to their interaction shortly.
Sliding Filament Model of Contraction
We almost always think “shortening” when we hear the
, but to physiologists the term refers only
to the activation of myosin’s cross bridges, which are the
force-generating sites. Shortening occurs if and when the
cross bridges generate enough tension on the thin ﬁlaments
to exceed the forces that oppose shortening. Contraction
ends when the cross bridges become inactive, the tension
declines, and then the muscle ﬁber relaxes.
In a relaxed muscle ﬁber, the thin and thick ﬁlaments overlap
only at the ends of the A band (
ﬁlament model of contraction
states that during contraction
the thin ﬁlaments slide past the thick ones so that the actin and
myosin ﬁlaments overlap to a greater degree:
When the nervous system stimulates muscle ﬁbers, the my-
osin heads on the thick ﬁlaments latch onto myosin-binding
sites on actin in the thin ﬁlaments, and the sliding begins.
Tese cross bridge attachments form and break several times
during a contraction, acting like tiny ratchets to generate tension
and propel the thin ﬁlaments toward the center of the sarcomere.
As this event occurs simultaneously in sarcomeres through-
out the cell, the muscle cell shortens.
Notice that as the thin ﬁlaments slide centrally, the Z discs to
which they attach are pulled
the M line (Figure 9.6
Overall, as a muscle cell shortens: (1) the I bands shorten, (2) the
distance between successive Z discs shortens, (3) the H zones
disappear, and (4) the contiguous A bands move closer together
but their length does not change.
Check Your Understanding
How does the term epimysium relate to the role and position
of this connective tissue sheath?
Which myoﬁlaments have binding sites for calcium? What
speciﬁc molecule binds calcium?
Which region or organelle—cytosol, mitochondrion, or
SR—contains the highest concentration of calcium ions in
a resting muscle ﬁber? Which structure provides the ATP
needed for muscle activity?
For answers, see Appendix H.
Physiology of Skeletal Muscle Fibers
Explain how muscle ﬁbers are stimulated to contract by
describing events that occur at the neuromuscular junction.
Describe how an action potential is generated.
Fully relaxed sarcomere of a muscle ﬁber
Fully contracted sarcomere of a muscle ﬁber
Sliding ﬁlament model of contraction.
indicate events in a
relaxed and a
fully contracted sarcomere.
At full contraction, the Z discs abut the thick ﬁlaments and the thin
ﬁlaments overlap each other. The photomicrographs (top view in
each case) show enlargements of 33,000