Chapter 9
Muscles and Muscle Tissue
285
9
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
action potential
, in its sarcolemma.
Triad Relationships
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-
lease Ca
2
1
. We will return to their interaction shortly.
Sliding Filament Model of Contraction
We almost always think “shortening” when we hear the
word
contraction
, 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 (
Figure 9.6
1
). Te
sliding
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
toward
the M line (Figure 9.6
2
).
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
3.
How does the term epimysium relate to the role and position
of this connective tissue sheath?
4.
Which myofilaments have binding sites for calcium? What
specific molecule binds calcium?
5.
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.
I
I
A
Z
Z
H
1
Fully relaxed sarcomere of a muscle fiber
I
I
A
Z
Z
2
Fully contracted sarcomere of a muscle fiber
Figure 9.6
Sliding filament model of contraction.
The numbers
indicate events in a
1
relaxed and a
2
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
3
.
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