Page 81 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
I band region are flexible and function as “elas- force of cardiac muscle at rest is greater than
tic bands” that counteract passive stretching of that of skeletal muscle (! E1, 2).
a muscle and influence its shortening velocity. Skeletal muscle normally functions in the
plateau region of its length–force curve,
The extensibility of titin molecules, which can
stretch to up to around ten times their normal length whereas cardiac muscle tends to operate in the
in skeletal muscle and somewhat less in cardiac ascending limb (below L max) of its length–force
muscle, is mainly due to frequent repetition of the curve without a plateau (! C, E1, 2). Hence,
PEVK motif (proline-glutamate-valine-lysine). In very the ventricle responds to increased diastolic
strong muscle extension, which represents the filling loads by increasing its force develop-
Nerve and Muscle, Physical Work stretches, the more sudden and crude this type of steeper curve (! E2). 2+ +
steepest part of the resting extensibility curve (! D),
ment (Frank–Starling mechanism; ! p. 204).
globular chain elements called immunoglobulin C2
In cardiac muscle, extension also affects
domains also unfold. The quicker the muscle
2+
troponin’s sensitivity to Ca , resulting in a
“shock absorber” action will be.
Action potentials in cardiac muscle are of
The length (L) and force (F) or “tension” of a
much longer duration than those in skeletal
muscle are closely related (! C, E). The total
muscle (! p. 59 A) because g K temporarily
force of a muscle is the sum of its active force
decreases and g Ca increases for 200 to 500 ms
and its extension force at rest, as was ex-
after rapid inactivation of Na channels. This
plained above. Since the active force is deter-
allows the slow influx of Ca , causing the ac-
myosin interactions, it varies in accordance
refractory period does not end until a contrac-
with the initial sarcomere length (! C, D).
tion has almost subsided (! p. 59 A). There-
Skeletal muscle can develop maximum active
2 mined by the magnitude of all potential actin- tion potential to reach a plateau. As a result, the
fore, tetanus cannot be evoked in cardiac
(isometric) force (F 0) from its resting length muscle.
(L max; sarcomere length ca. 2 to 2.2 µm; ! C). Unlike skeletal muscle, cardiac muscle has
When the sarcomeres shorten (L ! L max), part no motor units. Instead, the stimulus spreads
of the thin filaments overlap, allowing only across all myocardial fibers of the atria and
forces smaller than F 0 to develop (! C). When subsequently of the ventricles generating an
L is 70% of L max (sarcomere length: 1.65µm), all-or-none contraction of both atria and,
the thick filaments make contact with the Z thereafter, both ventricles.
disks, and F becomes even smaller. In addition, In cardiac muscle but not in skeletal muscle,
a greatly pre-extended muscle (L " L max) can the duration of an action potential can change
develop only restricted force, because the the force of contraction, which is controlled by
number of potentially available actin–myosin the variable influx of Ca 2+ into the cell.
bridges is reduced (! C). When extended to The greater the force (load), the lower the
130% or more of the L max, the extension force at velocity of an (isotonic) contraction (see velo-
rest becomes a major part of the total muscle city–force diagram, F1). Maximal force and a
force (! E). small amount of heat will develop if shorten-
The length–force curve corresponds to the ing does not occur. The maximal velocity (bi-
cardiac pressure–volume diagram in which ceps: ca. 7 m/s) and a lot of heat will develop in
ventricular filling volume corresponds to muscle without a stress load. Light loads can
muscle length, and ventricular pressure corre- therefore be picked up more quickly than
sponds to muscle force; ! p. 202. Changes in heavy loads (! F2). The total amount of energy
the cytosolic Ca 2+ concentration can modify consumed for work and heat is greater in
the pressure–volume relationship by causing a isotonic contractions than in isometric ones.
change in contractility (! p. 203 B2). Muscle power is the product of force and the
Other important functional differences be- shortening velocity: N · m · s – 1 = W (! F1,
tween cardiac muscle and skeletal muscle are colored areas).
listed below (see also p. 59 A):
Since skeletal muscle is more extensible
68 than the cardiac muscle, the passive extension
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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