Page 79 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Mechanical Features of Skeletal with the magnitude of depolarization. The
Muscle magnitude of contraction of tonus fibers is
regulated by variation of the cytosolic Ca con-
2+
Action potentials generated in muscle fibers centration (not by action potentials!)
increase the cytosolic Ca 2+ concentration In contrast, the general muscle tone (reflex
2+
[Ca ] i, thereby triggering a contraction tone), or the tension of skeletal muscle at rest,
(skeletal muscle; ! p. 36 B; myocardium; is attributable to the arrival of normal action
! p. 194). In skeletal muscles, gradation of potentials at the individual motor units. The
contraction force is achieved by variable re- individual contractions cannot be detected be-
Nerve and Muscle, Physical Work release and, thus, to a maximum single twitch in this involuntary state of tension. Resting
cause the motor units are alternately (asyn-
cruitment of motor units (! p. 58) and by
chronously) stimulated. When apparently at
changing the action potential frequency. A
2+
rest, muscles such as the postural muscles are
single stimulus always leads to maximum Ca
reflexes
tone
is
muscle
regulated
by
of skeletal muscle fiber if above threshold (all-
or-none response). Nonetheless, a single
(! p. 318ff.) and increases as the state of atten-
stimulus does not induce maximum shorten-
tiveness increases.
ing of muscle fiber because it is too brief to
Types of contractions (! B). There are
keep the sliding filaments in motion long
different types of muscle contractions. In
isometric contractions, muscle force (“ten-
enough for the end position to be reached.
stimulus arrives before the muscle has
mains constant. (In cardiac muscle, this also
completely relaxed after the first stimulus.
represents isovolumetric contraction, because
2 Muscle shortening continues only if a second sion”) varies while the length of the muscle re-
This type of stimulus repetition leads to in-
the muscle length determines the atrial or
cremental mechanical summation or super- ventricular volume.) In isotonic contractions,
position of the individual contractions (! A). the length of the muscle changes while muscle
Should the frequency of stimulation become force remains constant. (In cardiac muscle, this
so high that the muscle can no longer relax at also represents isobaric contraction, because
all between stimuli, sustained maximum con- the muscle force determines the atrial or
traction of the motor units or tetanus will ventricular pressure.) In auxotonic contrac-
occur (! A). This occurs, for example, at 20 Hz tions, muscle length and force both vary simul-
in slow-twitch muscles and at 60–100 Hz in taneously. An isotonic or auxotonic contrac-
fast-twitch muscles (! p. 58). The muscle tion that builds on an isometric one is called an
force during tetanus can be as much as four afterloaded contraction.
times larger than that of single twitches. The Muscle extensibility. A resting muscle con-
Ca 2+ concentration, which decreases to some taining ATP can be stretched like a rubber
extent between superpositioned stimuli, re- band. The force required to start the stretching
mains high in tetanus. action (! D, E; extension force at rest) is very
Rigor (! p. 2.13) as well as contracture, small, but increases exponentially when the
another state characterized by persistent muscle is under high elastic strain (see resting
muscle shortening, must be distinguished tension curve, ! D). A muscle’s resistance to
from tetanus. Contracture is not caused by ac- stretch, which keeps the sliding filaments in
tion potentials, but by persistent local depolari- the sarcomeres from separating, is influenced
zation due, for example, to increased extra- to a small extent by the fascia (fibrous tissue).
+
cellular K concentrations (K contracture) or The main factor, however, is the giant filamen-
+
2+
drug-induced intracellular Ca release, e.g., in tous elastic molecule called titin (or connectin;
response to caffeine. The contraction of so- 1000 nm long, M r = 3 to 3.7 MDa) which is in-
called tonus fibers (specific fibers in the exter- corporated in the sarcomere (6 titin molecules
nal eye muscles and in muscle spindles; per myosin filament). In the A band region of
! p. 318) is also a form of contracture. Tonus the sarcomere (! p. 61 B), titin lies adjacent to
fibers do not respond to stimuli according to a myosin filament and helps to keep it in the
66 the all-or-none law, but contract in proportion center of the sarcomere. Titin molecules in the
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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