Page 75 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Contraction of Striated Muscle ATP (! p. 72) is essential for filament sliding
and, hence, for muscle contraction. Due to
Stimulation of muscle fibers. The release of their ATPase activity, the myosin heads
acetylcholine at the motor end-plate of skeletal (! p. 60) act as the motors (motor proteins) of
muscle leads to an end-plate current that this process. The myosin-II and actin filaments
spreads electrotonically and activates voltage- of a sarcomere (! p. 60) are arranged in such a
gated Na + channels in the sarcolemma way that they can slide past each other. The
(! p. 56). This leads to the firing of action myosin heads connect with the actin filaments
potentials (AP) that travel at a rate of 2 m/s at a particular angle, forming so-called cross-
Nerve and Muscle, Physical Work (! B). In the skeletal muscle, this process which is increased by concerted movement of
bridges (! C1). Due to a conformational change
along the sarcolemma of the entire muscle
in the region of the nucleotide binding site of
fiber, and penetrate rapidly into the depths of
myosin-II (! p. 61 C), the spatial extent of
the fiber along the T system (! A).
The conversion of this excitation into a con-
traction is called electromechanical coupling
the neck region, the myosin head tilts down,
drawing the thin filament a length of roughly
begins with the action potential exciting volt-
4 nm (! C2). The second myosin head may also
receptors
dihydropyridine
move an adjacent actin filament. The head
age-sensitive
(DHPR) of the sarcolemma in the region of the
then detaches and “tenses” in preparation for
the next “oarstroke” when it binds to actin
triads. The DHPR are arranged in rows, and
Kinesin, another motor protein (! pp. 42 u.
brane of the sarcoplasmic reticulum (SR) are
channels called ryanodine recep-
2+
58), independently advances on the micro-
rows of Ca
2 directly opposite them in the adjacent mem- anew (! C3).
tubule by incremental movement of its two
tors (type 1 in skeletal muscle: RYR1). Every
other RYR1 is associated with a DHPR (! B2). heads (8 nm increments), as in tug-of-war. In
RYR1 open when they directly “sense” by me- this case, fifty percent of the cycle time is
chanical means an AP-related conformational “work time” (duty ratio = 0.5). Between two
change in the DHPR. In the myocardium, on the consecutive interactions with actin in skeletal
other hand, each DHPR is part of a voltage- muscle, on the other hand, myosin-II “jumps”
gated Ca 2+ channel of the sarcolemma that 36 nm (or multiples of 36, e.g. 396 nm or more
opens in response to an action potential. Small in rapid contractions) to reach the next (or the
quantities of extracellular Ca 2+ enter the cell 11th) suitably located actin binding site (! C3,
through this channel, leading to the opening of jump from a to b). Meanwhile, the other my-
myocardial RYR2 (so-called trigger effect of osin heads working on this particular actin
Ca 2+ or Ca 2+ spark; ! B3). Ca 2+ ions stored in filament must make at least another 10 to 100
the SR now flow through the opened RYR1 or oarstrokes of around 4 nm each. The duty ratio
RYR2 into the cytosol, increasing the cytosolic of a myosin-II head is therefore 0.1 to 0.01. This
2+
Ca 2+ concentration [Ca ] i from a resting value division of labor by the myosin heads ensures
of ca. 0.01µmol/L to over 1 µmol/L (! B1). In that a certain percentage of the heads will al-
skeletal muscle, DHPR stimulation at a single ways be ready to generate rapid contractions.
site is enough to trigger the coordinated open- When filament sliding occurs, the Z plates
ing of an entire group of RYR1, thereby increas- approach each other and the overlap region of
ing the reliability of impulse transmission. The thick and thin filaments becomes larger, but
increased cytosolic Ca 2+ concentration satu- the length of the filaments remains un-
rates the Ca 2+ binding sites on troponin-C, changed. This results in shortening of the
thereby canceling the troponin-mediated in- I band and H zone (! p. 60). When the ends of
hibitory effect of tropomyosin on filament the thick filaments ultimately bump against
sliding (! D). It is still unclear whether this the Z plate, maximum muscle shortening oc-
type of disinhibition involves actin–myosin curs, and the ends of the thin filaments overlap
binding or the detachment of ADP and P i, as (! p. 67 C). Shortening of the sarcomere there-
described below. fore occurs at both ends of the myosin bundle,
62 but in opposite directions.
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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