Page 77 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
Contraction cycle (! C and D). Each of the Parvalbumin, a protein that occurs in the cy-
two myosin heads (M) of a myosin-II molecule tosol of fast-twitch muscle fibers (! type F;
bind one ATP molecule in their nucleotide p. 58), accelerates muscle relaxation after
binding pocket. The resulting M-ATP complex short contractions by binding cytosolic Ca 2+ in
2+
lies at an approx. 90! angle to the rest of the exchange for Mg . Parvalbumin’s binding af-
myosin filament (! D4). In this state, myosin finity for Ca 2+ is higher than that of troponin,
has only a weak affinity for actin binding. Due but lower than that of SR’s Ca -ATPase. It
2+
to the influence of the increased cytosolic Ca 2+ therefore functions as a “slow” Ca 2+ buffer.
The course of the filament sliding cycle as
concentration on the troponin – tropomyosin described above mainly applies to isotonic
Nerve and Muscle, Physical Work plex (! D1). Detachment of P i (inorganic isometric contractions where muscular ten-
complex, actin (A) activates myosin’s ATPase,
contractions, that is, to contractions where
resulting in hydrolysis of ATP (ATP ! ADP + P i)
muscle shortening occurs. During strictly
and the formation of an A-M-ADP-P i com-
sion increases but the muscle length remains
phosphate) from the complex results in a con-
formational change of myosin that increases
unchanged, the sliding process tenses elastic
the actin–myosin association constant by four
components of a muscle, e.g. titin (! p. 66),
powers of ten (binding affinity now strong).
and then soon comes to a halt. Afterwards, the
A-M-ATP complex (! D3) probably transforms
The myosin heads consequently tilt to a 40!
angle (! D2a), causing the actin and myosin
directly into A-M-ADP-P i (! D1).
The muscle fibers of a dead body do not pro-
of ADP from myosin ultimately brings the my-
osin heads to their final position, a 45! angle
is no longer pumped back into the SR, and the
2 filaments to slide past each other. The release duce any ATP. This means that, after death, Ca 2+
(! D2b). The remaining A-M complex (rigor
ATP reserves needed to break down stable A-M
complex) is stable and can again be trans- complexes are soon depleted. This results in
formed into a weak bond when the myosin stiffening of the dead body or rigor mortis,
heads bind ATP anew (“softening effect” of ATP). which passes only after the actin and myosin
The high flexibility of the muscle at rest is im- molecules in the muscle fibers decompose.
portant for processes such as cardiac filling or
the relaxing of the extensor muscles during
rapid bending movement. If a new ATP is
bound to myosin, the subsequent weakening
of the actin–myosin bond allows the realign-
ment of the myosin head from 45! to 90!
(! D3, 4), the position preferred by the M-ATP
2+
complex. If the cytosolic Ca concentration re-
mains " 10 – 6 mol/L, the D1 to D4 cycle will
begin anew. This depends mainly on whether
subsequent action potentials arrive. Only a
portion of the myosin heads that pull actin fila-
ments are “on duty” (low duty ratio; see p. 62)
to ensure the smoothness of contractions.
The Ca 2+ ions released from the sarco-
plasmic reticulum (SR) are continuously
pumped back to the SR due to active transport
2+
by Ca -ATPase (! pp. 17 A and 26), also called
SERCA (! p. 16). Thus, if the RYR-mediated re-
2+
lease of Ca from the SR is interrupted, the cy-
tosolic Ca concentration rapidly drops below
2+
10 – 6 mol/L and filament sliding ceases (resting
position; ! D, upper left corner).
64
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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