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7. A physiology experiment is conducted in which a skeletal
muscle twitch is initiated using an electrical stimulator.
Which of the following mechanisms best explains why a
skeletal muscle twitch thus initiated will come to an end?
(A) Active return of calcium to the sarcoplasmic reticulum
occurs
(B) Calcium release channels (ryanodine receptors)
close
© Dihydropyridine receptors are stimulated
(D) Muscle ATP stores are exhausted
(E) Sarcoplasmic reticulum is depleted of calcium
(F) Transverse tubules close
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a.
and d is the cause f rigor mortis?
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yep A seems correct if it the test is conducted in living tissue, invivo
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The correct answer is A. During the excitation-contraction
process, the calcium released (mainly from the terminal
cisternae of the sarcoplasmic reticulum) diffuses
toward troponin located on thin filaments (actin filaments).
When calcium binds to troponin, tropomyosin
moves, exposing attachment sites for the myosin heads,
thus allowing the formation of cross bridges between
actin and myosin filaments. Cross-bridge cycling (i.e.,
contraction) will continue for as long as the calcium
remains attached to the troponin. Thus, contraction is
terminated when the calcium is returned to the sarcoplasmic
reticulum. This is accomplished by an active
pumping of calcium by a calcium-dependent ATPase in
the membranes of the sarcoplasmic reticulum.
The muscle action potential activates dihydropyridine
receptors (choice C), which opens ryanodine calciumrelease
channels (choice B) in the terminal cisternae of
the sarcoplasmic reticulum. Calcium then exits the sarcoplasmic
reticulum and enters the sarcoplasm.
Closure of the channels stops further entry of calcium
into the sarcoplasm, but cannot decrease calcium levels
in the sarcoplasm and thus cannot terminate the muscle
contraction.
When muscle ATP levels are exhausted (choice D) after
death, the muscles remain in a state of rigor mortis for
several hours.
