Which of the following statements is true regarding cross-bridges?
a. A cross-bridge is the basic unit of active muscle tension.
b. A cross-bridge is formed when head groups on the actin myofilament bind with tropomyosin on the myosin filament.
c. Cross-bridges do not occur in lengthening contractions.
d. A release of sodium is necessary to reveal the binding site on the actin.
ANS: A
A cross-bridge is the basic unit of muscle tension formed by movement of the thin actin filaments
by the head groups of the thicker myosin filaments. The movement of actin via the myosin head
groups occurs in both concentric and eccentric contractions. A release of calcium is necessary to
initiate this process. The calcium causes a shift by the tropomyosin on the actin filament that
uncovers the binding sites on the actin for the head groups to attach. The cycling of cross-bridges
will continue to occur as long as there is a sufficient supply of calcium, adenosine triphosphate
(ATP), and action potential to initiate the process.
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