Page 344 - Fundamentals of anatomy physiology
P. 344
Watch the > ™ on the Cross-Bridge Cycle.
4 Myosin Head Pivoting 5 Cross-Bridge Detachment 6 Myosin Reactivation
After cross-bridge formation, When another ATP binds to the Myosin reactivation occurs
the energy that was stored in the myosin head, the link between the when the free myosin head
resting state is released as the myosin head and the active site on splits ATP into ADP and P.
myosin head pivots toward the M the actin molecule is broken. The The energy released is used
line. This action is called the power active site is now exposed and able to recock the myosin head.
stroke; when it occurs, to form another cross-bridge.
the bound ADP and phosphate
group are released.
ADP + P Ca2+ ATP Ca2+ ADP Ca2+
Ca2+ ADP + P Ca2+ ATP +P ADP
Ca2+ P+
CONTRACTED SARCOMERE
The entire cycle is repeated several
times each second, as long as Ca2+
concentrations remain elevated and
ATP reserves are sufficient. Calcium ion
levels will remain elevated only as long
as action potentials continue to pass
along the T tubules and stimulate the
terminal cisternae. Once that stimulus
is removed, the calcium channels in the
SR close and calcium ion pumps pull
Ca2+ from the cytosol and store it
within the terminal cisternae. Troponin
molecules then shift
position, swinging the
tropomyosin strands
over the active sites
and preventing
further cross-bridge
formation.
331
