Myocyte Contraction and Relaxation. 

At rest, crossbridge

cycling and generation of force do not occur

because either the myosin heads are blocked from physically

reacting with the thin filament or they are only

weakly bound to actin (Fig. 20-14).16 Cross-bridge cycling

is initiated on binding of Ca2+ to TnC, which increases

TnC-TnI interaction and decreases the inhibitory TnIactin

interaction. 

These events, which ensue from the

binding of Ca2+ to TnC, lead to conformational changes in

tropomyosin and permit attachment of the myosin head

to actin. Cross-bridging involves the detachment of the

myosin head from actin and a reattachment of myosin

to another actin on hydrolysis of ATP by myosin ATPase.

Binding of ATP to the nucleotide pocket of the myosin

head leads to the activation of myosin ATPase,31-33 ATP

hydrolysis, and changes in the configuration of the myosin

head, all of which facilitate binding of the myosin

head to actin and the generation of the power stroke of

the myosin head. Based on this model, the rate of crossbridge

cycling is dependent on the activity of myosin

ATPase.36 Turnoff of cross-bridge cycling is largely initiated

by the decrease in cytosolic Ca2+.