steps in cross bridge cycle
a. in resting state myosin is thought to have partially hydrolyzed ATP.
- When ca is released from the terminal cisternae of the SR, it binds to troponin C, which promotes movement on the actin filament such that myosin binding sites on actin are exposed.
b.-This then allows the "energized" myosin head to bind to the underlying actin
c.-myosin next undergoes a conformational change termed "rachet action" that pulls the actin filament toward the center of the sarcomere
-myosin releases ADP and P during the transition to state
d.-binding of ATP to myosin decreases the affinity of myosin for actin, thereby resulting in the release of myosin from the actin filament
-myosin then partially hydrolyzes the ATP is used to recock the head and return to the resting state
-if intracellular ca is still elevated, myosin will undergo another cross-bridge cycle and produce further contraction of the muscle
-cycle continues until SERCA pumps ca back into the SR
-as ca levels fall, ca dissociates from troponin c and troponin-tropomyosin complex moves and blocks the myosin binding sites on the actin filaments