(A) As an action potential travels down a transverse tubule,
Ca2+ ions are released from the terminal cisternae of the sarcoplasmic reticulum.
The Ca2+ ions then diffuse into nearby sarcomeres and bind to the TnC subunits
of troponin molecules. (B) The Ca2+ causes troponins to change shape (conformational),
which pulls attached tropomyosins away from myosin binding sites on neighboring
G-actins. (C) As soon as the sites are exposed, the heads of the nearby myosin
molecules bond to the G-actins. The association of myosin and actin causes ADP+P
to leave the myosin heads. (D) In response, the myosin heads move into a low
energy position, dragging the attached G-actins towards the center of the sarcomere.
(E) When new ATPs attach, the myosin heads pull away from the G-actins. (F) As
soon as they attach, the ATPs hydrolyze to ADP + P, and the myosin heads move
into a high energy position. If enough Ca2+ is still present, myosin heads bind
to other actin molecules & the contraction cycle repeats.
