Figure 16.25: When each end of the myosin thick filament moves along the actin filament,
the two actin filaments at opposite sides of the sacromere are drawn closer together and the
sarcomere shortens. ( 10 )
calcium ions (Ca++). The sarcoplasmic reticulum is a special type of smooth endo-
plasmic reticulum found in smooth and skeletal muscle that contains large amounts of
Ca++.- The calcium ions bind to actin filaments of the myofibrils and activate the actin for
attachment by the myosin heads filaments. - Activated myosin binds strongly to the actin filament. Upon strong binding, myosin
rotates at the myosin-actin junction, which bends a region in the “neck” of the myosin
“head,” as shown in theFigurebelow. - Shortening of the muscle fiber occurs when thebending neckof the myosin region pulls
the actin and myosin filaments across each other. Meanwhile, the myosin heads remain
attached to the actin filament, as shown inFigure16.26. - The binding of ATP allows the myosin heads to detach from actin. While detached,
ATP breaks down to adenosine diphosphate and an inorganic phosphate (ADP + Pi).
The breaking of the chemical bond in ATP gives energy to the myosin head, allowing
it to bind to actin again. - Steps 4 and 5 repeat as long as ATP is available and Ca++is present on the actin
filament. The collective bending of numerous myosin heads (all in the same direction)
moves the actin filament relative to the myosin filament which causes a shortening of
the sacromere. Overall, this process results in muscle contraction. The sarcoplasmic
reticulum actively pumps Ca++back into itself. Muscle contraction stops when Ca++
is removed from the immediate environment of the myofilaments.