The Steps of Muscle
Contraction: The Sliding
Filament Theory
Key events that must take place
before the contraction of a
muscle fiber begins:
• SR releases calcium ions into the cytosol
• The calcium ions will bind to troponin which
•
•
causes the troponin-tropomyosin complexes to
move away from the myosin binding sites on
actin.
Once this happens contraction can begin.
The process is similar to rowing, each myosin
head rows forward using the actin as a base to
pull from.
Step 1: ATP hydrolysis by Myosin
• Myosin heads contain a site for ATP
binding and ATPase enzymes
• ATP is hydrolyzed into ADP freeing up a
Phosphate molecule, this energizes the
myosin head.
Step 2: Myosin Attaches to Actin
• Myosin attaches to
•
the myosin binding
site on actin and
releases the free
phosphate group
This forms the crossbridge between the
actin and myosin
threads
Step 3: The Powerstroke
• Phosphate release by myosin triggers the
actual power contraction stroke
• ADP pocket opens up on the myosin head
• Myosin head rotates and releases the ADP
• As the head rotates it moves in a ratchet
like motion and pulls the thin filaments
closer to the center of the M line of the
sarcomere
Step 4: Detachment of Myosin
• When the ratchet motion is over the
myosin awaits another ATP so it can be
released from the actin binding site
• If an ATP is available the myosin detaches
and begins another stroke of the cycle
• If not the Myosin remains locked in place
and the muscle remains locked up as well
this is called “rigor-mortis”
Summary Points
• Calcium ions are needed to start the cycle
• The movement of myosin and actin is like
rowers moving across water
• Myosin heads will keep binding with actin
sites until the ATP runs out or the Calcium
levels decrease