1. The Steps of Muscle Contraction: The Sliding Filament Theory

2. 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.

3. 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.

4. Step 2: Myosin Attaches to Actin
Myosin attaches to the myosin binding site on actin and releases the free phosphate group
This forms the cross- bridge between the actin and myosin threads

5. 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

6. 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”

7. 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