Contraction-Relaxation Cycle: Sliding Filament Theory

Question 1

What happens in Step 1?

Answer 1

1. Myosin binds to actin b/c high binding affinity

2. Myosin head is in a “cocked” position ready to do work (stored potential energy)

Question 2

What happens in Step 2?

Answer 2

1. Pi releases from myosin (now it’s lost in ICF)
2. Myosin undergoes “powerstroke” (releasing potential E into mechanical E)
   - >myosin pulls itself along the actin (potential E is converted to kinetic E and myosin performs work)
   - >change in length of muscle
   - >production of force
3. ADP releases from myosin @ end of powerstroke

Question 3

What happens in Step 3?

Answer 3

1. ATP binds to myosin and decreases myosin binding affinity to actin
2. Myosin releases from actin

Question 4

What happens in Step 4?

Answer 4

1. Myosin ATPase hydrolyzes ATP and releases ENERGY
2. Chemical E is converted to mechanical E
   - >myosin head “cocks” head back to its high stored potential E state
3. Myosin binding affinity to actin goes back up
   - >myosin is now ready to bind to actin again and perform work

Question 5

What must occur for Relaxation to occur?

Answer 5

1. Tropomyosin needs to re-block the myosin binding sites on actin
   - >Ca2+ needs to be transported back to the S.R
   - –>Ca2+ ATPase (active transport)
   - –>Ca2+ unbinds from troponin
   - –>troponin moves tropomyosin back over actin’s binding sites
2. ATP needs to bind to myosin head to release myosin from actin