Sliding filament theory

Question 1

Step 1 (contraction cycle begins):

Answer 1

The contraction cycle involves a series of interrelated steps. The cycle begins with electrical events in the sarcolemma that trigger the release of calcium from the terminal cisternae of the sarcoplasmic reticulum. These calcium ions enter the zone of overlap.

Question 2

Step 2 (active site exposure):

Answer 2

calcium ions bind to troponin in the troponin-tropomyosin complex. The tropomyosin molecule then rolls away from the active sites on the actin molecules of the thin filaments.

Question 3

Step 3 (cross bridge formation):

Answer 3

Once the active sites are exposed, the myosin heads of adjacent thick filaments bind to them, forming cross-bridges.

Question 4

Step 4 (myosin head pivoting):

Answer 4

After cross bridge formation, energy is released as myosin heads pivot toward the M line.

Question 5

step 5 (cross bridge detachment):

Answer 5

ATP then binds to the myosin heads, breaking the cross bridges between the myosin heads and the actin molecules.

Question 6

Step 6 (myosin reactivation):

Answer 6

ATP provides the energy to reactivate the myosin heads and return them to their original positions. Now the entire cycle can be repeated as long as calcium ion concentrations remain elevated and ATP reserves are sufficient.