The Sliding Filament Theory of Muscle Contraction

Question 1

2.

Excitation-Coupling Phase

Answer 1

• A signal from the nervous system causes calcium to be released into the muscle.
• Calcium binds to a protein called troponin, moving another protein (tropomyosin) out of the way, so myosin can attach to actin.

Question 1

1.

Resting Phase

Answer 1

• Muscles are relaxed, and actin and myosin aren’t touching.
• Calcium is stored away in the muscle, and the myosin heads are ready but not connected to actin.

so NOTHING is happening

Question 2

3.

Contraction Phase (Power Stroke)

Answer 2

• Myosin heads attach to actin and pull, sliding the filaments past each other to shorten the muscle.
• This is the actual “contraction” where the muscle gets shorter.

Question 3

4.

Recharging Phase

Answer 3

• After pulling, myosin lets go of actin, gets energy from ATP (a molecule that stores energy), and prepares to grab onto actin again if needed.

Question 4

5.

Relaxation Phase

Answer 4

• When the signal stops, calcium is pumped back into storage.
• Without calcium, myosin can’t attach to actin anymore, and the muscle relaxes.