Question 1

What is the sliding filament theory?

Accepted Answer

The sliding filament theory states that muscle contraction occurs because actin (thin) filaments slide past myosin (thick) filaments, increasing their overlap. Neither filament changes length — the sarcomere shortens purely by sliding, which is why the A band stays constant while the I band and H zone narrow.

Question 2

Which band does not change during muscle contraction?

Accepted Answer

The A band, whose width equals the length of the thick (myosin) filament. Since myosin filaments do not shorten, the A band remains constant. In contrast, the I band (thin-filament-only region) and the H zone (thick-filament-only region) both shrink during contraction.

Question 3

What is the role of calcium in muscle contraction?

Accepted Answer

Calcium ions released from the sarcoplasmic reticulum bind to troponin, the regulatory protein on the thin filament. This makes tropomyosin shift position, exposing the myosin-binding sites on actin so cross-bridges can form. When Ca²⁺ is pumped back into the SR, the sites are re-masked and the muscle relaxes.

Question 4

Why is ATP needed to detach the myosin head?

Accepted Answer

After the power stroke, the myosin head remains locked onto actin. Only the binding of a fresh ATP molecule to the myosin head breaks the cross-bridge (detachment). ATP hydrolysis then re-cocks (re-energises) the head for the next cycle. This is why ATP is required for detachment, not attachment.

Question 5

What causes rigor mortis?

Accepted Answer

After death, ATP production stops. Without ATP, myosin heads cannot detach from actin, so cross-bridges remain locked and the muscles become permanently stiff. Rigor mortis is living proof that ATP's role in the cross-bridge cycle is detachment, not attachment.

Sliding Filament Theory — Nothing Shortens, Everything Slides

Practice

Learned it? Now prove it under the clock.