Which description correctly identifies actin and myosin filaments?

• Actin forms thick filaments with pulling heads; myosin forms thin filaments.
• Both proteins form thin filaments with pulling heads; neither forms thick filaments.
• Actin forms thin filaments; myosin forms thick filaments with pulling heads.

As a sarcomere shortens, how do its bands change?

• The I bands and H zone narrow; the A band keeps its width.
• The I bands keep their width; the A band and H zone both become narrower.
• The I bands and H zone become wider; the A band becomes progressively narrower.

How does calcium expose myosin-binding sites on actin?

• It binds actin, dissolving thin filaments and exposing the thick myosin filaments.
• It binds troponin, changing its shape and moving tropomyosin aside.
• It binds myosin, permanently joining thick filaments to the surrounding thin filaments.

Which statement correctly describes ATP's roles in contraction and relaxation?

• ATP binding resets myosin; ATP splitting detaches it; calcium pumps work without ATP.
• ATP binding detaches myosin; ATP splitting shortens actin; calcium pumps work without ATP.
• ATP binding detaches myosin; ATP splitting resets it; calcium pumps also use ATP.

Start with a reset myosin head attaching to actin. Which sequence follows?

• ATP binds and detaches the head; power stroke; ATP splits and resets the head.
• Power stroke; ATP binds and detaches the head; ATP splits and resets the head.
• ATP splits and resets the head; ATP binds and detaches the head; power stroke.

Why does the A band's width stay constant during sarcomere shortening?

• It spans the thick myosin filaments, whose length does not change.
• It spans the gap between the Z lines, which keep the same distance apart.
• It spans the thin actin filaments, which become progressively shorter as they slide.

In a muscle preparation, calcium is lowered to stop new cross-bridges from forming. No ATP is available. Myosin heads already attached to actin will therefore detach normally.

A student says calcium travels from the motor neuron through T tubules to troponin. Which correction fits skeletal muscle signalling?

• T tubules carry acetylcholine into the fibre, where it binds troponin and exposes sites on actin.
• T tubules carry a muscle action potential that triggers calcium release from the fibre's internal store.
• T tubules carry calcium from the neuron, without a muscle action potential or release from an internal calcium store.