The sliding filament model and cross-bridge cycle

Calcium binding changes troponin's shape, which lets tropomyosin roll off the myosin-binding sites on actin so myosin heads can bind.

• Correct: Tropomyosin moves off the myosin-binding sites on actin:
• Myosin releases its ATP:
• The sarcoplasmic reticulum closes its release channels:

The A band spans the full length of the thick filaments. The I band has thin filaments only, and the H zone is the middle of the A band where only thick filaments are.

• The I band:
• The H zone:
• Correct: The A band:

Hydrolysis breaks off the last phosphate group, giving ADP plus inorganic phosphate and releasing energy the cell can use.

• It gains a phosphate and stores energy:
• Correct: It splits into ADP and a phosphate and releases energy:
• It becomes glucose:

The A band is the length of the thick filaments, and the thick filaments do not change length. The thin filaments slide in over them, so the A band keeps its width while everything defined by a gap narrows.

• The I band: The I bands are clearly narrower in the contracted sarcomere, because thick filaments now reach farther toward the Z discs.
• The H zone: The H zone has almost vanished, because the thin filaments from both ends now nearly meet.
• Correct: The A band: Correct. The A band, the thick filaments' length, is unchanged.
• The distance between Z discs: The Z discs are closer together; that is the shortening of the sarcomere.

Without ATP, the cross-bridge cycle stops after the power stroke. Heads that bind actin cannot let go, the calcium pumps fail so the binding sites stay uncovered, and the locked filaments make the muscles stiff: rigor mortis.

• Myosin heads detaching from actin: down. A head lets go of actin only when a new ATP binds it; with no ATP, attached heads stay attached.
• Number of cross-bridges attached: up. Calcium leaking into the sarcoplasm uncovers actin, heads bind and complete their stroke, and none can detach.
• Stiffness of the muscle: up. Locked cross-bridges bolt the thick and thin filaments together, so the muscle cannot be stretched.
• Calcium pumped into the SR: down. The SR calcium pumps are ATPases; without ATP they stop.

The I band is the part of the sarcomere not covered by thick filaments: 2.2 − 1.6 = 0.6 µm, split as 0.3 µm on each side of the sarcomere.

• 1.6 µm: 1.6 µm is the width of the A band, which equals the thick filament length.
• Correct: 0.6 µm: Correct. Sarcomere length minus A band: 2.2 − 1.6 = 0.6 µm.
• 3.8 µm: This adds the two lengths instead of subtracting; the I band must be shorter than the sarcomere.
• 0.2 µm: This comes from subtracting two thin filament lengths (2 × 1.0 µm), which gives the H zone if the thin filaments are 1.0 µm long, not the I band.

After death, ATP runs out. Calcium leaks into the sarcoplasm and uncovers actin, heads bind and complete their stroke, and without ATP to bind them they cannot let go. The locked cross-bridges hold the filaments together: rigor mortis.

• Continuous action potentials in his muscle fibers: Nerves and muscle membranes stop firing after death; the stiffness is in the filaments, not the membranes.
• Correct: Myosin heads locked onto actin: Correct. Rigor is locked cross-bridges.
• Acetylcholine left bound to the end plates: Acetylcholine acts only on the end plate's channels; it does not hold filaments together.
• Tropomyosin locked over the binding sites on actin: Covered binding sites would leave the muscle relaxed and easy to stretch, the opposite of rigor.

Thin filaments slide toward the M line, so overlap increases, the Z discs are pulled closer, and the I bands and H zone, which are gaps between filaments, narrow. The A band and the filaments themselves keep their lengths.

• Correct: The Z discs move closer together: Correct. The thin filaments are anchored to the Z discs and pull them inward.
• Correct: The I bands narrow: Correct. The thick filaments now reach closer to the Z discs.
• The A band narrows: The A band is the length of the thick filaments, which does not change.
• Correct: The H zone narrows: Correct. The thin filaments from both ends move toward each other.
• The thin filaments get shorter: Filaments slide; neither kind shortens.
• Correct: The overlap of thick and thin filaments increases: Correct. Sliding increases the overlap.

The A band spans the thick filaments from end to end. If the thick filaments shortened, the A band would narrow. Because it stays the same while the I band shrinks, the thin filaments must be sliding in over thick filaments of constant length.

• The A band is made of thin filaments, which would have to lengthen: The A band is set by the thick filaments; the thin filaments reach into it from the ends.
• The I band contains the thick filaments, and its narrowing shows they slide: The I band contains thin filaments only.
• Correct: The A band spans the thick filaments, and it did not narrow: Correct. A constant A band means constant thick filaments.
• Filaments made of protein cannot change length: Some proteins do change shape; the evidence here comes from the bands, not from a rule about proteins.

Splitting ATP recocks the head into a bent, high-energy position, like setting a mousetrap. The power stroke releases that stored energy as movement, after the head binds actin and lets go of its phosphate.

• In the bond between the myosin head and actin: Binding actin triggers the stroke, but the energy was stored in the head's shape before it bound.
• In the calcium bound to troponin: Calcium acts as a switch that uncovers the binding sites; it does not supply energy for the stroke.
• In a new ATP molecule waiting to bind the head: A new ATP binds after the stroke and releases the head; its energy is used for the next recocking.
• Correct: In the cocked shape of the myosin head: Correct. The energy is stored in the cocked head.

Rigor passes off over one to three days as enzymes released inside the dead fibers break down the muscle proteins. The locked cross-bridges are not released; the structure they hold together falls apart.

• Calcium is pumped back into the SR: The pumps need ATP, and there is none; calcium does not return to the SR after death.
• The myosin heads slowly recock and then let go of actin one by one: Letting go requires ATP to bind, and recocking requires ATP to be split; neither is available.
• Correct: Enzymes in the dead cells break down the muscle proteins: Correct. Protein breakdown, not cross-bridge release, ends rigor.
• New ATP is made by anaerobic respiration: Anaerobic breakdown of glycogen stops within hours of death, before rigor is even complete.