Questions: Skeletal Muscle Anatomy and Contraction

Question 1

During a muscle contraction that shortens the sarcomere, what happens to the lengths of the individual actin and myosin filaments?

• A. Both filaments shorten as the protein molecules compress under the contractile force
• B. Myosin shortens by coiling, while actin remains the same length
• C. Actin filaments shorten as they are reeled in by myosin cross-bridges
• D. Neither filament changes length — the filaments slide past each other, increasing their overlap

Answer: (D)

This is the central claim of the sliding filament model: the filaments themselves do not shorten. Myosin cross-bridges attach to actin and pull the actin filaments toward the center of the sarcomere (M-line), increasing filament overlap while the Z-lines move closer together. The sarcomere shortens; the filament lengths are unchanged. This can be verified microscopically: the A-band (where myosin resides) stays the same width during contraction, while the I-band and H-zone narrow as overlap increases.

Question 2

A person dies and their muscles enter rigor mortis — a rigid, locked state. What explains this at the molecular level of the cross-bridge cycle?

• A. Calcium floods out of the sarcoplasmic reticulum and cannot be pumped back, locking troponin in the activated state indefinitely
• B. Actin filaments polymerize further after death, rigidly linking adjacent sarcomeres
• C. Without ATP, myosin heads cannot detach from actin after completing the power stroke, freezing cross-bridges in the attached state
• D. ATP floods the cell after death, causing all available myosin heads to simultaneously undergo the power stroke

Answer: (C)

The cross-bridge cycle requires ATP for two distinct purposes: (1) to cock the myosin head into its high-energy configuration before binding actin, and (2) to allow the myosin head to DETACH from actin after the power stroke. Without ATP, detachment cannot occur — myosin heads remain rigidly attached to actin filaments. This produces the characteristic stiffness of rigor mortis. Muscle relaxation thus requires ATP not just for contraction but as a 'release factor' for every cross-bridge.

Question 3

Calcium ions initiate muscle contraction by binding directly to myosin heads, enabling them to reach and attach to actin.

• T. True
• F. False

Answer: False

Calcium acts on the THIN filament, not on myosin. At rest, tropomyosin physically blocks myosin-binding sites on actin. When calcium is released from the sarcoplasmic reticulum, it binds to troponin (a protein associated with tropomyosin on the actin filament), causing a conformational change that shifts tropomyosin away from the binding sites. This exposes the sites, allowing myosin heads — which were already in their high-energy cocked state — to bind. The regulation system gates access to actin, not activity of myosin.

Question 4

ATP is required for both the active (power stroke) phase and the relaxation phase of the cross-bridge cycle.

• T. True
• F. False

Answer: True

ATP plays two distinct roles in the cross-bridge cycle. First, it is hydrolyzed to ADP + Pi to cock the myosin head into its high-energy configuration — this provides the energy for the subsequent power stroke. Second, a NEW ATP molecule must bind to myosin AFTER the power stroke to allow the head to detach from actin. Without this second ATP, detachment cannot occur (as in rigor mortis). Relaxation also requires ATP to actively pump calcium back into the sarcoplasmic reticulum via the SERCA pump.

Question 5

Explain why the sliding filament model means that sarcomere shortening does not require protein filaments to shorten. What does change during contraction, and how does filament sliding produce force?

Model answer: In the sliding filament model, actin and myosin filaments maintain constant length throughout contraction. What changes is the degree of overlap between them. Myosin heads (cross-bridges) bind to actin, then perform a power stroke — rotating approximately 70° to pull actin toward the center of the sarcomere. This increases overlap and draws the Z-lines (which anchor actin) closer together, shortening the sarcomere. Hundreds of cross-bridges cycling repeatedly per second across thousands of sarcomeres in series generate macroscopic shortening and force.

The sliding filament model resolved a key controversy in mid-20th century muscle research. It predicts specific microscopic observations: the A-band (myosin) width is constant during contraction; the I-band (actin without overlap) and H-zone (myosin without overlap) both narrow as overlap increases. These predictions were confirmed by electron microscopy, providing strong evidence for the model. The key insight is that mechanical work comes from the cross-bridge power stroke — a conformational change in the myosin molecule — not from filament shortening.