Questions: Skeletal Muscle Contraction

Question 1

At which step in the cross-bridge cycle does ATP most directly cause myosin to detach from actin?

• A. During the power stroke, when ADP and Pi are released from myosin
• B. When a new ATP molecule binds to myosin immediately after the rigor state
• C. When Ca²⁺ binds to troponin C and exposes the myosin-binding sites on actin
• D. When SERCA pumps Ca²⁺ back into the sarcoplasmic reticulum during relaxation

Answer: (B)

After the power stroke, myosin is in the rigor state — tightly bound to actin with no nucleotide. It is the binding of a fresh ATP molecule to the myosin head that induces a conformational change causing detachment from actin. ATP hydrolysis (to ADP + Pi) then re-cocks the myosin head, preparing it for the next cycle. This is why ATP is required for both contraction and relaxation: without it, myosin cannot release actin.

Question 2

During maximal muscle contraction, the actin and myosin filaments themselves shorten, which is why the sarcomere gets shorter.

• T. True
• F. False

Answer: False

The sliding filament model specifies that the filaments do NOT change length. The sarcomere shortens because actin filaments slide further over the myosin filaments, increasing the overlap between them. The A band (thick filament length) stays constant; the I bands and H zone narrow as actin slides inward. This was the key experimental insight from Huxley and Hanson in the 1950s.

Question 3

Explain why rigor mortis occurs after death, using the mechanism of the cross-bridge cycle.

Model answer: After death, ATP production (oxidative phosphorylation and glycolysis) ceases as oxygen and substrate supplies fail. Without ATP, myosin heads that have completed a power stroke cannot detach from actin — ATP binding is what causes detachment. All cross-bridges become permanently locked in the rigor state, causing muscles to stiffen. Rigor resolves hours later as proteolytic enzymes degrade the contractile proteins.

This is the most direct clinical application of the cross-bridge cycle mechanism. It illustrates that ATP is not 'fuel for contraction' in a simple sense — it is specifically required for the detachment step. Muscles actually lock in a shortened, contracted-like state, not in relaxation, because that is the state myosin occupies when cross-bridges are formed.