API: Case Study-Muscle Contraction Mechanisms Quiz

The diagram shows acetylcholine being released and binding to receptors but NOT being broken down by acetylcholinesterase (AChE). What is the most likely result?

• Muscle cannot contract

• Muscle contracts briefly then stops

• Continuous muscle contraction

• No sodium enters the cell

• Calcium cannot be released

Answer explanation

Answer: C Continuous muscle contraction

Explanation: Without AChE, ACh continues stimulating receptors → persistent depolarization → sustained contraction (spasm).

The diagram shows an action potential reaching the sarcolemma but not traveling down T tubules. What step fails next?

• ACh release

• Sodium influx

• Calcium release from SR

• Myosin detachment

• ATP production

Answer explanation

Answer: C Calcium release from SR

Explanation: T-tubules carry the signal to the SR. Without this → no Ca²⁺ release → no contraction.

The diagram shows no calcium released from the sarcoplasmic reticulum. What happens next?

• ATP cannot bind myosin

• Active sites remain covered

• Myosin cannot hydrolyze ATP

• Sodium cannot enter

• ACh cannot bind

Answer explanation

Answer: Active sites remain covered

Without Ca²⁺ → troponin not activated → tropomyosin blocks actin sites → no cross-bridges.

A muscle fiber has calcium present, but ATP is unavailable. What happens to the muscle?

• It cannot contract

• It contracts normally

• It becomes stuck in contraction

• It cannot form cross bridges

• ACh cannot be released

Answer explanation

C It becomes stuck in contraction.

Explanation: ATP is required for detachment → without it → rigor-like state (locked contraction).

Prompt:
A student claims:
"Muscle contraction happens because actin and myosin filaments get shorter."

Question:
What is the correct correction?

• Actin shortens

• Myosin shortens

• Both shorten

• Sarcomere shortens as filaments slide

• Filaments dissolve

Answer explanation

Answer: Sarcomere shortens as filaments slide

Explanation: Filaments do NOT shorten—the sarcomere shortens due to sliding.

Prompt:
A diagram shows a relaxed biceps muscle but the arm has not straightened. A student says:
"When a muscle relaxes, it automatically returns to its original length."

Question:
What is the error?

• Relaxation requires ATP

• Calcium must increase

• Muscles actively lengthen

• Opposing muscle must contract

• Myosin pushes actin back

Answer explanation

Opposing muscle must contract to return to full relaxed position.

Muscles do not actively lengthen—antagonistic muscles restore length.

Analysis shows that myosin heads are attached to actin with no ATP present. What condition does this represent?

• Normal contraction

• Muscle relaxation

• Rigor mortis

• NMJ signaling failure

• Calcium depletion

Answer explanation

Answer: Rigor mortis

ATP is required for detachment → without it → cross-bridges remain locked (rigor mortis).