Free Muscle Contraction MCQs with Answers

1. The functional contractile unit of a myofibril is the

• A. myofilament
• B. sarcomere
• C. sarcolemma
• D. sarcoplasmic reticulum

Explanation: A sarcomere is the region between two Z lines, containing overlapping thick and thin filaments, and the whole muscle shortens because thousands of sarcomeres in series each shorten a little. A myofilament is a single actin or myosin strand, so it is a component rather than the unit. The sarcolemma is the cell membrane and the sarcoplasmic reticulum is the calcium store.

2. During the contraction of a skeletal muscle, which of the following does NOT change in length?

• A. I band
• B. H zone
• C. Sarcomere
• D. A band

Explanation: The A band is the full length of the thick myosin filaments, and since the filaments themselves do not shorten, the A band stays the same width throughout contraction. The I band and the H zone both narrow because the thin filaments are pulled inwards over the thick ones, and the sarcomere as a whole therefore shortens. This constant A band is the strongest single piece of evidence for the sliding filament model.

3. According to the sliding filament theory, a muscle shortens because

• A. actin filaments slide inwards over the myosin filaments
• B. both actin and myosin filaments themselves shorten
• C. the sarcomere is stretched by the load
• D. the Z lines dissolve and reform further apart

Explanation: Myosin heads bind actin, swivel, and drag the thin filaments towards the centre of the sarcomere, then detach and repeat, so the filaments overlap more without any of them changing length. The idea that the filaments contract is the older, wrong picture the theory replaced, and the unchanged A band disproves it. Each cycle of attachment and release needs one molecule of ATP.

4. Calcium ions start muscle contraction by binding to

• A. the myosin heads
• B. tropomyosin directly
• C. troponin, which moves tropomyosin off the binding sites on actin
• D. ATP, activating it

Explanation: In a relaxed fibre tropomyosin lies across the myosin binding sites of actin. Calcium released from the sarcoplasmic reticulum binds troponin, troponin changes shape and pulls tropomyosin aside, and only then can cross bridges form. Calcium therefore acts as the switch rather than as the energy source, and pumping it back into the reticulum is what causes relaxation.

5. Rigor mortis, the stiffening of muscles after death, occurs because

• A. calcium ions disappear from the sarcoplasm
• B. ATP is no longer available to detach the myosin heads from actin
• C. the muscle proteins are denatured by the fall in body temperature
• D. acetylcholine continues to be released at the neuromuscular junction

Explanation: ATP is needed not to form a cross bridge but to break one, so when respiration stops and ATP runs out the myosin heads stay locked on actin and the muscle cannot relax. Calcium in fact leaks out of the sarcoplasmic reticulum after death and floods the sarcoplasm, which is the opposite of the first option and is what allows the bridges to form in the first place.

6. The neurotransmitter released at the neuromuscular junction of skeletal muscle is

• A. noradrenaline
• B. dopamine
• C. acetylcholine
• D. GABA

Explanation: Acetylcholine diffuses across the synaptic cleft and depolarises the sarcolemma, and the resulting action potential spreads down the T tubules to release calcium. The enzyme cholinesterase then destroys it quickly, so each nerve impulse produces one twitch rather than continuous contraction. Nerve gases and some insecticides are lethal because they block that enzyme.

7. The sliding filament theory involves

• A. Actin and myosin
• B. Tropomyosin only
• C. Troponin only
• D. Collagen

Explanation: Muscle contraction involves sliding of actin (thin) filaments past myosin (thick) filaments.

8. The functional unit of skeletal muscle is

• A. Myofibril
• B. Sarcomere
• C. Muscle fiber
• D. Tendon

Explanation: The sarcomere (between two Z-lines) is the basic functional unit of muscle contraction.

9. Calcium binds to

• A. Actin
• B. Myosin
• C. Troponin
• D. Tropomyosin

Explanation: Ca2+ binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

10. ATP is required for muscle contraction for

• A. Only cross-bridge formation
• B. Only cross-bridge detachment
• C. Both formation and detachment
• D. Neither

Explanation: ATP is required for myosin head cocking (after power stroke) and detachment from actin.