Unit 02 — Nerve & Muscle · Question Bank

TMU Physiology · Nerve & Muscle Cells · Guyton 14e Ch 6–8
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Q1
Which protein forms the thick filaments of the sarcomere?
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A. myosin
B. actin
C. tropomyosin
D. troponin
✅ Answer: A. myosin
Thick filaments = myosin (tail backbone + globular cross-bridge heads). Thin filaments = actin + tropomyosin + troponin.
Q2
The neurotransmitter at the neuromuscular junction is:
A. noradrenaline
B. acetylcholine
C. glutamate
D. dopamine
✅ Answer: B. acetylcholine
Motor neurons release acetylcholine, which binds nicotinic (N) receptors on the motor end-plate.
Q3
The receptor on the motor end-plate is a:
A. muscarinic ACh receptor
B. β-adrenergic receptor
C. nicotinic ACh receptor (ligand-gated Na⁺ channel)
D. voltage-gated Ca²⁺ channel
✅ Answer: C. nicotinic ACh receptor (ligand-gated Na⁺ channel)
The end-plate nicotinic receptor is a ligand-gated cation channel; ACh opens it → Na⁺ influx → end-plate potential.
Q4
In the sliding-filament theory, during contraction:
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A. the A band shortens
B. the thick filaments shorten
C. both filaments shorten equally
D. the I band shortens while the A band stays constant
✅ Answer: D. the I band shortens while the A band stays constant
Filaments slide (don't shorten): the I band and H zone narrow and the sarcomere shortens, but the A band (= thick-filament length) is unchanged.
Q5
Ca²⁺ triggers contraction by binding to:
A. troponin-C
B. tropomyosin
C. myosin head
D. actin
✅ Answer: A. troponin-C
Ca²⁺ binds troponin-C → troponin–tropomyosin shifts → exposes actin's myosin-binding sites → cross-bridge cycling begins.
Q6
During excitation–contraction coupling, Ca²⁺ that triggers contraction is released from the:
A. extracellular fluid
B. sarcoplasmic reticulum
C. mitochondria
D. T-tubule lumen
✅ Answer: B. sarcoplasmic reticulum
The AP runs down the T-tubule → DHP receptor signals the RyR on the SR terminal cisternae → Ca²⁺ floods from the SR into the sarcoplasm.
Q7
Relaxation of skeletal muscle requires:
A. continued Ca²⁺ release
B. ATP depletion
C. SERCA pumping Ca²⁺ back into the SR
D. acetylcholine release
✅ Answer: C. SERCA pumping Ca²⁺ back into the SR
The SR Ca²⁺-ATPase (SERCA) actively re-uptakes Ca²⁺ → troponin releases it → tropomyosin re-covers the sites → cross-bridges detach.
Q8
Rigor mortis (rigid muscle after death) occurs because:
A. excess Ca²⁺ is released
B. AChE is inhibited
C. SR is overloaded
D. there is no ATP to detach myosin from actin
✅ Answer: D. there is no ATP to detach myosin from actin
ATP is needed to release the myosin head from actin. Without ATP after death the cross-bridges stay attached → muscle is rigid.
Q9
In an isometric contraction, the muscle:
A. develops increasing tension without shortening
B. shortens at constant tension
C. lengthens
D. shortens then lengthens
✅ Answer: A. develops increasing tension without shortening
Isometric = same length: tension rises but the muscle does not shorten (e.g. pushing a wall, posture). Isotonic = shortens at constant load.
Q10
A smooth, sustained, maximal contraction with no relaxation between stimuli is:
A. a single twitch
B. complete (fused) tetanus
C. incomplete (unfused) tetanus
D. a fasciculation
✅ Answer: B. complete (fused) tetanus
High-frequency stimulation fuses the twitches with no relaxation between them = complete tetanus (up to ~4× twitch tension).
Q11
A motor unit consists of:
A. one muscle fibre and all its neurons
B. one sarcomere
C. one motor neuron and all the muscle fibres it innervates
D. the neuromuscular junction only
✅ Answer: C. one motor neuron and all the muscle fibres it innervates
All fibres of a motor unit contract together. Small units (few fibres) give fine control; large units give power.
Q12
Acetylcholine action at the NMJ is terminated by:
A. reuptake into the terminal
B. diffusion only
C. nicotinic receptor internalisation
D. acetylcholinesterase hydrolysing it
✅ Answer: D. acetylcholinesterase hydrolysing it
AChE in the synaptic cleft splits ACh into acetate + choline; choline is recycled. This resets the end-plate for the next impulse.
Q13
The end-plate potential (EPP) is:
A. a graded local depolarization that triggers the muscle AP
B. an all-or-none action potential
C. a hyperpolarization
D. produced by Cl⁻ influx
✅ Answer: A. a graded local depolarization that triggers the muscle AP
The EPP is a graded local potential (Na⁺ influx through nicotinic channels); its large size (safety factor) reaches threshold and fires the muscle AP.
Q14
Myasthenia gravis is caused by autoantibodies against:
A. presynaptic Ca²⁺ channels
B. postsynaptic nicotinic ACh receptors
C. acetylcholinesterase
D. the SR Ca²⁺ pump
✅ Answer: B. postsynaptic nicotinic ACh receptors
Loss of end-plate nicotinic receptors → small EPPs fail to reach threshold → fatigable weakness; improved by AChE inhibitors (neostigmine).
Q15
Compared with skeletal muscle, smooth muscle uses which Ca²⁺-binding protein for contraction?
A. troponin-C
B. calsequestrin
C. calmodulin (→ MLCK)
D. parvalbumin
✅ Answer: C. calmodulin (→ MLCK)
Smooth muscle has no troponin; Ca²⁺ + calmodulin activate myosin light-chain kinase (MLCK), which phosphorylates myosin to allow cycling.
Q16
The boundary of a sarcomere is the:
A. M line
B. A band
C. H zone
D. Z disc (line)
✅ Answer: D. Z disc (line)
The sarcomere runs from one Z disc to the next; Z discs anchor the thin (actin) filaments and move closer together during contraction.
Q17
In the cross-bridge cycle, the power stroke is associated with release of:
A. ATP
B. Pi then ADP
C. Ca²⁺
D. acetylcholine
✅ Answer: B. Pi then ADP
The energised head binds actin, then releases Pi and ADP as it pivots (power stroke). A new ATP then detaches the head.
Q18
Why can skeletal muscle be tetanised but cardiac muscle cannot?
A. cardiac muscle has no T-tubules
B. cardiac muscle has a very long refractory period
C. skeletal muscle lacks SR
D. cardiac muscle has no actin
✅ Answer: B. cardiac muscle has a very long refractory period
The cardiac AP plateau gives a long refractory period (lasts almost the whole contraction), preventing summation/tetanus — so the heart always relaxes to refill.
Q19
The active tension a muscle can develop is greatest at the optimal length because:
A. all SR Ca²⁺ is released
B. ATP is highest
C. thin and thick filament overlap is maximal for cross-bridge formation
D. the membrane is most excitable
✅ Answer: C. thin and thick filament overlap is maximal for cross-bridge formation
The length–tension relationship: optimal overlap → maximal number of cross-bridges → maximal force. Too short or too stretched → fewer cross-bridges → less force (basis of Frank–Starling).
Q20
Preload of a muscle is defined as the:
A. resistance it contracts against
B. velocity of shortening
C. frequency of stimulation
D. stretch (initial length) before it contracts
✅ Answer: D. stretch (initial length) before it contracts
Preload = the load that stretches the resting muscle (sets initial length/overlap). Afterload = the resistance the muscle works against during contraction.
1Motor unit+
One motor neuron together with all the skeletal muscle fibres it innervates. All fibres of the unit contract simultaneously when the neuron fires; graded whole-muscle force comes from recruiting more units and raising firing rate.
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2Neuromuscular junction (motor end-plate)+
The chemical synapse between a motor-neuron terminal and a skeletal muscle fibre. ACh released by Ca²⁺-triggered exocytosis binds nicotinic receptors → end-plate potential → muscle action potential; AChE then terminates the signal.
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3Sarcomere+
The fundamental contractile unit of striated muscle, from one Z disc to the next. It contains interdigitating thin (actin) and thick (myosin) filaments; the A band, I band, H zone, M line and Z disc are its repeating landmarks.
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4Excitation–contraction coupling+
The process linking the muscle action potential to contraction: AP → T-tubule → SR Ca²⁺ release → Ca²⁺ binds troponin-C → tropomyosin shifts → actin–myosin cross-bridge cycling. Ca²⁺ is the coupling messenger.
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5Single twitch+
The brief contraction–relaxation produced by a single action potential, with a latent period, a contraction phase and a relaxation phase. Because the AP is much shorter than the twitch, repeated stimuli can summate.
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6Tetanus (complete vs incomplete)+
A summated contraction from repetitive stimulation. Incomplete (unfused) tetanus has partial relaxation between peaks (wavy); complete (fused) tetanus has no relaxation between responses — a smooth maximal contraction (~4× twitch tension).
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Essay 1
Describe the process of transmission of excitation at the neuromuscular junction.
10 marks

Structure

Presynaptic terminal (ACh vesicles + voltage-gated Ca²⁺ channels), synaptic cleft (containing acetylcholinesterase), and the folded postsynaptic end-plate packed with nicotinic ACh receptors.

Sequence

  1. The nerve action potential reaches the terminal → opens voltage-gated Ca²⁺ channels.
  2. Ca²⁺ influx triggers exocytosis of ACh into the cleft.
  3. ACh diffuses across and binds nicotinic receptors on the end-plate.
  4. Receptor channels open → Na⁺ influx → graded end-plate potential (EPP).
  5. The large EPP (high safety factor) reaches threshold → a muscle action potential propagates along the sarcolemma.
  6. Acetylcholinesterase rapidly hydrolyses ACh → the end-plate repolarises, ready for the next impulse.
Marking guide (10 marks): Ca²⁺ influx triggers release 2 · ACh released by exocytosis 1.5 · binds nicotinic receptor 1.5 · Na⁺ influx → EPP (graded) 2 · EPP reaches threshold → muscle AP 1.5 · AChE terminates 1.5
Essay 2
Explain the sliding-filament mechanism of skeletal muscle contraction.
10 marks

Filaments

Thin filaments (actin + tropomyosin + troponin) anchored at the Z disc; thick filaments (myosin, with cross-bridge heads) at the centre.

Sliding, not shortening

During contraction the thin filaments slide over the thick filaments toward the M line, increasing overlap. Therefore the I band and H zone narrow and the sarcomere shortens, while the A band stays constant; the filament lengths themselves are unchanged.

Cross-bridge cycle

  1. Energised myosin head (ADP + Pi) binds exposed actin.
  2. Pi then ADP release → head pivots (power stroke) → thin filament pulled inward.
  3. New ATP binds → head detaches.
  4. ATP hydrolysis re-cocks the head. Cycles repeat while Ca²⁺ is present.
Marking guide (10 marks): filament composition 2 · filaments slide (I/H shorten, A constant) 3 · cross-bridge cycle steps 4 · role of ATP 1
Essay 3
Describe excitation–contraction coupling in skeletal muscle.
10 marks

From membrane to Ca²⁺ release

  1. The muscle AP propagates over the sarcolemma and down the T-tubules.
  2. The T-tubule voltage sensor (DHP receptor) signals the SR terminal cisternae.
  3. The ryanodine receptor (RyR) opens → Ca²⁺ released from the SR into the sarcoplasm.
  4. Ca²⁺ binds troponin-C → troponin–tropomyosin shifts → actin sites exposed → contraction.

Relaxation

Ca²⁺ is pumped back into the SR by SERCA → Ca²⁺ leaves troponin → tropomyosin re-covers the sites → cross-bridges detach → muscle relaxes.

Marking guide (10 marks): AP down T-tubule 2 · DHP → RyR → SR Ca²⁺ release 3 · Ca²⁺ → troponin-C → tropomyosin shift 3 · SERCA re-uptake for relaxation 2
Essay 4
Distinguish a single twitch, incomplete tetanus and complete tetanus, and explain why summation is possible.
10 marks

Single twitch

One AP → one brief contraction (latent, contraction, relaxation phases), with full relaxation before the next stimulus.

Why summation is possible

The muscle AP (~1–2 ms) and its refractory period are far shorter than the mechanical twitch (~100 ms). A second stimulus can therefore arrive during the ongoing contraction, and the mechanical responses add up.

Incomplete vs complete tetanus

  • Incomplete (unfused) tetanus: moderate frequency → partial relaxation between summated peaks (wavy trace).
  • Complete (fused) tetanus: high frequency → no relaxation → smooth, sustained, maximal contraction (~4× twitch tension).
Marking guide (10 marks): single twitch + phases 2 · AP/refractory period << twitch → summation 3 · incomplete tetanus 2 · complete tetanus + ~4× tension 3
Essay 5
Describe the cross-bridge cycle and the role of ATP in contraction and relaxation.
10 marks

Cross-bridge cycle

  1. Attachment: the energised myosin head (carrying ADP + Pi) binds exposed actin.
  2. Power stroke: Pi then ADP are released; the head pivots, pulling the thin filament toward the M line → force/shortening.
  3. Detachment: a new ATP binds myosin → the head releases actin.
  4. Re-cocking: myosin ATPase hydrolyses ATP → ADP + Pi re-energise and re-cock the head, ready to re-attach. Cycling continues while cytosolic Ca²⁺ is high.

Role of ATP

ATP is needed (i) to detach myosin from actin and (ii) to power SERCA Ca²⁺ re-uptake for relaxation. Without ATP the muscle stays contracted — the basis of rigor mortis.

Marking guide (10 marks): attachment 1.5 · power stroke (Pi/ADP release) 2.5 · detachment needs ATP 2 · re-cocking by ATP hydrolysis 2 · ATP also for SERCA/relaxation + rigor 2