Muscle Physiology (Skeletal, Smooth, Motor Unit) MCQs

Physiology · 39 free questions with answers & explanations.

1. During skeletal muscle contraction, calcium released from the sarcoplasmic reticulum binds to which molecule to initiate cross-bridge cycling?
2. A physiotherapist notes that a patient can sustain posture for prolonged periods without fatigue. This reflects the predominance of which motor unit type in postural muscles?
3. A muscle physiologist records the isometric tension produced by a motor unit at different stimulation frequencies. At 10 Hz, individual twitches are visible. At 50 Hz, a smooth fused tetanus is produced with much higher tension. What accounts for the marked increase in tension during tetanus compared to a single twitch?
4. Smooth muscle in the intestinal wall undergoes slow-wave spontaneous depolarizations. Under which condition do these slow waves generate action potentials and produce contraction?
5. The velocity of shortening in skeletal muscle follows the Hill force-velocity relationship: as load increases, velocity decreases. At zero load (Vmax), which structural event limits the maximum rate of cross-bridge cycling?
6. In vascular smooth muscle, Ca²⁺-calmodulin activates myosin light chain kinase (MLCK), which phosphorylates myosin light chain (MLC20) to initiate contraction. Which mechanism maintains 'latch state' (sustained contraction at low energy cost) in smooth muscle?
7. Size principle of motor unit recruitment (Henneman's principle) states that smaller motor units (slow-twitch type I) are recruited before larger (fast-twitch type II) units. The basis for this orderly recruitment is:
8. The latch state in smooth muscle allows sustained force generation with low energy expenditure. What is the molecular basis of the latch state?
9. During an isometric twitch contraction of a single muscle fibre, the 'series elastic component' (SEC) must be stretched before external force is recorded. Which structure primarily constitutes the SEC in skeletal muscle?
10. A patient with myasthenia gravis has an impaired neuromuscular junction. The safety factor of the NMJ refers to which concept, and how does it explain the characteristic fatigability?
11. In smooth muscle, what is the critical kinase responsible for initiating contraction, and how is it regulated differently from skeletal muscle crossbridge cycling?
12. Fiber type transformation in skeletal muscle (e.g., Type II → Type I with endurance training) is primarily controlled by which transcriptional regulator?
13. In excitation-contraction coupling of skeletal muscle, the dihydropyridine receptor (DHPR) of the T-tubule and the ryanodine receptor (RyR1) of the SR terminal cisternae are functionally coupled. In skeletal muscle (unlike cardiac muscle), how does DHPR activation cause RyR1 opening?
14. A patient with Lambert-Eaton myasthenic syndrome (LEMS) has autoantibodies against voltage-gated calcium channels (VGCC) at the presynaptic motor nerve terminal. The clinical consequence that distinguishes LEMS from myasthenia gravis is:
15. In smooth muscle, calmodulin-dependent myosin light chain kinase (MLCK) phosphorylates the 20-kDa myosin light chain (MLC20) at Ser19 to initiate contraction. Dephosphorylation of MLC20 by myosin light chain phosphatase (MLCP) relaxes smooth muscle. Rho-kinase (ROCK) inhibits MLCP by phosphorylating its regulatory subunit MYPT1. This mechanism is called calcium sensitization. Which clinical agent exploits this pathway for vasodilation?
16. A sprinter uses type IIx (fast-twitch glycolytic) muscle fibers during a 100-m sprint. These fibers fatigue rapidly because they rely primarily on anaerobic glycolysis. The specific biochemical explanation for why these fibers are more fatigue-prone than type I (slow-twitch oxidative) fibers is:
17. In excitation-contraction coupling of skeletal muscle, which structural interaction between the transverse tubule voltage sensor and the sarcoplasmic reticulum Ca²⁺ release channel is responsible for Ca²⁺ release WITHOUT a requirement for extracellular Ca²⁺ influx?
18. The force–velocity relationship of skeletal muscle (Hill equation) shows that maximum velocity of shortening (Vmax) occurs at zero external load, and maximum isometric force (P₀) occurs at zero velocity. What is the physiological basis of the inverse relationship between force and velocity?
19. Single-unit (visceral) smooth muscle differs from multi-unit smooth muscle in its electrical coupling and contractile behavior. Which feature is responsible for the spontaneous rhythmicity and phasic contractions of single-unit smooth muscle (e.g., intestinal)?
20. In smooth muscle, contraction is initiated by Ca²⁺-calmodulin activation of myosin light chain kinase (MLCK). The mechanism by which alpha-smooth muscle cells sustain tonic contraction at low energy cost (latch state) involves:
21. Duchenne muscular dystrophy is caused by dystrophin deficiency. Dystrophin connects the intracellular actin cytoskeleton to the extracellular matrix via the dystrophin-associated protein complex (DAPC). The primary pathological consequence of dystrophin loss is:
22. During a sprint, fast-twitch (type IIX) muscle fibers are recruited. Compared to slow-twitch (type I) fibers, fast-twitch fibers differ in which key metabolic and mechanical property?
23. A physiological tremor at 8–12 Hz is noted in a patient's outstretched hand. This frequency corresponds to the natural resonance of which oscillatory loop?
24. During an isometric contraction, a muscle develops maximum tension at its resting length (Lo). This length-tension relationship is explained at the sarcomere level by:
25. A patient with Duchenne muscular dystrophy lacks dystrophin. Which biomechanical consequence directly causes membrane fragility and progressive myofiber death?
26. During sustained isometric contraction of the quadriceps, blood flow to the muscle paradoxically decreases. Which physiological mechanism explains this?
27. In a patient with myasthenia gravis, the decremental response on repetitive nerve stimulation is diagnostic. Which of the following correctly explains the electrophysiological basis of the decremental response at 3 Hz stimulation?
28. Smooth muscle, unlike skeletal muscle, can maintain sustained contraction (tone) at very low energy consumption — the so-called 'latch' state. Which molecular mechanism is responsible for the latch state?
29. A patient with myasthenia gravis has anti-AChR antibodies. On repetitive nerve stimulation (RNS) at 3 Hz, there is a >10% decrement in compound muscle action potential (CMAP) amplitude. Which physiological mechanism explains this decrement?
30. Smooth muscle contraction differs from skeletal muscle because it uses a calmodulin-MLCK pathway rather than troponin-tropomyosin. A patient receiving a calcium channel blocker for hypertension shows reduced vascular tone. Which molecular step is directly inhibited by reducing intracellular Ca²⁺ in vascular smooth muscle?
31. A motor unit consists of a single alpha motor neuron and all the muscle fibres it innervates. Slow-twitch (Type I) motor units differ from fast-twitch (Type IIx) motor units in which of the following respects?
32. In smooth muscle, tonic contraction is maintained after initial Ca2+ signalling through which mechanism that is absent in skeletal muscle?
33. In smooth muscle, unlike skeletal muscle, cross-bridge cycling is regulated by phosphorylation of myosin light chains. The key enzyme that phosphorylates myosin and initiates contraction is:
34. The force-velocity relationship in skeletal muscle describes an inverse relationship between load and velocity of shortening. Which of the following BEST explains why maximum force is generated at zero velocity (isometric contraction)?
35. A patient with myasthenia gravis has antibodies against nicotinic acetylcholine receptors at the neuromuscular junction. Which electromyographic (EMG) finding is characteristic on repetitive nerve stimulation at low frequency (3 Hz)?
36. The mechanism of smooth muscle contraction differs from skeletal muscle in several key ways. Which statement is TRUE regarding regulation of smooth muscle contraction?
37. In smooth muscle, contraction is initiated by Ca2+ binding to calmodulin (CaM) rather than troponin C. The downstream phosphorylation target is:
38. The Henneman size principle of motor unit recruitment states that during graded muscle contractions:
39. The resting membrane potential of a typical neuron lies closest to the equilibrium potential of which ion, reflecting the membrane's high resting permeability to it?