USMLE Step 1 Muscle Physiology practice questions
Muscle physiology on USMLE Step 1 centers on the molecular mechanisms that convert an action potential into force generation and relaxation across skeletal, cardiac, and smooth muscle. Key themes include the sliding filament model of cross-bridge cycling, excitation-contraction coupling (the dihydropyridine receptor-ryanodine receptor link in skeletal muscle vs. calcium-induced calcium release in cardiac muscle vs. the calmodulin-myosin light-chain kinase pathway in smooth muscle), the length-tension relationship, and the metabolic and contractile differences between Type I and Type II skeletal muscle fibers.
A 68-year-old man dies of cardiac arrest. At autopsy, performed 8 hours after death, the medical examiner notes generalized stiffening of the skeletal musculature that developed gradually after death. This stiffening is attributed to depletion of a small molecule within the myocytes. Which of the following processes is most directly affected by depletion of this molecule?
A 22-year-old healthy volunteer participates in a physiology study in which electrical activity of the tibialis anterior muscle is recorded during voluntary contraction. Investigators explain that depolarization of the T-tubule membrane produces a conformational change in a voltage-sensing membrane protein that is physically linked to a calcium release channel on the adjacent sarcoplasmic reticulum. Which of the following best describes this mechanism of excitation-contraction coupling?
In a muscle physiology laboratory exercise, an isolated skeletal muscle fiber is passively stretched to a sarcomere length at which the thick and thin filaments no longer overlap. The fiber is then electrically stimulated. Which of the following is the most likely effect on the active force generated by the fiber compared with stimulation at its optimal resting length?
A 30-year-old competitive marathon runner undergoes a research muscle biopsy of the gastrocnemius. Histochemical analysis shows fibers with abundant mitochondria, high myoglobin content, and dense surrounding capillary networks. Compared with the fibers that predominate in an elite 100-meter sprinter, which of the following properties is most characteristic of the fibers identified in this runner's biopsy?
A 26-year-old woman at term is in active labor with strong, regular uterine contractions. Which of the following most directly initiates cross-bridge cycling in the uterine smooth muscle cells responsible for these contractions?
Muscle Physiology, answered
What's the key difference between excitation-contraction coupling in skeletal muscle and cardiac muscle?
In skeletal muscle, the dihydropyridine receptor and ryanodine receptor are mechanically linked, so T-tubule depolarization directly opens the sarcoplasmic reticulum calcium channel without needing extracellular calcium. In cardiac muscle, calcium entering through L-type calcium channels triggers additional calcium release from the sarcoplasmic reticulum, a process called calcium-induced calcium release.
How much do I need to know about smooth muscle contraction for Step 1?
Focus on the calcium-calmodulin-myosin light-chain kinase pathway and how it differs from the troponin-tropomyosin system in skeletal and cardiac muscle. Knowing that smooth muscle contraction is regulated at the level of the myosin light chain, rather than by exposing the actin-binding site, covers most testable points.
Do I need to memorize the entire length-tension curve?
You mainly need the general concept: force depends on the degree of thick and thin filament overlap, with an optimal sarcomere length producing maximal force, and force falling off when a sarcomere is either overstretched or overly shortened. This concept also underlies the Frank-Starling relationship in cardiac physiology, which is tested more often than the isolated skeletal muscle curve.
What's the most efficient way to remember Type I versus Type II fiber differences?
Anchor the comparison to endurance versus sprint activity: Type I fibers are slow, oxidative, mitochondria- and myoglobin-rich, and fatigue-resistant, while Type II fibers are fast, glycolytic, and better suited to brief, powerful contractions but fatigue more quickly. Most questions test this contrast through a clinical or athletic scenario rather than isolated recall.
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