MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3b Muscular System Contraction
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3b Muscular System ContractionQuestion 1 of 20

During repetitive stimulation of a skeletal muscle at 50 Hz, intracellular ATP falls from 5.0 mM to 2.0 mM while inorganic phosphate rises. The muscle begins to fail to relax fully between stimuli, producing a sustained increase in baseline tension. Assume cytosolic free Ca2+^{2+} transients remain normal in amplitude.

Which factor is most likely to limit muscle contraction in the described conditions?

Impaired detachment of myosin from actin due to insufficient ATP binding to myosin heads.
Failure of acetylcholinesterase to degrade acetylcholine because ATP is required for synaptic hydrolysis.
Reduced opening of voltage-gated Na+^+ channels because ATP directly gates the channels in skeletal muscle.
Reduced Ca2+^{2+} binding to troponin because ATP is the ligand that activates troponin C.
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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3b Muscular System Contraction

Practice 3b Muscular System Contraction in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on 3b Muscular System Contraction, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

During repetitive stimulation of a skeletal muscle at 50 Hz, intracellular ATP falls from 5.0 mM to 2.0 mM while inorganic phosphate rises. The muscle begins to fail to relax fully between stimuli, producing a sustained increase in baseline tension. Assume cytosolic free Ca2+^{2+} transients remain normal in amplitude.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Impaired detachment of myosin from actin due to insufficient ATP binding to myosin heads. (correct answer)
  2. Failure of acetylcholinesterase to degrade acetylcholine because ATP is required for synaptic hydrolysis.
  3. Reduced opening of voltage-gated Na+^+ channels because ATP directly gates the channels in skeletal muscle.
  4. Reduced Ca2+^{2+} binding to troponin because ATP is the ligand that activates troponin C.

Explanation: This question probes the role of ATP in the cross-bridge cycle during sustained muscle activity, particularly in relaxation and repeated contractions. ATP binding to myosin heads causes detachment from actin, allowing relaxation and re-cocking for new cycles. During repetitive stimulation, falling ATP and rising phosphate impair this detachment, leading to incomplete relaxation and sustained tension. Choice A rightly attributes the limitation to impaired myosin detachment, consistent with the cross-bridge cycle requiring ATP for release. Choice D is misleading as it claims ATP activates troponin, but troponin is calcium-dependent, not ATP-dependent, confusing regulatory mechanisms. To solve similar fatigue-related queries, review metabolic changes and their impact on contractile steps. Validate by confirming if calcium transients are normal, isolating the issue to post-activation processes.

Question 2

In an experiment on skeletal muscle, extracellular Mg2+^{2+} is increased substantially. The primary observed effect is reduced quantal content of acetylcholine release at the NMJ, with smaller end-plate potentials. Muscle fiber resting potential is unchanged.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Increased contraction because Mg2+^{2+} depolarizes the end plate by opening nicotinic receptors directly.
  2. Reduced troponin C activation because Mg2+^{2+} competitively inhibits Ca2+^{2+} binding to actin.
  3. Reduced myosin binding because Mg2+^{2+} replaces ATP at the myosin active site, preventing attachment.
  4. Reduced presynaptic Ca2+^{2+} entry and vesicle fusion probability, decreasing acetylcholine release per nerve impulse. (correct answer)

Explanation: This question tests understanding of the neuromuscular junction (NMJ) and the role of ions in neurotransmitter release during skeletal muscle contraction. The key physiological principle is that calcium influx into the presynaptic terminal triggers vesicle fusion and acetylcholine (ACh) release, with magnesium ions acting as antagonists to calcium channels, reducing this influx. In the scenario, elevated extracellular Mg²⁺ reduces quantal content of ACh release, leading to smaller end-plate potentials without altering muscle fiber resting potential, indicating a presynaptic effect. The correct answer, choice D, follows logically because decreased presynaptic Ca²⁺ entry due to Mg²⁺ blockade limits ACh release per impulse, thereby restricting the depolarization needed for muscle contraction. A common distractor, choice B, is incorrect as it misattributes the effect to postsynaptic troponin C inhibition, ignoring the primary presynaptic reduction in quantal content described. For similar questions, always distinguish between pre- and postsynaptic mechanisms by noting effects on potentials or release metrics. Additionally, verify ion roles by recalling antagonists like Mg²⁺ primarily affect voltage-gated calcium channels at the NMJ.

Question 3

A motor neuron innervating a fast-twitch skeletal muscle is exposed to a toxin that selectively blocks presynaptic voltage-gated Ca2+^{2+} channels at the neuromuscular junction (NMJ). End-plate potentials recorded in the muscle fiber fall below threshold despite normal motor neuron action potentials. Extracellular ion concentrations remain physiological: [Na+]o=145[\mathrm{Na^+}]_o=145 mM, [K+]o=4[\mathrm{K^+}]_o=4 mM, [Ca2+]o=1.2[\mathrm{Ca^{2+}}]_o=1.2 mM.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Reduced acetylcholine release due to impaired presynaptic Ca2+^{2+}-triggered vesicle fusion. (correct answer)
  2. Reduced myosin ATPase activity because Ca2+^{2+} is required to hydrolyze ATP at the myosin head.
  3. Reduced Na+^+ driving force across the sarcolemma because extracellular Ca2+^{2+} normally maintains the Na+^+ gradient.
  4. Failure of tropomyosin to bind actin because Ca2+^{2+} normally stabilizes the actin filament in the relaxed state.

Explanation: This question assesses knowledge of neuromuscular transmission and the role of presynaptic calcium in synaptic vesicle release at the neuromuscular junction. Calcium influx through voltage-gated channels in the presynaptic terminal triggers acetylcholine vesicle fusion and release, essential for postsynaptic depolarization. Here, the toxin blocks these calcium channels, impairing acetylcholine release despite normal motor neuron action potentials. Choice A accurately identifies reduced acetylcholine release as the limiting factor, leading to subthreshold end-plate potentials and failed contraction. Choice B is wrong because it incorrectly states that calcium directly hydrolyzes ATP at the myosin head, overlooking that calcium acts via troponin while ATP powers myosin. For similar problems, map the pathway from nerve impulse to muscle force and pinpoint the blocked step. Confirm by evaluating if downstream processes like action potentials or calcium transients remain intact.

Question 4

A skeletal muscle fiber is exposed to a local anesthetic that blocks voltage-gated Na+^+ channels in the sarcolemma but does not affect nicotinic receptors. Motor neuron stimulation still produces an end-plate potential, but no propagated muscle action potential occurs.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Excessive Ca2+^{2+} influx through Na+^+ channels, causing sustained contraction and tetany.
  2. Failure of acetylcholine to bind its receptor because Na+^+ channels are required for receptor activation.
  3. Failure to generate a propagated action potential, preventing T-tubule depolarization and subsequent SR Ca2+^{2+} release. (correct answer)
  4. Increased force because blocking Na+^+ channels reduces ATP consumption during repolarization.

Explanation: This question assesses knowledge of action potential propagation in skeletal muscle and its link to excitation-contraction coupling. The core principle is that voltage-gated sodium channels enable the propagation of action potentials along the sarcolemma and into T-tubules, which triggers calcium release from the sarcoplasmic reticulum (SR) via dihydropyridine and ryanodine receptors. Here, the local anesthetic blocks these Na⁺ channels, allowing an end-plate potential from nicotinic receptor activation but preventing propagated action potentials. Choice C is correct because without propagation, T-tubule depolarization fails, halting SR Ca²⁺ release essential for contraction. Choice B is a distractor that wrongly assumes Na⁺ channels are needed for acetylcholine binding, confusing ligand-gated receptor function with voltage-gated propagation. To approach similar problems, map the sequence from NMJ depolarization to SR release and identify the disrupted step. Confirm by considering that end-plate potentials are local and require Na⁺ channel amplification for full muscle activation.

Question 5

In an isolated frog skeletal muscle fiber, a researcher voltage-clamps the sarcolemma to evoke a single action potential and measures peak cytosolic [Ca2+][\mathrm{Ca^{2+}}] with a fluorescent indicator. The fiber is then treated with a ryanodine receptor (RyR1) inhibitor that prevents Ca2+^{2+} release from the sarcoplasmic reticulum (SR) without affecting the action potential. Peak cytosolic [Ca2+][\mathrm{Ca^{2+}}] falls from 1.2 μ\muM to 0.15 μ\muM (resting [Ca2+][\mathrm{Ca^{2+}}] is 0.10 μ\muM), and twitch force decreases by 85%.

Which statement best describes the role of calcium in this process?

  1. Ca2+^{2+} binding to myosin heads causes their detachment from actin, so reduced Ca2+^{2+} increases force by prolonging attachment.
  2. Ca2+^{2+} hydrolysis provides the energy for the myosin power stroke; reduced Ca2+^{2+} directly lowers ATP availability.
  3. Ca2+^{2+} influx through nicotinic acetylcholine receptors is the primary source of activator Ca2+^{2+} for skeletal muscle contraction.
  4. Ca2+^{2+} binds to troponin C, shifting tropomyosin to expose myosin-binding sites on actin and permitting cross-bridge cycling. (correct answer)

Explanation: This question tests understanding of the mechanisms of excitation-contraction coupling in skeletal muscle, specifically the role of calcium release from the sarcoplasmic reticulum. In skeletal muscle contraction, calcium ions are released from the SR via ryanodine receptors (RyR1) in response to depolarization, binding to troponin to initiate cross-bridge formation. In this scenario, the RyR1 inhibitor prevents SR calcium release, resulting in markedly reduced cytosolic calcium elevation during the action potential. Choice D correctly explains that calcium binds to troponin C, shifting tropomyosin to allow myosin-actin interactions, and the observed force reduction follows from insufficient calcium for this process. Choice B is incorrect as it misattributes the energy source for the power stroke to calcium hydrolysis rather than ATP hydrolysis by myosin, confusing the roles of calcium and ATP. To approach similar questions, recall the sequence of excitation-contraction coupling and identify which step is disrupted. Verify the outcome by considering whether the intervention affects calcium availability or downstream contractile machinery.

Question 6

A patient receives an injection of botulinum toxin into a hyperactive skeletal muscle. The toxin cleaves SNARE proteins in the presynaptic terminal. Motor neuron action potentials remain intact, and muscle nicotinic receptors are normal.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Decreased myosin-actin overlap because botulinum toxin shortens thick filaments.
  2. Decreased Ca2+^{2+} binding to troponin because SNARE proteins normally buffer Ca2+^{2+} in the sarcoplasm.
  3. Decreased acetylcholine release due to impaired synaptic vesicle fusion, reducing end-plate potentials. (correct answer)
  4. Increased contraction because blocking SNARE proteins prevents acetylcholine breakdown at the synapse.

Explanation: This question examines botulinum toxin's effect on presynaptic release mechanisms. SNARE proteins facilitate vesicle fusion; cleavage impairs acetylcholine release, reducing end-plate potentials. This limits contraction despite intact downstream elements. Choice C properly identifies reduced release as key. Choice D is wrong as blocking SNAREs prevents release, not breakdown. For synaptic toxin problems, identify pre- or postsynaptic sites. Validate by direct stimulation bypassing the junction.

Question 7

A researcher measures force in a single skeletal muscle fiber while holding sarcomere length constant. When cytosolic Ca2+^{2+} is clamped high (1 μ\muM), the fiber generates steady force. A myosin ATPase inhibitor is then added; Ca2+^{2+} remains high, but force declines over time.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Increased actin depolymerization because myosin ATPase normally stabilizes actin filaments at rest.
  2. Reduced Ca2+^{2+} binding to troponin C because myosin ATPase activity is required to expose troponin sites.
  3. Reduced acetylcholine release because myosin ATPase provides energy for synaptic vesicle fusion.
  4. Reduced cross-bridge cycling because myosin cannot hydrolyze ATP to perform repeated power strokes. (correct answer)

Explanation: This question investigates ATP's role in sustaining force via myosin ATPase activity. Myosin ATPase hydrolyzes ATP to drive repeated power strokes in cross-bridge cycling. Inhibiting it prevents energy release, causing force decline despite high calcium. Choice D correctly identifies reduced cycling as the limiter, tied to ATPase function. Choice B misconnects ATPase to troponin binding, but troponin is calcium-regulated, not ATPase-dependent. In enzymatic inhibition studies, link the enzyme to its contractile step. Confirm by ensuring calcium clamping isolates the issue to myosin function.

Question 8

A researcher shortens a skeletal muscle fiber to different initial sarcomere lengths before delivering a single supramaximal stimulus. At 2.2 μ\mum sarcomere length, peak twitch force is maximal. At 3.6 μ\mum, peak twitch force is markedly reduced, despite a normal action potential and normal peak cytosolic Ca2+^{2+}.

Based on the scenario, which outcome is most consistent with the sliding filament theory?

  1. Reduced force at 3.6 μ\mum results from decreased ATP synthesis at longer lengths due to reduced mitochondrial density.
  2. Reduced force at 3.6 μ\mum results from increased Ca2+^{2+} binding to troponin, which inhibits cross-bridge formation.
  3. Reduced force at 3.6 μ\mum results from decreased actin-myosin overlap, reducing the number of possible cross-bridges. (correct answer)
  4. Reduced force at 3.6 μ\mum results from failure of acetylcholine release because stretched sarcomeres inhibit vesicle fusion.

Explanation: This question tests the length-tension relationship in skeletal muscle within the framework of the sliding filament theory. Optimal sarcomere length maximizes actin-myosin overlap, allowing the most cross-bridges for force generation. At extended length (3.6 μm), overlap decreases, reducing possible cross-bridges despite normal excitation and calcium. Choice C accurately explains the force reduction due to diminished overlap, fitting the theory's prediction for non-optimal lengths. Choice B errs by suggesting calcium inhibits cross-bridges, but calcium enables them via troponin, confusing activation with inhibition. In similar biomechanics questions, relate sarcomere structure to force output. Verify by ensuring excitation-contraction coupling remains intact, isolating length as the variable.

Question 9

During repetitive stimulation of a skeletal muscle at 50 Hz, intracellular metabolites are monitored. In one condition, creatine phosphate buffering is inhibited, causing ATP to transiently fall from 5 mM to 2 mM while ADP rises. Cytosolic free Ca2+\text{Ca}^{2+} transients remain normal in amplitude and timing. The muscle produces less peak force over time compared with control.

Which factor is most likely to limit muscle contraction in the described conditions?

  1. Increased sarcomere shortening because lower ATP causes more rapid detachment of myosin from actin
  2. Increased binding of calcium to troponin C, which blocks actin–myosin interactions and reduces force
  3. Failure of acetylcholine receptors to open because ATP is required to gate the ligand-binding domain
  4. Reduced availability of ATP for myosin head cycling, decreasing the number of force-generating cross-bridges per unit time (correct answer)

Explanation: This question tests understanding of how ATP availability affects force generation during muscle fatigue. During high-frequency stimulation, ATP consumption increases dramatically for cross-bridge cycling, and when creatine phosphate buffering is inhibited, ATP levels can fall significantly. With ATP dropping from 5 mM to 2 mM, there is reduced availability of ATP for myosin ATPase activity, which decreases the rate of cross-bridge cycling and reduces the number of force-generating cross-bridges active at any given time. The question specifies that calcium transients remain normal, ruling out problems with excitation-contraction coupling, so the limitation is specifically at the level of the contractile machinery. Choice B is incorrect because it suggests calcium blocks contraction, when actually calcium binding to troponin C enables (not blocks) actin-myosin interactions. When analyzing metabolic factors in muscle fatigue, consider how changes in ATP, ADP, Pi, and H+ can affect different steps of the cross-bridge cycle, with ATP depletion primarily affecting the cycling rate rather than calcium regulation.

Question 10

A skeletal muscle is stimulated with two identical supramaximal impulses separated by 20 ms. The second twitch produces greater peak force than the first, even though the second action potential is unchanged. Cytosolic Ca2+^{2+} after the second stimulus is higher due to incomplete Ca2+^{2+} reuptake.

Which statement best describes the role of calcium in this process?

  1. Residual Ca2+^{2+} increases force only in cardiac muscle because skeletal muscle lacks troponin.
  2. Residual Ca2+^{2+} decreases force by promoting myosin detachment from actin, shortening the power stroke.
  3. Residual Ca2+^{2+} increases force by directly providing energy for the power stroke through Ca2+^{2+} hydrolysis.
  4. Residual Ca2+^{2+} increases troponin occupancy, increasing the fraction of available actin sites and enhancing force (temporal summation). (correct answer)

Explanation: This question explores temporal summation in muscle force generation through calcium dynamics. Residual calcium from prior stimuli adds to new releases, increasing troponin occupancy and force. Closer impulses cause summation, enhancing the second twitch. Choice D correctly describes summation via higher calcium and more sites. Choice B errs by saying calcium promotes detachment, but it enables attachment. In summation queries, relate calcium kinetics to force potentiation. Check by varying intervals, expecting less summation with longer gaps.

Question 11

In a controlled experiment, a skeletal muscle fiber is exposed to a drug that selectively inhibits the sarcoplasmic reticulum Ca2+\text{Ca}^{2+}-ATPase (SERCA) by 80% without directly affecting ryanodine receptors, troponin, or myosin ATPase. A single muscle action potential is elicited. Assume resting cytosolic free Ca2+\text{Ca}^{2+} is ∼100 nM\sim 100\ \text{nM} and peak free Ca2+\text{Ca}^{2+} during a twitch is ∼10 μM\sim 10\ \mu\text{M} under control conditions.

Which statement best describes the role of calcium in this process?

  1. Reduced SERCA activity prolongs elevated cytosolic Ca2+\text{Ca}^{2+}, delaying relaxation by maintaining troponin C occupancy (correct answer)
  2. Reduced SERCA activity prevents initial Ca2+\text{Ca}^{2+} release by blocking depolarization of the T-tubule membrane
  3. Reduced SERCA activity decreases acetylcholine release because presynaptic vesicle fusion requires sarcoplasmic reticulum reuptake
  4. Reduced SERCA activity increases cross-bridge detachment because ATP is no longer required when calcium remains high

Explanation: This question tests understanding of SERCA's role in muscle relaxation and calcium homeostasis. SERCA (sarcoplasmic reticulum Ca2+-ATPase) is responsible for pumping calcium from the cytosol back into the sarcoplasmic reticulum after contraction, which is essential for muscle relaxation. When SERCA is inhibited by 80%, calcium removal from the cytosol is severely impaired, causing cytosolic calcium to remain elevated for a prolonged period after the initial release. Since calcium binding to troponin C is what keeps the myosin-binding sites on actin exposed, this prolonged elevation of cytosolic calcium maintains troponin C occupancy and delays relaxation. The question confirms that ryanodine receptors function normally, so initial calcium release and contraction can still occur. Choice D is incorrect because ATP is still required for cross-bridge cycling regardless of calcium levels, and high calcium actually promotes continued cycling rather than detachment. When analyzing muscle relaxation problems, remember that relaxation requires both calcium removal (by SERCA) and ATP for cross-bridge detachment - disrupting either process will impair relaxation.

Question 12

A toxin is applied to the presynaptic terminal at the NMJ and selectively cleaves SNARE proteins required for synaptic vesicle fusion. Motor neuron action potentials still invade the terminal, and voltage-gated Ca2+\text{Ca}^{2+} channels open normally. The muscle fiber membrane and contractile apparatus are otherwise intact. Resting membrane potential is −90 mV-90\ \text{mV}, and a typical EPP in control is sufficient to trigger a muscle action potential.

Based on the scenario, which outcome is most consistent with the sliding filament theory?

  1. Sarcomeres shorten normally because actin and myosin can slide even without neurotransmitter release
  2. Sarcomeres fail to shorten because insufficient postsynaptic depolarization prevents Ca2+\text{Ca}^{2+} release needed to permit actin–myosin interaction (correct answer)
  3. Sarcomeres shorten but the A band decreases in length because myosin filaments depolymerize when SNAREs are cleaved
  4. Sarcomeres relax more slowly because acetylcholine persists longer in the synaptic cleft when vesicle fusion is blocked

Explanation: This question tests understanding of synaptic transmission failure and its consequences for muscle contraction. When SNARE proteins are cleaved (as occurs with botulinum toxin), synaptic vesicles cannot fuse with the presynaptic membrane despite normal calcium influx, preventing acetylcholine release. Without acetylcholine release, there is no activation of postsynaptic nicotinic receptors, no end-plate potential generation, and therefore no muscle fiber depolarization. Without reaching the action potential threshold, there is no propagation through T-tubules, no calcium release from the sarcoplasmic reticulum, and consequently no calcium available to bind troponin C. Without calcium-troponin C binding, tropomyosin remains blocking the myosin-binding sites on actin, preventing cross-bridge formation and sarcomere shortening. Choice A is incorrect because it ignores the requirement for neural activation - sarcomeres cannot contract without the excitation-contraction coupling cascade initiated by neurotransmitter release. When analyzing neuromuscular junction blockade, trace the complete pathway from presynaptic vesicle fusion through to sarcomere activation to identify where the cascade is interrupted.

Question 13

In an isolated frog skeletal muscle preparation, the motor neuron is stimulated at 20 Hz for 2 s. The bathing solution contains normal extracellular ions and glucose. A competitive antagonist of nicotinic acetylcholine receptors (nAChRs) is added to the neuromuscular junction (NMJ) at a concentration that reduces end-plate current amplitude by ~70% without affecting axonal action potential propagation. Resting membrane potential of the muscle fiber is approximately −90 mV-90\ \text{mV}, and the threshold for triggering a muscle fiber action potential is approximately −55 mV-55\ \text{mV}. The measured end-plate potential (EPP) peak depolarization decreases from 40 mV (control) to 12 mV (with antagonist) relative to rest.

Based on this scenario, which factor is most likely to limit muscle contraction during stimulation in the presence of the antagonist?

  1. Reduced probability that the muscle fiber reaches action potential threshold, decreasing sarcoplasmic reticulum Ca2+\text{Ca}^{2+} release (correct answer)
  2. Increased acetylcholine breakdown by acetylcholinesterase, causing prolonged depolarization and sustained contraction
  3. Direct inhibition of ryanodine receptors in the sarcoplasmic reticulum, preventing Ca2+\text{Ca}^{2+} release despite normal EPPs
  4. Decreased ATP binding to myosin heads, preventing cross-bridge detachment and thereby preventing force generation

Explanation: This question tests understanding of neuromuscular transmission and the relationship between end-plate potentials and muscle action potentials. In normal conditions, acetylcholine binding to nicotinic receptors causes an end-plate potential (EPP) that depolarizes the muscle membrane from -90 mV to about -50 mV (a 40 mV change), which exceeds the -55 mV threshold for triggering an action potential. With the competitive antagonist reducing the EPP to only 12 mV, the membrane would only reach -78 mV (-90 + 12), which is well below the -55 mV threshold needed to trigger an action potential. Without reaching threshold, the muscle fiber cannot generate an action potential, preventing the spread of depolarization through T-tubules and subsequent calcium release from the sarcoplasmic reticulum. Choice B is incorrect because it suggests prolonged depolarization, but competitive antagonists actually reduce depolarization, and acetylcholinesterase normally terminates (not prolongs) the signal. When analyzing neuromuscular transmission problems, always check whether the EPP amplitude is sufficient to reach the action potential threshold - this is the critical checkpoint for excitation-contraction coupling.

Question 14

A skinned skeletal muscle fiber is activated at constant intracellular free Ca2+ (1.0 µM) and then exposed to a myosin ATPase inhibitor that markedly slows ATP hydrolysis but does not affect actin binding directly. ADP and Pi are initially low, and total ATP is clamped at 5.0 mM. The physiological process assessed is ATP utilization in the cross-bridge cycle during contraction and relaxation. Which factor is most likely to limit muscle contraction in the described conditions?

  1. Reduced rate of myosin head detachment from actin due to absence of Ca2+ binding to troponin
  2. Reduced frequency of power strokes because ATP hydrolysis is required to cock the myosin head for force generation (correct answer)
  3. Reduced acetylcholine release because ATP hydrolysis is required for Ca2+ influx through postsynaptic receptors
  4. Increased sarcomere shortening because ATP hydrolysis normally prevents actin–myosin overlap

Explanation: This question tests understanding of ATP's role in the cross-bridge cycle during muscle contraction. ATP hydrolysis by myosin ATPase is essential for cocking the myosin head into its high-energy conformation, preparing it for the power stroke that generates force. The scenario describes an ATPase inhibitor that slows this hydrolysis step, which would reduce the frequency of power strokes because fewer myosin heads can be reset to their pre-power stroke state. With calcium present and binding sites exposed, myosin can still bind actin, but without efficient ATP hydrolysis, the cycling rate decreases, limiting force generation. Option A is incorrect because myosin head detachment requires ATP binding (not hydrolysis), and calcium's role is to expose binding sites, not directly affect detachment. When analyzing cross-bridge cycle questions, trace through each step: binding, power stroke, ATP binding for detachment, and ATP hydrolysis for resetting.

Question 15

A researcher compares two conditions in isolated skeletal muscle fibers at the same sarcomere length (2.1 µm) and temperature (30°C). Condition 1: intracellular ATP = 5.0 mM. Condition 2: ATP is rapidly depleted to 0.05 mM while intracellular free Ca2+ is held at 1.0 µM in both conditions. Under Condition 2, fibers develop high passive stiffness and fail to relax even after Ca2+ is lowered. The physiological process assessed is ATP's role in the contraction–relaxation cycle. Which factor is most likely to limit muscle contraction in the described conditions?

  1. Inability of myosin to detach from actin due to insufficient ATP binding, leading to rigor-like persistent cross-bridges (correct answer)
  2. Failure of Ca2+ to bind troponin because ATP is required for troponin C expression
  3. Reduced acetylcholine synthesis at the motor neuron terminal because ATP is the ligand for nAChRs
  4. Increased SERCA pumping because low ATP increases the affinity of SERCA for Ca2+, accelerating relaxation

Explanation: This question tests understanding of ATP's essential role in cross-bridge detachment during the contraction-relaxation cycle. When ATP is depleted to very low levels, myosin heads that are bound to actin cannot detach because ATP binding to myosin is required to break the actin-myosin bond. This creates a rigor state where cross-bridges remain persistently attached, causing high muscle stiffness even when calcium is lowered. The muscle cannot relax because the mechanical linkages between thick and thin filaments cannot be broken without ATP. Option D is incorrect because SERCA activity would be reduced (not increased) with low ATP, and this would impair calcium removal rather than accelerate relaxation. To understand ATP's roles in muscle function, remember it is needed for: cross-bridge detachment, myosin head resetting, and active calcium pumping by SERCA.

Question 16

In a skeletal muscle fiber, a drug selectively blocks the sarcoplasmic reticulum Ca2+-ATPase (SERCA) that pumps Ca2+ from the cytosol back into the SR. After a brief tetanic stimulation, cytosolic free Ca2+ remains elevated for longer than normal (peak ~ 2 µM; decay time constant increases from 50 ms to 300 ms), while ATP remains at 4 mM. The physiological process assessed is Ca2+-dependent relaxation following contraction. Which statement best describes the role of calcium in this process?

  1. Persistently elevated Ca2+ directly shortens thick filaments, increasing the A band length and preventing force generation
  2. Persistently elevated Ca2+ prevents ATP from binding myosin, forcing immediate cross-bridge detachment and faster relaxation
  3. Reduced Ca2+ reuptake increases acetylcholine release, which is the primary determinant of relaxation rate in skeletal muscle
  4. Persistently elevated Ca2+ maintains troponin C occupancy, prolonging exposure of actin binding sites and delaying relaxation (correct answer)

Explanation: This question tests understanding of calcium regulation during muscle relaxation and the role of SERCA pumps. During normal relaxation, SERCA pumps rapidly remove calcium from the cytosol back into the sarcoplasmic reticulum, lowering calcium concentration and allowing troponin C to release calcium. When SERCA is blocked, calcium remains elevated in the cytosol for a prolonged period, maintaining troponin C occupancy and keeping myosin-binding sites on actin exposed. This prolongs the activated state of the muscle, delaying relaxation even though ATP is available for cross-bridge cycling. Option B is incorrect because calcium does not prevent ATP from binding myosin; rather, elevated calcium maintains the thin filament in its activated state. For relaxation questions, remember that both calcium removal (via SERCA) and ATP availability (for cross-bridge detachment) are required for normal muscle relaxation.

Question 17

In an isolated frog skeletal muscle preparation, the motor neuron is stimulated with a single action potential. End-plate recordings show a normal miniature end-plate potential frequency, but the evoked end-plate potential amplitude is reduced after adding a competitive antagonist that binds postsynaptic nicotinic acetylcholine receptors (nAChRs) without opening the channel. Extracellular ionic conditions are held constant (Na+o = 145 mM, K+o = 4.0 mM, Ca2+o = 2.0 mM), temperature is 25°C, and intracellular ATP is not limiting. The physiological process assessed is neuromuscular junction activation leading to excitation–contraction coupling. Which factor is most likely to limit muscle contraction in the described conditions?

  1. Reduced sarcoplasmic reticulum Ca2+ release due to direct inhibition of ryanodine receptors by the antagonist
  2. Decreased postsynaptic depolarization, reducing the probability of reaching threshold for a muscle action potential (correct answer)
  3. Increased acetylcholine breakdown by acetylcholinesterase, preventing vesicle fusion at the presynaptic terminal
  4. Failure of myosin ATPase activity to hydrolyze ATP, preventing cross-bridge detachment

Explanation: This question tests understanding of neuromuscular junction physiology and the role of nicotinic acetylcholine receptors in excitation-contraction coupling. At the neuromuscular junction, acetylcholine binds to postsynaptic nicotinic receptors, causing sodium influx and membrane depolarization that must reach threshold to trigger a muscle action potential. The scenario describes a competitive antagonist blocking these receptors, which would reduce the amplitude of the end-plate potential by preventing acetylcholine from opening ion channels. Since the end-plate potential is smaller, it is less likely to reach the threshold needed to trigger a muscle action potential, thereby limiting subsequent contraction. Option A is incorrect because the antagonist acts at the neuromuscular junction, not directly on ryanodine receptors in the sarcoplasmic reticulum. To approach similar questions, identify where in the excitation-contraction coupling sequence the intervention acts, then trace the downstream effects systematically.

Question 18

A patient with fluctuating muscle weakness is suspected to have myasthenia gravis. In a repetitive nerve stimulation test at 3 Hz, the compound muscle action potential amplitude declines with successive stimuli. Serum testing detects antibodies that accelerate internalization of postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction. Resting membrane potential of the muscle is −90 mV, and extracellular ions are normal (Na+o 145 mM, K+o 4.0 mM, Ca2+o 2.0 mM). The physiological process assessed is neuromuscular transmission leading to muscle fiber action potentials. Which factor is most likely to limit muscle contraction in the described conditions?

  1. Decreased end-plate current, making it less likely that the end-plate potential reaches threshold to trigger a muscle action potential (correct answer)
  2. Increased Ca2+ release from the sarcoplasmic reticulum, causing sustained tetany and fatigue
  3. Failure of smooth muscle calmodulin activation, preventing myosin light-chain phosphorylation
  4. Inhibition of Na+/K+ ATPase in the T-tubule, causing immediate depletion of ATP needed for cross-bridge cycling

Explanation: This question tests understanding of myasthenia gravis pathophysiology and neuromuscular transmission failure. Myasthenia gravis involves antibodies that reduce the number of functional nicotinic acetylcholine receptors at the neuromuscular junction through accelerated internalization. With fewer receptors available, the same amount of acetylcholine release produces a smaller end-plate current and thus a smaller end-plate potential. The decremental response to repetitive stimulation occurs because the reduced safety factor makes it increasingly likely that end-plate potentials will fail to reach threshold for triggering muscle action potentials. Option B is incorrect because the problem is insufficient depolarization at the neuromuscular junction, not excessive calcium release from the sarcoplasmic reticulum. To approach neuromuscular disease questions, consider whether the defect affects presynaptic release, synaptic transmission, or postsynaptic response, then trace the consequences for action potential generation.

Question 19

A single skeletal muscle fiber is stimulated to contract while its length is held constant (isometric). During activation, measured band changes include: A band remains constant at 1.6 µm; I band decreases from 0.6 µm to 0.3 µm; H zone decreases from 0.2 µm to ~0 µm. Cytosolic free Ca2+ rises from 0.1 µM to 1.0 µM, and ATP is 5.0 mM. The physiological process assessed is force generation explained by the sliding filament theory. Based on the scenario, which outcome is most consistent with the sliding filament theory?

  1. Thick filaments shorten during contraction, decreasing the A band length in proportion to force
  2. Thin filaments slide toward the M line, increasing overlap with thick filaments while filament lengths remain constant (correct answer)
  3. Z lines move apart because actin polymerizes at the Z disc during activation, increasing I band width
  4. Cross-bridges form only after ATP binds actin, so lowering ATP would increase overlap and increase the I band

Explanation: This question tests understanding of the sliding filament theory through analysis of sarcomere band patterns during isometric contraction. The data shows classic changes consistent with sliding filaments: the A band (thick filament length) remains constant at 1.6 μm, while the I band (thin filament only regions) decreases and the H zone (thick filament only regions) nearly disappears. These changes occur because thin filaments slide toward the M line, increasing their overlap with thick filaments while both filament types maintain constant length. The decrease in I band and H zone with constant A band is the hallmark evidence for the sliding filament mechanism. Option A is incorrect because thick filaments do not shorten; the constant A band measurement directly contradicts this. When analyzing sarcomere data, use the principle that A bands track thick filament length (constant) while I and H zones reflect the degree of thick-thin filament overlap.

Question 20

A permeabilized skeletal muscle fiber is placed in solutions with controlled free cytosolic Ca2+ while ATP is maintained at 5.0 mM and Mg2+ at 1.0 mM. At baseline (pCa 8; [Ca2+]=10−8[\mathrm{Ca^{2+}}] = 10^{-8} M), the fiber generates negligible tension. When free Ca2+ is increased to pCa 5 ([Ca2+]=10−5[\mathrm{Ca^{2+}}] = 10^{-5} M), tension rises rapidly without changing sarcomere length. The physiological process assessed is Ca2+-regulated activation of the thin filament during contraction. Which statement best describes the role of calcium in this process?

  1. Ca2+ binds to tropomyosin, causing it to detach from actin and shorten the thick filament
  2. Ca2+ binds directly to myosin heads to initiate ATP hydrolysis and the power stroke
  3. Ca2+ activates acetylcholinesterase at the neuromuscular junction to prolong end-plate depolarization
  4. Ca2+ binds to troponin C, shifting tropomyosin to expose myosin-binding sites on actin (correct answer)

Explanation: This question tests understanding of calcium's role in regulating skeletal muscle contraction at the molecular level. In skeletal muscle, calcium binds to troponin C on the thin filament, causing a conformational change that shifts tropomyosin away from myosin-binding sites on actin. The experiment uses a permeabilized fiber where calcium concentration can be directly controlled, bypassing the neuromuscular junction and membrane depolarization steps. When calcium increases from 10^-8 M to 10^-5 M, it binds troponin C and exposes actin binding sites, allowing myosin heads to bind and generate force. Option B is incorrect because calcium does not bind directly to myosin heads; rather, myosin ATPase activity is constitutive when binding sites are available. For similar questions, remember that calcium's primary regulatory role in skeletal muscle is through the troponin-tropomyosin complex on thin filaments, not through direct effects on thick filaments.