Muscle Metabolism and Fatigue — MCQs
Which of the following statements is NOT true regarding red muscle fibers?
What are the products of the isocitrate to α-ketoglutarate conversion in the TCA cycle?
Which of the following processes primarily utilizes lactate produced anaerobically?
In severe exercise, decrease in pH is due to:
In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?
The most significant immediate result of lowered serum calcium is
Protein connecting Z-lines to M-lines is:
Most common nerve used for nerve conduction study in H reflex -
Inverse stretch reflex is mediated :
What is the primary site of action of tetanospasmin in the nervous system?
Muscle Metabolism and Fatigue Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following statements is NOT true regarding red muscle fibers?
- A. Decreased glycolytic enzymes
- B. Used for aerobic activity
- C. Increased blood flow
- D. Increased muscle fiber length (Correct Answer)
Explanation: ***Increased muscle fiber length*** - The length of muscle fibers is generally determined by the muscle's anatomical structure and function, not by whether they are red or white fibers. - While red muscle fibers (slow-twitch) are optimized for **endurance** and **sustained contractions**, this optimization does not involve an inherent increase in the length of individual muscle fibers. *Decreased glycolytic enzymes* - Red muscle fibers, also known as slow-oxidative fibers, primarily rely on **aerobic metabolism** for ATP production. - They have a lower content of glycolytic enzymes compared to white muscle fibers, which are specialized for **anaerobic glycolysis**. *Increased blood flow* - Red muscle fibers have a rich capillary supply, leading to **increased blood flow**, which is essential for delivering oxygen and nutrients for sustained aerobic activity. - This extensive vascularization contributes to their characteristic red appearance and their ability to resist fatigue. *Used for aerobic activity* - Red muscle fibers are well-suited for prolonged, low-intensity activities due to their high concentration of **mitochondria**, myoglobin, and oxidative enzymes. - They are primarily responsible for maintaining posture and performing **endurance activities** such as long-distance running.
Question 2: What are the products of the isocitrate to α-ketoglutarate conversion in the TCA cycle?
- A. GTP, CO2
- B. NADPH, H2O
- C. FADH2, ATP
- D. NADH, CO2 (Correct Answer)
Explanation: ***NADH, CO2*** - The conversion of **isocitrate to α-ketoglutarate** is an oxidative decarboxylation step catalyzed by **isocitrate dehydrogenase**. - This reaction produces **NADH** (from NAD+) and **carbon dioxide (CO2)**, as a carbon atom is lost. *GTP, CO2* - **GTP** is produced during the conversion of **succinyl-CoA to succinate** in a substrate-level phosphorylation step, not during the isocitrate to α-ketoglutarate conversion. - While CO2 is produced in the latter, GTP is not. *NADPH, H2O* - **NADPH** is primarily generated in the **pentose phosphate pathway** and is used for reductive biosynthesis, not directly produced in the TCA cycle. - **H2O** is consumed or produced in other steps of the TCA cycle but not as a direct product of this specific reaction. *FADH2, ATP* - **FADH2** is produced during the conversion of **succinate to fumarate** by succinate dehydrogenase. - **ATP** (or GTP which can be converted to ATP) is produced in the succinyl-CoA to succinate step, not at the isocitrate dehydrogenase step.
Question 3: Which of the following processes primarily utilizes lactate produced anaerobically?
- A. Cori cycle (Correct Answer)
- B. Gluconeogenesis
- C. TCA cycle
- D. Glycolysis
Explanation: ***Cori cycle*** - The **Cori cycle** (lactic acid cycle) involves the transport of **lactate** produced during anaerobic metabolism in muscles to the liver. - In the **liver**, this lactate is then converted back to **glucose** via gluconeogenesis, which can be returned to the muscles. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of glucose from non-carbohydrate precursors, one of which is lactate. - While it uses lactate, it is only one component of the broader **Cori cycle**, which describes the inter-organ cooperation. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, which can then be converted to lactate under anaerobic conditions. - This process *produces* lactate but does not *utilize* it, acting upstream of lactate production. *TCA cycle* - The **TCA cycle** (Krebs cycle) is a central part of aerobic respiration that oxidizes acetyl-CoA to produce ATP, NADH, and FADH2. - It does not directly utilize lactate; instead, lactate is typically converted to pyruvate before potentially entering the TCA cycle under aerobic conditions.
Question 4: In severe exercise, decrease in pH is due to:
- A. Respiratory acidosis
- B. H+ retention
- C. HCO3 excretion
- D. Lactic acidosis (Correct Answer)
Explanation: ***Lactic acidosis*** - During **severe exercise**, particularly anaerobic activity, muscles produce **lactic acid** secondary to **anaerobic glycolysis**. - **Lactic acid** dissociates into **lactate** and **hydrogen ions (H+)**, leading to an increase in H+ concentration and a decrease in pH. *Respiratory acidosis* - **Respiratory acidosis** results from **hypoventilation**, leading to CO2 retention and an increase in carbonic acid, which lowers pH. - During severe exercise, individuals typically **hyperventilate** to increase oxygen intake and expel CO2, thus preventing respiratory acidosis. *H+ retention* - **H+ retention** would imply that the body is failing to excrete hydrogen ions. While an accumulation of H+ ions does occur, it's primarily due to their overproduction (e.g., from lactic acid) rather than a simple failure of excretion mechanisms at the systemic level during exercise. - The mechanism is direct production, not just failure to excrete. *HCO3 excretion* - **Bicarbonate (HCO3-)** is a crucial buffer in the blood that helps maintain pH. Its excretion would reduce buffering capacity. - However, in cases of metabolic acidosis (like lactic acidosis), the body tries to **conserve** HCO3- or uses it to buffer excess H+ ions, rather than excrete it, until its stores are depleted.
Question 5: In the context of muscle physiology, which structure is described as a threadlike component that extends along the length of a muscle fiber?
- A. Sarcomere
- B. Sarcolemma
- C. Myofibril (Correct Answer)
- D. Myofilament
Explanation: ***Myofibril*** - A **myofibril** is a cylindrical organelle that runs longitudinally inside a muscle fiber and contains **contractile proteins**. - Myofibrils are composed of repeating units called **sarcomeres**, which are the fundamental units of muscle contraction. *Sarcomere* - A **sarcomere** is the basic contractile unit of a muscle fiber, extending from one Z-disc to the next. - While it is a key component for muscle contraction, it is a **segment within a myofibril**, not the threadlike component that extends the entire length of the fiber. *Sarcolemma* - The **sarcolemma** is the cell membrane of a muscle fiber, responsible for transmitting nerve impulses to the muscle cell. - It encloses the muscle fiber but is not an internal, threadlike contractile component. *Myofilament* - **Myofilaments** are the individual protein filaments (actin and myosin) that make up a sarcomere within a myofibril. - They are the **smallest contractile elements**, but they are not the threadlike structure that extends along the entire muscle fiber.
Question 6: The most significant immediate result of lowered serum calcium is
- A. Decalcification of bones
- B. Decalcification of teeth
- C. Weakened heart action
- D. Hyperirritability of nerves and muscles (Correct Answer)
Explanation: ***Hyperirritability of nerves and muscles*** - Lowered serum calcium (hypocalcemia) decreases the threshold potential of excitable cells, leading to **increased neuronal and muscular excitability**. - This can manifest as **tetany**, muscle cramps, paresthesias, and in severe cases, seizures. *Decalcification of bones* - **Chronic hypocalcemia** can lead to secondary hyperparathyroidism, which may eventually cause bone decalcification. - This is a **long-term effect**, not an immediate significant result of acutely lowered serum calcium. *Decalcification of teeth* - Tooth decalcification is primarily associated with **fluoride deficiency**, poor oral hygiene, or acidic erosion, not directly with acute systemic hypocalcemia. - The calcium in teeth is **highly stable** and less readily mobilized than bone calcium in response to acute serum calcium changes. *Weakened heart action* - While severe **hypocalcemia can impair myocardial contractility** and lead to a weakened heart action, it is often preceded or accompanied by significant neuromuscular symptoms. - **Hyperkalemia** is more classically associated with immediate life-threatening cardiac dysfunction, while hypocalcemia primarily affects nerve and muscle excitability first.
Question 7: Protein connecting Z-lines to M-lines is:
- A. Kinin
- B. Desmin
- C. Titin (Correct Answer)
- D. Actin
Explanation: ***Titin*** - **Titin** is a giant protein that extends from the **Z-disc to the M-line** in the sarcomere, acting as a molecular spring. - It maintains the **structural integrity** of the sarcomere and provides passive elasticity to muscles. *Kinin* - **Kinin** is a protein involved in **inflammation and blood pressure regulation**, not a structural component of muscle sarcomeres. - Examples include **bradykinin**, which mediates pain and vasodilation. *Desmin* - **Desmin** is an **intermediate filament protein** that forms a scaffold around the Z-discs, linking myofibrils together. - While it connects myofibrils, it does not directly span between the Z-line and M-line within a single sarcomere. *Actin* - **Actin** is a primary component of **thin filaments** in the sarcomere and participates in muscle contraction, but it does not connect the Z-line to the M-line. - Thin filaments are anchored at the **Z-disc** but only extend partway into the A-band.
Question 8: Most common nerve used for nerve conduction study in H reflex -
- A. Median nerve
- B. Tibial nerve (Correct Answer)
- C. Peroneal nerve
- D. Ulnar nerve
Explanation: ***Tibial nerve*** - The **tibial nerve** is the most commonly used nerve for H-reflex studies because it reliably activates the **soleus muscle's Ia afferents**, which are crucial for generating a strong and measurable H-reflex. - The H-reflex of the **tibial nerve** is the most robust and easily elicitable, making it a standard for assessing **monosynaptic reflex arcs** and **S1 radiculopathy**. *Median nerve* - While the **median nerve** can elicit an H-reflex, it is less common and harder to obtain consistently compared to the tibial nerve due to its smaller Ia afferent input to distal muscles. - H-reflexes in the upper limbs, including those of the median nerve, are generally smaller and more difficult to interpret clinically. *Peroneal nerve* - The **peroneal nerve** is primarily a motor nerve to the anterior and lateral compartments of the leg; it does not produce a significant or clinically useful H-reflex. - An H-reflex is typically an electromyographic response to stimulation of **Ia afferent fibers**, and the peroneal nerve lacks the prominent Ia afferent pool necessary for a robust reflex. *Ulnar nerve* - Similar to the median nerve, the **ulnar nerve** can produce an H-reflex, but it is typically small and less reliable than the tibial nerve due to fewer Ia afferent fibers involved in a monosynaptic arc compared to the soleus. - Its clinical utility for H-reflex studies is limited due to the difficulty in eliciting a consistent and large response.
Question 9: Inverse stretch reflex is mediated :
- A. Unmyelinated C fibres
- B. Dorsal Column
- C. Muscle spindle
- D. Golgi tendon organ (Correct Answer)
Explanation: ***Golgi tendon*** - The **Golgi tendon organ (GTO)** is a **proprioceptor** located at the junction of muscle fibers and tendons, sensitive to changes in muscle tension. - When muscle tension becomes excessive, the GTO is activated, inhibiting the alpha motor neurons innervating that muscle, leading to muscle relaxation, which is the **inverse stretch reflex**. *Unmyelinated C fibres* - These fibers are primarily involved in transmitting **slow, dull pain** and **temperature sensations**, but not proprioceptive reflexes. - Their conduction velocity is much slower than that required for rapid protective reflexes. *Dorsal Column* - The dorsal column-medial lemniscus pathway is responsible for transmitting **fine touch, vibration, and proprioception** to the brain, but it is an ascending sensory pathway and does not directly mediate spinal reflexes. - This pathway is involved in conscious perception, not the direct arc of a reflex. *Muscle spindle* - The **muscle spindle** is responsible for the **stretch reflex** (myotatic reflex), which causes muscle contraction in response to stretch. - It detects changes in **muscle length and rate of change of length**, which is distinct from the inverse stretch reflex mediated by the GTO.
Question 10: What is the primary site of action of tetanospasmin in the nervous system?
- A. Presynaptic terminals of the spinal cord (Correct Answer)
- B. Postsynaptic terminals of the spinal cord
- C. Neuromuscular junction
- D. Muscle fibers
Explanation: ***Presynaptic terminals of the spinal cord*** - **Tetanospasmin** is transported via **retrograde axonal transport** to the central nervous system, specifically targeting the **presynaptic terminals** of inhibitory interneurons in the spinal cord. - It interferes with the release of **inhibitory neurotransmitters** like **GABA** and **glycine**, leading to uncontrolled muscle spasms. *Postsynaptic terminals of the spinal cord* - This is incorrect because tetanospasmin acts by preventing the release of inhibitory neurotransmitters from the **presynaptic terminal**, rather than directly affecting the postsynaptic receptor. - While the absence of inhibition is perceived at the postsynaptic terminal, the direct mechanism of action is presynaptic. *Neuromuscular junction* - This is incorrect because **tetanospasmin** does not primarily act at the neuromuscular junction; that is the site of action for toxins like **botulinum toxin**. - Tetanospasmin is transported to the central nervous system to exert its effects. *Muscle fibers* - This is incorrect as **tetanospasmin** does not act directly on muscle fibers. - Its action is on the **nervous system**, leading to altered neuronal signaling that indirectly affects muscle contraction.
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