Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 201: Which of the following are properties of fast twitch muscle fibers?

A. Abundant mitochondria
B. Extensive sarcoplasmic reticulum (Correct Answer)
C. Extensive blood supply
D. Large glycolytic pathway

Explanation: ### Explanation Skeletal muscle fibers are classified into **Type I (Slow-twitch)** and **Type II (Fast-twitch)** based on their metabolic and contractile properties. **Why Option B is Correct:** Fast-twitch fibers (Type II) are designed for rapid, powerful bursts of activity. To achieve high-speed contraction, they require a rapid release and sequestration of calcium ions ($Ca^{2+}$). This is facilitated by an **extensive sarcoplasmic reticulum (SR)** and highly developed T-tubules. The abundance of $Ca^{2+}$-ATPase pumps in the SR allows for the quick termination of contraction, enabling high-frequency stimulation. **Analysis of Incorrect Options:** * **A & C (Abundant mitochondria & Extensive blood supply):** These are characteristics of **Type I (Slow-oxidative)** fibers. These fibers rely on aerobic metabolism for sustained activity (e.g., posture) and thus require high oxygen delivery and mitochondrial density. Fast-twitch fibers have fewer mitochondria and a lower capillary density, making them appear "white." * **D (Large glycolytic pathway):** While Fast-twitch fibers (Type IIb) *do* rely heavily on glycolysis, the question asks for the "most" defining structural property among the choices provided in standard physiological texts (like Guyton). However, in many competitive contexts, if "Extensive SR" is the marked key, it highlights the structural adaptation for speed rather than just the metabolic pathway. *Note: In some classifications, both B and D are true, but the SR development is the hallmark of "fast" contractile mechanics.* **High-Yield Clinical Pearls for NEET-PG:** * **Type I (Red):** "One Slow Red Ox" — Type **I**, **Slow**-twitch, **Red** (high myoglobin), **Ox**idative (high mitochondria). * **Type II (White):** Fast-twitch, high glycogen content, high myosin ATPase activity, prone to fatigue. * **Myosin ATPase:** The velocity of contraction is directly proportional to the Myosin ATPase activity of the fiber. * **Back Muscles:** Predominantly Type I (postural). * **Extraocular Muscles:** Predominantly Type II (rapid movement).

Question 202: Compression of a nerve leading to temporary paraesthesia suggests the involvement of which type of nerve fiber?

A. Aα (Correct Answer)
B. Aδ
C. C
D. B

Explanation: This question tests your knowledge of **Erlanger-Gasser classification** and the susceptibility of different nerve fibers to external stressors like pressure, hypoxia, and local anesthetics [1]. ### **Explanation** The correct answer is **Aα** because of the differential sensitivity of nerve fibers to pressure. According to the susceptibility rules: * **Pressure/Compression:** Large-diameter, myelinated fibers are affected first. **Type A** fibers (especially Aα) are the most sensitive to mechanical compression [1]. * **Hypoxia:** Type B fibers are the most sensitive [1]. * **Local Anesthetics:** Small-diameter, unmyelinated fibers are affected first. **Type C** fibers are the most sensitive [1]. When a nerve is compressed (e.g., "Saturday Night Palsy" or a limb "falling asleep"), the large Aα fibers—which carry motor and proprioceptive signals—are blocked first, leading to motor weakness and the tingling sensation known as **paraesthesia**. ### **Analysis of Incorrect Options** * **B (Aδ):** These are smaller myelinated fibers responsible for fast pain and temperature [3]. While they are affected by pressure, they are less sensitive than the larger Aα fibers. * **C (C fibers):** These are the smallest, unmyelinated fibers carrying slow pain [2]. They are the **least sensitive** to pressure but the **most sensitive** to local anesthesia. * **D (B fibers):** These are preganglionic autonomic fibers. They are the most sensitive to **hypoxia**, not pressure [1]. ### **NEET-PG High-Yield Pearls** * **Order of Susceptibility to Pressure:** A > B > C (Large to Small) [1]. * **Order of Susceptibility to Local Anesthesia:** C > B > A (Small to Large) [1]. * **Order of Susceptibility to Hypoxia:** B > A > C [1]. * **Aα fibers** are the thickest, have the fastest conduction velocity, and are primarily involved in somatic motor function and proprioception [1],[2].

Question 203: Maximum number of Na+ channels per square micrometer is present in which of the following locations?

A. Cell body
B. Axon terminal
C. Surface of myelin
D. Nodes of Ranvier (Correct Answer)

Explanation: ### Explanation The density of voltage-gated sodium (Na+) channels is the primary determinant of the threshold for excitation and the velocity of impulse conduction. **Why Nodes of Ranvier is correct:** In myelinated neurons, the **Nodes of Ranvier** contain the highest concentration of voltage-gated Na+ channels, estimated at **2,000 to 12,000 per μm²**. This extreme density is essential for **saltatory conduction**, allowing the action potential to "jump" from node to node. This mechanism ensures rapid signal transmission while conserving energy, as depolarization is restricted to these small gaps rather than the entire axonal membrane. **Why the other options are incorrect:** * **Cell body (Soma):** The density here is relatively low (approx. 50–75 per μm²), as the soma is primarily involved in metabolic functions rather than rapid signal propagation. * **Axon terminal:** While it contains Na+ channels, the terminal is more densely packed with voltage-gated **calcium (Ca²+) channels** to facilitate neurotransmitter release. * **Surface of myelin:** Myelin acts as an insulator. The axonal membrane underneath the myelin sheath is nearly devoid of Na+ channels; instead, it contains potassium (K+) channels in the juxtaparanodal regions. **High-Yield Clinical Pearls for NEET-PG:** * **Axon Hillock:** This is the site where the action potential is typically **initiated** because it has the lowest threshold for excitation (density ~350–500 per μm²). * **Demyelinating Diseases:** In conditions like **Multiple Sclerosis**, the loss of myelin exposes the internodal membrane (which lacks Na+ channels), leading to conduction block or slowing. * **Unmyelinated Axons:** The Na+ channel density is uniform but low (approx. 110 per μm²) compared to the Nodes of Ranvier.

Question 204: Which nerve fibers are involved in proprioception?

A. Type A fiber (Correct Answer)
B. Type B fiber
C. Type C fiber
D. Type IV fiber

Explanation: **Explanation:** The Erlanger-Gasser classification categorizes nerve fibers based on their diameter, myelination, and conduction velocity. **Type A fibers** are large, myelinated fibers with the fastest conduction velocities. They are further subdivided into alpha ($\alpha$), beta ($\beta$), gamma ($\gamma$), and delta ($\delta$). **Type A-alpha ($\alpha$)** fibers are specifically responsible for **proprioception** (position sense) and somatic motor function. They have the largest diameter and highest velocity, which is essential for the brain to receive instantaneous feedback regarding muscle length and joint position to maintain balance and coordinate movement. **Analysis of Incorrect Options:** * **Type B fibers:** These are medium-sized, myelinated preganglionic autonomic fibers. They are primarily involved in autonomic functions, not sensory perception like proprioception. * **Type C fibers:** These are small, unmyelinated fibers with the slowest conduction velocity. They transmit "slow" pain, temperature, and postganglionic autonomic signals. * **Type IV fibers:** In the Lloyd-Hunt classification (used for sensory fibers), Type IV is synonymous with Type C fibers, carrying slow pain and itch sensations. **High-Yield NEET-PG Pearls:** * **Fastest to Slowest:** Type A$\alpha$ > A$\beta$ > A$\gamma$ > A$\delta$ > B > C. * **Susceptibility to Block:** * **Hypoxia:** Type A fibers are most sensitive. * **Pressure:** Type A fibers are most sensitive (e.g., "Saturday Night Palsy"). * **Local Anesthetics:** Type C fibers are most sensitive (size-dependent block). * **Proprioception Receptors:** Muscle spindles (Ia) and Golgi tendon organs (Ib) both utilize Type A$\alpha$ fibers.

Question 205: What is the ratio of actin : troponin : tropomyosin molecules in a thin filament?

A. 1:1:1
B. 1:3:5
C. 1:7:7
D. 7:1:1 (Correct Answer)

Explanation: ### Explanation **1. Understanding the Correct Answer (D: 7:1:1)** The thin filament of skeletal muscle is a complex polymer primarily composed of **F-actin**, **tropomyosin**, and the **troponin complex**. * **Actin:** Individual G-actin molecules polymerize to form two twisting strands of F-actin. * **Tropomyosin:** These are long, rod-like proteins that lie in the grooves of the actin helix. Each tropomyosin molecule covers exactly **7 actin monomers**. * **Troponin:** This is a complex of three subunits (I, T, and C) attached to one end of each tropomyosin molecule. Because one troponin complex and one tropomyosin molecule are associated with a specific segment of seven actin molecules, the functional unit ratio is **7 Actin : 1 Troponin : 1 Tropomyosin**. **2. Why Other Options are Incorrect** * **Option A (1:1:1):** This would imply a 1:1 stoichiometric relationship, which is physically impossible given that tropomyosin is a long fibrous protein spanning multiple actin globular heads. * **Option B (1:3:5):** These numbers do not correspond to any known structural arrangement in the sarcomere. * **Option C (1:7:7):** This incorrectly suggests there are seven troponin and tropomyosin molecules for every one actin molecule. The ratio is the inverse. **3. High-Yield Clinical Pearls for NEET-PG** * **Troponin Subunits:** * **Troponin T:** Binds to **T**ropomyosin. * **Troponin I:** **I**nhibits the ATPase activity of the actin-myosin interaction. * **Troponin C:** Binds to **C**alcium (requires 4 $Ca^{2+}$ ions to initiate contraction). * **Clinical Marker:** Cardiac Troponin I and T are highly specific markers for **Myocardial Infarction (MI)** because they are released into the blood when cardiac myocytes are damaged. * **The "Relaxed" State:** In a resting muscle, tropomyosin physically blocks the myosin-binding sites on the 7 actin molecules. Contraction only occurs when $Ca^{2+}$ binds to Troponin C, causing a conformational shift that moves tropomyosin out of the way.

Question 206: A single contraction of skeletal muscle is most likely to be terminated by which of the following actions?

A. Closure of the postsynaptic nicotinic acetylcholine receptor
B. Removal of acetylcholine from the neuromuscular junction
C. Removal of Ca++ from the terminal of the motor neuron
D. Removal of sarcoplasmic Ca++ (Correct Answer)

Explanation: ### Explanation **1. Why "Removal of sarcoplasmic Ca++" is Correct:** The termination of a skeletal muscle contraction (relaxation) is primarily dependent on the sequestration of calcium ions ($Ca^{2+}$) from the sarcoplasm back into the **Sarcoplasmic Reticulum (SR)**. This process is mediated by the **SERCA pump** (Sarco-Endoplasmic Reticulum Calcium ATPase). When cytosolic $Ca^{2+}$ levels drop, $Ca^{2+}$ dissociates from **Troponin C**. This allows the troponin-tropomyosin complex to return to its inhibitory state, physically blocking the myosin-binding sites on the actin filament, thereby ending the cross-bridge cycle. **2. Why the Other Options are Incorrect:** * **Options A & B:** While the closure of nicotinic receptors and the degradation of Acetylcholine (by Acetylcholinesterase) are essential to stop the *initiation* of new action potentials, they do not directly terminate a contraction that is already in progress. The "contractile machinery" continues as long as $Ca^{2+}$ is present in the sarcoplasm. * **Option C:** Removal of $Ca^{2+}$ from the motor neuron terminal prevents the *release* of neurotransmitters, thus preventing future contractions, but it does not stop the current contraction occurring within the muscle fiber itself. **3. High-Yield NEET-PG Pearls:** * **SERCA Pump:** This is an active transport mechanism (uses ATP). Therefore, **relaxation is an active process.** * **Rigor Mortis:** Occurs because the lack of ATP prevents the SERCA pump from removing $Ca^{2+}$ and prevents the detachment of myosin heads from actin. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR1)**, leading to excessive $Ca^{2+}$ release and sustained muscle contraction/heat production. * **Calsequestrin:** A protein within the SR that binds $Ca^{2+}$, allowing the SR to store high concentrations of calcium at low osmotic pressure.

Question 207: In a myelinated nerve fiber, if the refractory period is 1/2500 seconds, what is the maximum impulse rate per second?

A. 40 per sec
B. 250 per sec
C. 400 per sec
D. 2500 per sec (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** The maximum frequency of nerve impulses is determined by the **Absolute Refractory Period (ARP)**. During this period, the voltage-gated sodium channels are either already open or in an inactivated state, making it physiologically impossible for the nerve to fire another action potential, regardless of the stimulus strength. To calculate the maximum impulse rate (frequency), we use the formula: $$\text{Maximum Frequency} = \frac{1}{\text{Refractory Period (in seconds)}}$$ Given the refractory period is $1/2500$ seconds: $$\text{Frequency} = \frac{1}{1/2500} = 2500 \text{ impulses per second.}$$ **2. Why Other Options are Incorrect:** * **Option A (40 per sec):** This would correspond to a very long refractory period of $1/40$ sec (25 ms), which is more characteristic of cardiac muscle than a myelinated nerve fiber. * **Option B (250 per sec):** This would require a refractory period of $4$ ms ($1/250$ sec). While some slow fibers have this, it does not match the value provided in the question. * **Option C (400 per sec):** This would result from a refractory period of $2.5$ ms ($1/400$ sec). **3. Clinical Pearls & High-Yield Facts:** * **Myelination & Speed:** Myelination increases conduction velocity via **Saltatory Conduction** (jumping from one Node of Ranvier to the next) but does not directly dictate the refractory period; the density and kinetics of Na+ channels do. * **ARP vs. RRP:** During the *Absolute* Refractory Period, no stimulus can trigger an AP. During the *Relative* Refractory Period (RRP), a suprathreshold stimulus can trigger an AP, but the resulting impulse has a lower amplitude. * **Accommodation:** If a nerve is subjected to a slowly increasing constant current, the threshold for firing rises; this is known as accommodation, caused by the slow inactivation of Na+ channels. * **Fiber Types:** Type A fibers (large, myelinated) have the shortest refractory periods and highest conduction velocities, allowing for high-frequency signaling.

Question 208: Size of action potential is decreased as a result of?

A. Lower extracellular sodium (Correct Answer)
B. Raised extracellular calcium
C. Lower extracellular calcium
D. Raised extracellular sodium

Explanation: **Explanation:** The size (amplitude) of an action potential is primarily determined by the **electrochemical gradient of Sodium (Na⁺)**. During the depolarization phase, voltage-gated Na⁺ channels open, allowing Na⁺ to rush into the cell. This influx continues until the membrane potential approaches the **Equilibrium Potential for Sodium (E_Na)**, which is approximately +60 mV. **1. Why Option A is Correct:** When **extracellular sodium concentration is lowered**, the concentration gradient between the outside and inside of the cell decreases. According to the Nernst equation, this reduces the Equilibrium Potential for Sodium. Consequently, less Na⁺ enters the cell during depolarization, leading to a **lower peak (decreased amplitude)** of the action potential. **2. Why the other options are incorrect:** * **Option B & C (Extracellular Calcium):** Calcium levels primarily affect the **threshold** for firing an action potential, not its size. Low Ca²⁺ (Hypocalcemia) makes the cell more excitable (lowers threshold), while high Ca²⁺ (Hypercalcemia) stabilizes the membrane (raises threshold). * **Option D (Raised extracellular sodium):** Increasing extracellular Na⁺ would increase the concentration gradient, theoretically increasing the amplitude of the action potential (though physiological limits exist). **High-Yield Clinical Pearls for NEET-PG:** * **Amplitude vs. Velocity:** The *size* of the action potential depends on Na⁺ concentration, but the *conduction velocity* depends on myelination and axon diameter. * **All-or-None Law:** While an action potential follows the all-or-none law for a given set of conditions, changing the ionic environment (like hyponatremia) alters the "all" level. * **Hypokalemia:** Primarily affects the **Resting Membrane Potential (RMP)** by making it more negative (hyperpolarization), making it harder to initiate an action potential.

Question 209: What is the location of the lowest threshold potential in a motor nerve fiber?

A. Dendrite
B. Soma (cell body)
C. Axon hillock (Correct Answer)
D. Axon

Explanation: **Explanation:** The **Axon Hillock** (specifically the initial segment) is the site where the action potential is generated in a motor neuron. It possesses the **lowest threshold potential** because it contains the **highest density of voltage-gated sodium (Na+) channels** per unit area. Since the threshold for firing depends on the density of these channels, the axon hillock requires the least amount of depolarization (approximately -10 to -15 mV from resting membrane potential) to trigger an all-or-nothing response. **Analysis of Options:** * **Dendrites (A):** These are primarily involved in receiving inputs (EPSPs and IPSPs). They generally have a low density of voltage-gated Na+ channels, making them poor at generating action potentials; they conduct signals decrementally. * **Soma/Cell Body (B):** While the soma can conduct electrical impulses, its threshold is significantly higher than the axon hillock due to a lower concentration of Na+ channels. * **Axon (D):** Once the action potential is generated at the hillock, it is propagated along the axon. While the axon has enough channels to maintain the signal, the density is lower than at the initial segment/hillock. **High-Yield NEET-PG Pearls:** * **Trigger Zone:** The axon hillock is often referred to as the "integrative center" or "trigger zone" of the neuron. * **Threshold Values:** The typical threshold at the hillock is about **-55 mV**, whereas the rest of the soma may require depolarization to **-35 mV** to fire. * **Safety Factor:** The high density of Na+ channels at the hillock ensures a high "safety factor" for signal transmission from the cell body to the axon.

Question 210: A 45-year-old lady suffers from botulism poisoning. This toxin blocks the release of acetylcholine. What is the primary effect of this toxin on synaptic transmission?

A. Inhibition of adrenergic synapse transmission
B. Facilitation of adrenergic synapse transmission
C. Inhibition of cholinergic synapse transmission (Correct Answer)
D. No effect on synaptic transmission

Explanation: ### Explanation **Core Concept: Mechanism of Botulinum Toxin** Botulinum toxin, produced by the bacterium *Clostridium botulinum*, is a potent neurotoxin that targets the **SNARE proteins** (specifically synaptobrevin, SNAP-25, and syntaxin) at the presynaptic nerve terminal. These proteins are essential for the docking and fusion of synaptic vesicles with the presynaptic membrane. By cleaving these proteins, the toxin prevents the exocytosis of **Acetylcholine (ACh)** into the synaptic cleft. Since ACh is the primary neurotransmitter for cholinergic synapses (including the neuromuscular junction and autonomic ganglia), the toxin results in the **inhibition of cholinergic synapse transmission**, leading to flaccid paralysis. **Analysis of Options:** * **Option A & B (Adrenergic Synapses):** Adrenergic transmission involves the release of norepinephrine/epinephrine. Botulinum toxin specifically targets cholinergic terminals; it does not significantly interfere with the release of catecholamines at adrenergic synapses. * **Option D (No effect):** This is incorrect because the toxin causes a profound blockade of neurotransmission, which is clinically manifested as life-threatening respiratory failure and paralysis. **High-Yield Clinical Pearls for NEET-PG:** * **Target Protein:** Cleaves **SNARE proteins**, preventing vesicle fusion. * **Clinical Presentation:** Characterized by the "4 Ds": Diplopia, Dyspharthria, Dysphagia, and Dyspnea, followed by **symmetric descending flaccid paralysis**. * **Therapeutic Uses:** In small, controlled doses (Botox), it is used for conditions like achalasia cardia, strabismus, blepharospasm, and cosmetic wrinkle reduction. * **Contrast with Tetanus:** While both cleave SNARE proteins, Tetanus toxin travels retrogradely to the CNS to inhibit GABA/Glycine release (inhibitory neurons), causing **spastic** paralysis, whereas Botulinum acts peripherally causing **flaccid** paralysis.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

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Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

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Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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