Nerve and Muscle Physiology Practice Questions

Q: Arrange the following nerve fibers sequentially in the descending order of nerve impulse transmission velocity: A. C fiber B. Aa fiber C. B fiber D. Ad fiber

B-D-C-A. **Explanation** The velocity of nerve impulse transmission is primarily determined by two factors: **fiber diameter** and the **presence of myelin**. According to the Erlanger-Gasser classification, thicker and more heavily myelinated fibers conduct impulses faster than thinner, unmyelinated ones. **The Correct Sequence (Descending Order):** 1. **Aα (Alpha) fiber:** The thickest (12–20 µm) and most heavily myelinated; conducts at 70–120 m/s. 2. **Aδ (Delta) fiber:** Thinner than Aα (2–5 µm) but still myelinated; conducts at 12–30 m/s. 3. **B fiber:** Small, preganglionic autonomic fibers ( Aδ > B > C**, which corresponds to sequence **B-D-C-A**. **Analysis of Options:** * **Option B (Correct):** Correctly follows the hierarchy of diameter and myelination. * **Options A, C, and D:** These are incorrect because they place C fibers (slowest) or B fibers (intermediate) ahead of A-group fibers in velocity. **High-Yield NEET-PG Pearls:** * **Sensitivity to Local Anesthetics:** The order of blockade is **B > C > Aδ > Aγ > Aβ > Aα**. Note that B fibers are blocked first despite being myelinated because of their small diameter and anatomy. * **Sensitivity to Pressure:** A fibers are most sensitive (e.g., "limb falling asleep"). * **Sensitivity to Hypoxia:** B fibers are most sensitive. * **Function of C fibers:** They carry "slow pain" (dull, aching), temperature, and postganglionic sympathetic signals. Aδ fibers carry "fast pain" (sharp, localized).

Q: Which of the following best describes muscle B compared with muscle A?

Innervated by smaller nerve fibers. ***Innervated by smaller nerve fibers*** - According to **Henneman's size principle**, slow-twitch (Type I) muscle fibers are innervated by **smaller motor neurons** with smaller axon diameters. - Type I fibers require **lower threshold stimulation** and are recruited first during muscle activation, consistent with smaller nerve fiber innervation. *Adapted for rapid contraction* - This describes **fast-twitch (Type II) fibers**, not slow-twitch fibers; Type I fibers contract **slowly** and are built for endurance. - Fast-twitch fibers have rapid **calcium release and reuptake** mechanisms, enabling quick contraction-relaxation cycles. *Composed of larger muscle fibers* - **Type II (fast-twitch) fibers** are typically larger in diameter to generate more force quickly. - Type I fibers are generally **smaller** as they prioritize sustained activity over peak force production. *Fewer mitochondria* - Type I fibers have **abundant mitochondria** for sustained **aerobic metabolism** and fatigue resistance. - Type II fibers rely more on **anaerobic glycolysis** and have fewer mitochondria compared to oxidative Type I fibers.

Q: Which of the following specifically blocks sodium channels?

Tetrodotoxin. **Explanation:** The correct answer is **Tetrodotoxin (TTX)**. This potent neurotoxin, famously found in the liver and ovaries of the **Pufferfish (Fugu)**, acts by selectively and reversibly binding to the extracellular pores of **voltage-gated sodium channels (VGSCs)**. By blocking these channels, it prevents the influx of sodium ions required for the depolarization phase of an action potential, leading to muscle paralysis and respiratory failure. **Analysis of Options:** * **Nifedipine:** This is a **Dihydropyridine (DHP) Calcium Channel Blocker**. It primarily targets L-type calcium channels in vascular smooth muscle and the heart, used clinically for hypertension and angina. * **Tetraethylammonium (TEA):** This is a classic pharmacological tool used to block **Voltage-gated Potassium (K+) channels**. It inhibits the efflux of K+, thereby prolonging the repolarization phase of the action potential. * **Choline:** This is a precursor for the neurotransmitter **Acetylcholine**. It is not a channel blocker; rather, it is taken up by presynaptic neurons via a high-affinity transporter to synthesize new neurotransmitters. **High-Yield Facts for NEET-PG:** * **Saxitoxin:** Produced by red tide dinoflagellates; it has a mechanism identical to Tetrodotoxin (blocks Na+ channels). * **Batrachotoxin:** Found in poison dart frogs; it keeps Na+ channels **open**, preventing repolarization. * **Local Anesthetics (e.g., Lidocaine):** These also block Na+ channels but act on the **inner (cytoplasmic) side** of the channel, unlike TTX which acts on the outer surface. * **Dendrotoxin:** A snake toxin (Mamba) that blocks K+ channels.

Q: The refractory period is minimum for which of the following nerve fibers?

A alpha fibers. **Explanation:** The duration of the **refractory period** in a nerve fiber is inversely proportional to its **diameter** and **conduction velocity**. 1. **Why A alpha fibers are correct:** According to the Erlanger-Gasser classification, **A alpha (Aα) fibers** are the thickest (12–20 μm) and fastest (70–120 m/s) myelinated fibers. Because they have the largest diameter, they possess a shorter membrane time constant and a higher density of voltage-gated sodium channels at the Nodes of Ranvier. This allows them to recover from inactivation more rapidly, resulting in the **shortest absolute refractory period (ARP)**. A shorter refractory period allows these fibers to fire at higher frequencies. 2. **Why other options are incorrect:** * **A beta (Aβ) and A delta (Aδ) fibers:** These are progressively thinner and slower than A alpha fibers. As diameter decreases, the refractory period increases. * **C fibers:** These are the thinnest, unmyelinated fibers with the slowest conduction velocity (0.5–2 m/s). Consequently, they have the **longest refractory period** among all nerve fibers. **High-Yield Facts for NEET-PG:** * **Order of Refractory Period:** C > B > A (C fibers have the longest; A fibers have the shortest). * **Order of Velocity:** Aα > Aβ > Aγ > Aδ > B > C. * **Susceptibility to Blockade:** * **Hypoxia:** B fibers are most sensitive. * **Pressure:** A fibers are most sensitive (e.g., "Saturday Night Palsy"). * **Local Anesthetics:** C fibers are most sensitive (due to small diameter and lack of myelin).

Q: Botulinum toxin blocks neuromuscular transmission by which of the following mechanisms?

Closure of Ca++ channels at the presynaptic membrane. **Explanation:** **Mechanism of Action (The Correct Answer):** Botulinum toxin, produced by *Clostridium botulinum*, acts primarily at the **presynaptic terminal** of the neuromuscular junction (NMJ). Its primary mechanism involves the cleavage of **SNARE proteins** (such as synaptobrevin, SNAP-25, and syntaxin). These proteins are essential for docking and fusing acetylcholine (ACh) vesicles with the presynaptic membrane. By disrupting this process, the toxin effectively prevents the release of ACh into the synaptic cleft. In the context of ion channels, Botulinum toxin inhibits the **calcium-dependent exocytosis** of neurotransmitters, effectively mimicking a functional **closure or blockade of presynaptic Ca++ channels** (as Ca++ entry no longer triggers vesicle release). **Analysis of Incorrect Options:** * **Option B & D:** These refer to **postsynaptic** events. Botulinum toxin does not affect the postsynaptic membrane or its receptors (like the nicotinic ACh receptor). Postsynaptic blockade is characteristic of drugs like Curare (d-tubocurarine) or conditions like Myasthenia Gravis. * **Option C:** Opening presynaptic K+ channels would cause hyperpolarization and inhibit ACh release, but this is not the mechanism of Botulinum toxin. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Causes **flaccid paralysis** (descending) and "floppy baby syndrome" (infantile botulism via honey ingestion). * **Therapeutic Uses:** Used for focal dystonias, achalasia cardia, strabismus, and cosmetic reduction of wrinkles (Botox). * **Contrast with Tetanus Toxin:** While both cleave SNARE proteins, Tetanus toxin undergoes **retrograde axonal transport** to the CNS and inhibits GABA/Glycine release from Renshaw cells, causing **spastic paralysis**. * **Lambert-Eaton Syndrome:** Also affects the presynaptic terminal but specifically involves antibodies against voltage-gated Ca++ channels.

Q: Dynamic response is due to which type of muscle ending?

Primary ending. **Explanation:** The muscle spindle is a complex sensory organ that monitors muscle length and the rate of change in length. It contains two types of intrafusal fibers: **Nuclear Bag fibers** and **Nuclear Chain fibers**. 1. **Why Primary Ending is Correct:** Primary sensory endings (also known as **Type Ia fibers**) wrap around the central portions of both nuclear bag and nuclear chain fibers. Because nuclear bag fibers are highly sensitive to the *velocity* of stretch, the primary endings provide a **dynamic response**. This means they fire rapidly during the actual movement or change in length, signaling how fast the muscle is being stretched. 2. **Why Other Options are Incorrect:** * **Secondary Ending (Type II fibers):** These are located primarily on nuclear chain fibers. They are responsible for the **static response**, meaning they fire at a constant rate proportional to the *absolute length* of the muscle, rather than the speed of change. * **Tertiary Ending:** This term is not standard in muscle spindle physiology. Sensory innervation is categorized into Primary (Ia) and Secondary (II) endings. * **All of the above:** Incorrect because dynamic and static responses are mediated by distinct fiber types with different physiological properties. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Endings (Ia):** Responsible for the **monosynaptic stretch reflex** (e.g., Knee jerk). They are "velocity sensors." * **Secondary Endings (II):** Responsible for "position sensors." * **Gamma Motor Neurons:** Maintain spindle sensitivity during muscle contraction. **Gamma-dynamic** neurons affect bag fibers (dynamic response), while **Gamma-static** neurons affect chain fibers (static response). * **Golgi Tendon Organ (Ib):** Unlike spindles, these sense **muscle tension** and are arranged in series with extrafusal fibers.

Question 1

Arrange the following nerve fibers sequentially in the descending order of nerve impulse transmission velocity: A. C fiber B. Aa fiber C. B fiber D. Ad fiber

Accepted Answer

B-D-C-A

Answer

A-D-C-B

Answer

A-C-D-B

Answer

C-A-B-D

Question 2

In Myasthenia gravis, what is the fundamental defect at the neuromuscular junction?

Accepted Answer

Failure of Acetylcholine (ACh) to attach to the postsynaptic receptor

Answer

Deficiency of Acetylcholine (ACh)

Answer

Defect in the release of Acetylcholine (ACh)

Answer

Impeded presynaptic release of Acetylcholine (ACh)

Question 3

Where does the action potential in a spinal motor neuron generate from?

Accepted Answer

Initial segment

Answer

Cell body

Answer

Dendrites

Answer

Axon

Question 4

Which of the following best describes muscle B compared with muscle A?

Accepted Answer

Innervated by smaller nerve fibers

Answer

Adapted for rapid contraction

Answer

Composed of larger muscle fibers

Answer

Fewer mitochondria

Question 5

Which of the following statements regarding intrafusal fibers is incorrect?

Accepted Answer

Primary sensory endings are excited by both nuclear chain and nuclear bag fibers.

Answer

Nuclear chain fibers are shorter and thinner than nuclear bag fibers.

Answer

Nuclear bag fibers are fewer in number than nuclear chain fibers.

Answer

Secondary sensory endings are excited by nuclear chain fibers only.

Question 6

Post-tetanic facilitation is thought to be the result of which of the following?

Accepted Answer

A buildup of calcium in the presynaptic terminal

Answer

Opening of voltage-gated sodium channels

Answer

Opening of transmitter-gated potassium channels

Answer

Electrotonic conduction

Question 7

Which of the following specifically blocks sodium channels?

Accepted Answer

Tetrodotoxin

Answer

Nifedipine

Answer

Tetraethylammonium

Answer

Choline

Question 8

The refractory period is minimum for which of the following nerve fibers?

Accepted Answer

A alpha fibers

Answer

A beta fibers

Answer

A delta fibers

Answer

C fibers

Question 9

Botulinum toxin blocks neuromuscular transmission by which of the following mechanisms?

Accepted Answer

Closure of Ca++ channels at the presynaptic membrane

Answer

Closure of Na+ channels at the postsynaptic membrane

Answer

Opening of K+ channels at the presynaptic membrane

Answer

Opening of Cl– channels at the postsynaptic membrane

Question 10

Dynamic response is due to which type of muscle ending?

Accepted Answer

Primary ending

Answer

Secondary ending

Answer

Tertiary ending

Answer

All of the above

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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