Nerve and Muscle Physiology Practice Questions

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

Aα. 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].

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

Nodes of Ranvier. ### 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.

Q: Which nerve fibers are involved in proprioception?

Type A fiber. **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.

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

Removal of sarcoplasmic Ca++. ### 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.

Q: Size of action potential is decreased as a result of?

Lower extracellular sodium. **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.

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

Axon hillock. **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 1

Which of the following are properties of fast twitch muscle fibers?

Accepted Answer

Extensive sarcoplasmic reticulum

Answer

Abundant mitochondria

Answer

Extensive blood supply

Answer

Large glycolytic pathway

Question 2

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

Accepted Answer

Aα

Answer

Aδ

Answer

C

Answer

B

Question 3

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

Accepted Answer

Nodes of Ranvier

Answer

Cell body

Answer

Axon terminal

Answer

Surface of myelin

Question 4

Which nerve fibers are involved in proprioception?

Accepted Answer

Type A fiber

Answer

Type B fiber

Answer

Type C fiber

Answer

Type IV fiber

Question 5

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

Accepted Answer

7:1:1

Answer

1:1:1

Answer

1:3:5

Answer

1:7:7

Question 6

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

Accepted Answer

Removal of sarcoplasmic Ca++

Answer

Closure of the postsynaptic nicotinic acetylcholine receptor

Answer

Removal of acetylcholine from the neuromuscular junction

Answer

Removal of Ca++ from the terminal of the motor neuron

Question 7

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

Accepted Answer

2500 per sec

Answer

40 per sec

Answer

250 per sec

Answer

400 per sec

Question 8

Size of action potential is decreased as a result of?

Accepted Answer

Lower extracellular sodium

Answer

Raised extracellular calcium

Answer

Lower extracellular calcium

Answer

Raised extracellular sodium

Question 9

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

Accepted Answer

Axon hillock

Answer

Dendrite

Answer

Soma (cell body)

Answer

Axon

Question 10

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?

Accepted Answer

Inhibition of cholinergic synapse transmission

Answer

Inhibition of adrenergic synapse transmission

Answer

Facilitation of adrenergic synapse transmission

Answer

No effect on synaptic transmission

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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