Nerve and Muscle Physiology Practice Questions

Q: A person wakes up with pain, paresthesia, and tingling of the arms after sleeping with their arm below their head. Which nerve fibres are involved?

Type A fibres. ### Explanation The clinical scenario describes **"Saturday Night Palsy"** or **"Sleep Palsy,"** which occurs due to prolonged mechanical compression of a peripheral nerve (typically the radial nerve). The susceptibility of nerve fibers to different types of insults follows a specific order (Gasser-Erlanger classification): 1. **Pressure/Compression:** **Type A fibers** are the **most sensitive** to pressure, while Type C fibers are the least sensitive. 2. **Hypoxia:** Type B fibers are the most sensitive. 3. **Local Anesthetics:** Type C fibers are the most sensitive. In this case, mechanical compression leads to the blockage of Type A fibers first. Since Type A-alpha and A-beta fibers carry motor functions and touch/pressure sensations, their involvement results in the "pins and needles" sensation (paresthesia) and temporary weakness. #### Analysis of Options: * **Option A (Correct):** Type A fibers are large, myelinated fibers. Their large diameter makes them highly susceptible to mechanical deformation and ischemia caused by direct pressure. * **Option B (Incorrect):** Type B fibers (preganglionic autonomic) are most sensitive to **hypoxia**, not mechanical pressure. * **Option C & D (Incorrect):** Type C fibers are small, unmyelinated fibers. They are the **most resistant to pressure** but the most sensitive to **local anesthesia**. While Type C fibers do carry slow pain, the initial tingling and paresthesia from "falling asleep" on an arm are classic signs of Type A fiber compromise. #### NEET-PG High-Yield Pearls: * **Order of Sensitivity to Pressure:** A > B > C (Mnemonic: **P**ressure affects **P**rimary/A fibers). * **Order of Sensitivity to Hypoxia:** B > A > C. * **Order of Sensitivity to Local Anesthesia:** C > B > A (Mnemonic: **L**ocal **L**ast/C fibers). * **Neuropraxia:** This scenario is a form of Neuropraxia (Seddon’s classification), where there is a temporary conduction block without axonal degeneration.

Q: What is the study of the electrical activity of the muscle called?

Electromyogram (EMG). **Explanation:** The correct answer is **Electromyogram (EMG)**. **1. Why Electromyogram (EMG) is correct:** Electromyography is the diagnostic procedure used to assess the health of muscles and the nerve cells (motor neurons) that control them. It records the **electrical activity of muscle fibers** during rest, slight contraction, and forceful contraction. When a muscle fiber is stimulated by a motor neuron, it generates an action potential; the EMG electrodes detect these electrical signals and translate them into graphs or numerical data. **2. Why the other options are incorrect:** * **Electroencephalogram (EEG):** This records the electrical activity of the **brain** (cerebral cortex) using electrodes placed on the scalp. It is primarily used to diagnose epilepsy and sleep disorders. * **Electrocardiogram (ECG/EKG):** This records the electrical activity of the **heart** over a period of time. It is the gold standard for diagnosing arrhythmias and myocardial infarctions. * **Venn diagram:** This is a **mathematical/logical tool** used to show relationships between sets; it has no application in physiological electrical recording. **3. High-Yield Clinical Pearls for NEET-PG:** * **Motor Unit Action Potential (MUAP):** The fundamental unit studied in EMG. Changes in MUAP morphology help differentiate between **myopathic** (small, short-duration potentials) and **neurogenic** (large, long-duration polyphasic potentials) disorders. * **Fibrillation potentials and Fasciculations:** Spontaneous electrical activity seen in EMG during rest, often indicating denervation or lower motor neuron (LMN) lesions. * **Nerve Conduction Studies (NCS):** Often performed alongside EMG to measure the speed and strength of signals traveling through a nerve, helping to localize peripheral nerve entrapments (like Carpal Tunnel Syndrome).

Q: What is the velocity of conduction for A-alpha nerve fibers in m/second?

120. **Explanation:** The conduction velocity of a nerve fiber is directly proportional to its diameter and the presence of a myelin sheath. According to the **Erlanger-Gasser classification**, nerve fibers are categorized based on their diameter and velocity. **Why Option D is Correct:** **A-alpha (Aα) fibers** are the thickest and most heavily myelinated fibers in the human body. They have a diameter of **12–20 μm** and a conduction velocity ranging from **70 to 120 m/sec**. In the context of NEET-PG, when a range is provided, the maximum value (120 m/sec) is typically considered the defining characteristic for A-alpha fibers. These fibers are responsible for **proprioception** (muscle spindles and Golgi tendon organs) and **somatic motor function**. **Why Other Options are Incorrect:** * **Options A, B, and C (80, 70, 90 m/sec):** While these values fall within the functional *range* of A-alpha fibers, they do not represent the maximum potential velocity. In competitive exams, the upper limit of 120 m/sec is the standard "textbook" answer used to distinguish A-alpha from A-beta fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Hursh’s Factor:** Conduction velocity (m/s) = Diameter (μm) × 6. This is a quick rule of thumb to estimate velocity. * **Order of Sensitivity to Blockade:** * **Local Anesthetics:** Type C > Type B > Type A (Smallest/unmyelinated are blocked first). * **Pressure:** Type A > Type B > Type C (Largest are blocked first; e.g., "foot falling asleep"). * **Hypoxia:** Type B > Type A > Type C. * **Type C Fibers:** These are the only unmyelinated fibers, have the smallest diameter (0.4–1.2 μm), and the slowest velocity (0.5–2 m/sec). They carry slow pain and temperature.

Q: Z-lines are anchored to the sarcolemma by:

Desmin. **Explanation:** The correct answer is **Desmin**. In skeletal muscle, the structural integrity and spatial alignment of myofibrils are maintained by a complex network of intermediate filaments. **Desmin** is the primary intermediate filament that encircles the Z-lines of adjacent myofibrils, linking them to each other and anchoring the entire Z-disk complex to the sarcolemma (cell membrane) via costameres. This ensures that all myofibrils contract in synchrony and maintains the lateral registration of the sarcomeres. **Analysis of Incorrect Options:** * **Titin (Option A):** This is the largest known protein. It acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the muscle and centering the myosin filaments. * **Dystrophin (Option B):** While it also links the cytoskeleton to the sarcolemma, it specifically connects **F-actin** (not the Z-line directly) to the dystroglycan complex in the membrane. Its deficiency leads to Duchenne Muscular Dystrophy. * **Actinin (Option D):** Specifically **α-actinin**, this protein is located *within* the Z-line. Its primary role is to anchor the plus ends of actin (thin) filaments to the Z-disk, rather than anchoring the Z-disk to the sarcolemma. **High-Yield Clinical Pearls for NEET-PG:** * **Costameres:** These are the functional units (containing desmin, vinculin, and dystrophin) that couple the sarcomere to the extracellular matrix. * **Nebulin:** Acts as a "molecular ruler" to regulate the length of actin filaments. * **Desmin-related Myopathy:** Mutations in the desmin gene lead to myofibrillar myopathies characterized by muscle weakness and cardiac conduction blocks.

Q: Wallerian degeneration is observed in which part of the neuron after axonal injury?

Distal to the injury. **Explanation:** **Wallerian Degeneration** refers to the sequence of events that occur when an axon is severed from its metabolic source—the cell body (soma). **1. Why Option A is Correct:** The axon depends on the cell body for the synthesis of proteins and organelles, which are transported via axoplasmic flow. When an injury occurs, the segment **distal to the injury** is physically separated from the soma. Deprived of essential nutrients, the distal cytoskeleton and membrane disintegrate within 24–36 hours. This is followed by the infiltration of macrophages to clear myelin debris, creating a path for potential regeneration. **2. Why the Other Options are Incorrect:** * **Option B (Proximal to the injury):** The proximal segment remains attached to the cell body. While it may undergo limited "retrograde degeneration" (up to the nearest Node of Ranvier), the primary process of Wallerian degeneration is a distal phenomenon. * **Option C (Both ends):** Degeneration is asymmetrical. The distal end undergoes complete breakdown, whereas the proximal end prepares for repair. * **Option D (Cell body):** The cell body does not degenerate; instead, it undergoes **Chromatolysis** (swelling of the soma, displacement of the nucleus to the periphery, and dispersal of Nissl bodies) to ramp up protein synthesis for repair. **High-Yield Facts for NEET-PG:** * **Chromatolysis:** The characteristic change in the cell body after axonal injury. * **Rate of Regeneration:** Peripheral nerves typically regrow at a rate of **1–3 mm/day**. * **Schwann Cells:** In the PNS, these cells survive Wallerian degeneration and form **Bungner bands** (tubes) to guide the regenerating axon sprout. * **CNS vs. PNS:** Wallerian degeneration is much slower in the CNS because oligodendrocytes do not provide the same guidance, and myelin debris (which contains inhibitory factors like Nogo-A) persists longer.

Question 1

Action potential is initiated at the axon hillock/initial segment of a neuron because:

Accepted Answer

The threshold for excitation is lowest at this site.

Answer

Neurotransmitter is released at this site.

Answer

It is an unmyelinated segment.

Answer

It has the lowest concentration of voltage-gated Na+ channels.

Question 2

Saltatory conduction in myelinated axons results from the fact that?

Accepted Answer

Voltage-gated sodium channels are concentrated at the nodes of Ranvier

Answer

Salt concentration is increased beneath the myelin segments

Answer

Nongated ion channels are present beneath the segments of myelin

Answer

Membrane resistance is decreased beneath the segments of myelin

Question 3

A person wakes up with pain, paresthesia, and tingling of the arms after sleeping with their arm below their head. Which nerve fibres are involved?

Accepted Answer

Type A fibres

Answer

Type B fibres

Answer

Type C fibres (pain)

Answer

Type C fibres (postganglionic)

Question 4

What is the study of the electrical activity of the muscle called?

Accepted Answer

Electromyogram (EMG)

Answer

Electroencephalogram (EEG)

Answer

Venn diagram

Answer

Electrocardiogram (ECG)

Question 5

What is the velocity of conduction for A-alpha nerve fibers in m/second?

Accepted Answer

120

Answer

80

Answer

70

Answer

90

Question 6

Which of the following is a true statement regarding malignant hyperthermia?

Accepted Answer

Increased Ca++ transport through L-type calcium channels in T-tubules

Answer

Increased Ca++ transport through T-type calcium channels in T-tubules

Answer

Increased Ca++ transport through T-type calcium channels in L-tubules

Answer

Increased Ca++ transport through L-type calcium channels in L-tubules

Question 7

Z-lines are anchored to the sarcolemma by:

Accepted Answer

Desmin

Answer

Titin

Answer

Dystrophin

Answer

Actinin

Question 8

What is an auxotonic contraction of a muscle?

Accepted Answer

A contraction where the muscle length changes and the tension changes throughout the contraction.

Answer

A contraction where the tension developed changes when the length of the muscle changes.

Answer

A contraction where the muscle length is constant but tension changes.

Answer

A contraction where the muscle length changes but tension remains constant.

Question 9

Which of the following statements is true about myoglobin?

Accepted Answer

Myoglobin has a higher O2 affinity compared to that of hemoglobin.

Answer

Myoglobin binds to 4 moles of O2 per mole.

Answer

The myoglobin dissociation curve has a parabolic profile.

Answer

The myoglobin dissociation curve shows the Bohr effect.

Question 10

Wallerian degeneration is observed in which part of the neuron after axonal injury?

Accepted Answer

Distal to the injury

Answer

Proximal to the injury

Answer

At both ends

Answer

In the cell body (soma)

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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