Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 291: A 35-year-old man wakes up after sleeping with his arm draped over a chair and complains of pain. Which of the following accurately describes the order of susceptibility of nerve fibers in the given condition?

A. C < B < A
B. A < B < C
C. C > B > A
D. A > B > C (Correct Answer)

Explanation: ***A > B > C***- The clinical scenario describes **neuropraxia** (transient functional block) due to **compression and ischemia**, such as in 'Saturday night palsy'.- A fibers have the largest diameter and the heaviest myelination, making them the most vulnerable to conduction block resulting from **focal demyelination** caused by mechanical stress. *A < B < C*- This sequence incorrectly places the greatest susceptibility on the smallest, unmyelinated **C fibers**.- C fibers transmit **slow pain** and temperature and are known to be the most resilient nerve type to compression and ischemia. *C > B > A*- This order represents the susceptibility of nerve fibers to **local anesthetic agents** (pharmacologic block), not mechanical compression. - Local anesthetics preferentially block smaller, unmyelinated C fibers (pain and temperature sensation), followed by B and then A fibers (motor/proprioception). *C < B < A*- While mathematically consistent with **A being the most susceptible**, this alternative formatting is less commonly used to denote the decreasing order of susceptibility (A fibers > B fibers > C fibers) to compression injury.

Question 292: Which of the following ion movements is primarily responsible for the repolarization phase (Phase 3) of an action potential, as depicted in the image?

A. Efflux of K ions (Correct Answer)
B. Influx of Na ions
C. Efflux of Na ions
D. Resting membrane potential is maintained by the Na-K pump

Explanation: ***Efflux of K ions*** - Phase 3, the **repolarization** or falling phase, is initiated by the opening of voltage-gated **potassium (K+) channels** as the membrane potential peaks. - The outflow of positive K+ ions from the cell, known as **efflux**, causes the membrane potential to become negative again, returning it towards the resting state. *Efflux of Na ions* - The electrochemical gradient for **sodium (Na+)** strongly favors its movement into the cell (influx), not out of it (efflux). - While the **Na+/K+ pump** does move Na+ out of the cell, this is a slow, active process to maintain resting potential, not the cause of rapid repolarization. *Influx of Na ions* - The rapid influx (inflow) of **Na+** ions through voltage-gated channels is responsible for the **depolarization** phase (Phase 0), the sharp upstroke of the action potential. - During repolarization (Phase 3), these voltage-gated **Na+ channels** become inactivated, stopping the influx. *Resting membrane potential is maintained by the Na-K pump* - The **Na+/K+ pump** is crucial for establishing and maintaining the ion gradients for the **resting membrane potential** (Phase 4), not for the rapid repolarization phase itself. - Repolarization is a passive process resulting from ion flow through channels, which is much faster than the action of the Na+/K+ pump.

Question 293: What best describes step 3 in the given diagram?

A. Efflux of K ions (Correct Answer)
B. Efflux of Na ions
C. Influx of Na ions
D. Influx of K ions

Explanation: ***Efflux of K ions*** - Step 3 represents the **repolarization** phase of the action potential. This is caused by the opening of voltage-gated **K+ channels** and the inactivation of voltage-gated Na+ channels. - The opening of these channels allows a rapid **efflux** (outward flow) of positively charged K+ ions, which makes the membrane potential decrease from its positive peak back towards the negative resting potential. *Efflux of Na ions* - An efflux of Na+ ions is primarily driven by the **Na+/K+ pump** to maintain the resting potential over time, not to cause the rapid repolarization seen in step 3. - The significant movement of Na+ during the action potential is an **influx** during depolarization (step 2), not an efflux. *Influx of Na ions* - The influx of Na+ ions through voltage-gated channels is responsible for the **depolarization** phase (step 2), the rapid rising phase of the action potential. - During step 3, the voltage-gated **Na+ channels are inactivated**, preventing the influx of Na+ ions and allowing repolarization to occur. *Influx of K ions* - K+ ions move **outward** (efflux) during repolarization, not inward. - An influx of K+ would make the membrane potential more negative, but this is not the mechanism of repolarization in step 3.

Question 294: Which of the following changes occurs during muscle contraction while exercising, as shown in the image?

A. M length increase
B. M length increase and I decrease
C. A length decrease
D. I length decrease (Correct Answer)

Explanation: ***I length decrease*** - The **I band** is the region of the sarcomere containing only **thin filaments (actin)**. During contraction, these thin filaments slide over the thick filaments, causing the I band to shorten. - The shortening of the I band, along with the H zone, results in the **Z lines** moving closer together, which constitutes the shortening of the entire **sarcomere**. *M length increase* - The **M line** is a protein structure in the center of the H zone that anchors the **thick filaments (myosin)**. It is a line, not a band, and its own length does not change. - The region surrounding the M line, the **H zone**, actually *decreases* in width during contraction, it does not increase. *A length decrease* - The **A band** represents the entire length of the **thick myosin filaments**. The length of these filaments does not change during the sliding filament process of muscle contraction. - Because the thick filaments do not shorten, the length of the **A band remains constant** during both muscle contraction and relaxation. *M length increase and I decrease* - This option is partially correct, as the **I band** does decrease in length during contraction. - However, it is incorrect because the **M line** does not increase in length; it remains constant. The overall statement is therefore false.

Question 295: A single muscle twitch lasts 40 milliseconds. What is the minimum tetanization frequency required to produce a sustained (fused) contraction in this muscle?

A. 10 Hz
B. 20 Hz
C. 25 Hz (Correct Answer)
D. 40 Hz

Explanation: ***25 Hz*** - The **minimum tetanization frequency** (Critical Fusion Frequency) required to produce fused tetanus is calculated as the reciprocal of the total twitch duration: **f = 1/T** - With a complete muscle twitch duration of **40 milliseconds (0.04 seconds)**, the minimum frequency is: **1/0.04 s = 25 Hz** - At this frequency, each stimulus arrives **before the muscle can relax** from the previous contraction, causing **summation** and resulting in **smooth, sustained (fused) tetanus** - This represents the threshold where individual twitches fuse into continuous contraction *10 Hz* - This frequency provides one stimulus every **100 milliseconds (1/10 Hz)** - Since the twitch duration is only 40 ms, the muscle **completely relaxes between stimuli** - This results in **separate, discrete twitches** with no summation - Frequency is far below the critical fusion frequency of 25 Hz *20 Hz* - This frequency corresponds to a stimulus interval of **50 milliseconds (1/20 Hz)** - This interval is longer than the 40 ms twitch duration, allowing **partial relaxation** between stimuli - Results in **unfused (incomplete) tetanus** with visible oscillations in tension - Does not produce the smooth, sustained contraction characteristic of complete tetanus *40 Hz* - This frequency corresponds to an interval of **25 milliseconds (1/40 Hz)** between stimuli - While this frequency **does produce fused tetanus**, it exceeds the minimum requirement - At 40 Hz, stimuli arrive well before any relaxation occurs, but the question asks for the **minimum frequency required** - The minimum frequency for fused tetanus is 25 Hz, making this option incorrect as it is unnecessarily high

Question 296: Name the interneuron marked X in colour purple involved in tetanus. (Recent NEET Pattern 2016-17)

A. Renshaw cell (Correct Answer)
B. Basket cell
C. Purkinje cell
D. Anterior horn cell

Explanation: ***Renshaw cell*** - The image depicts a **Renshaw cell (X)**, which is an **inhibitory interneuron** in the spinal cord, regulating motor neuron activity. - In tetanus, the toxin **tetanospasmin** inhibits the release of neurotransmitters (glycine and GABA) from Renshaw cells, leading to **uncontrolled muscle spasms**. *Basket cell* - **Basket cells** are found in the **cerebellar cortex** and hippocampus, playing a role in inhibiting Purkinje cell activity. - They are not located in the spinal cord gray matter in the position marked X. *Purkinje cell* - **Purkinje cells** are large, distinctive neurons found exclusively in the **cerebellar cortex**, crucial for motor coordination. - They are not present in the spinal cord and are not interneurons in the context of spinal reflexes. *Anterior horn cell* - **Anterior horn cells** are **motor neurons** whose cell bodies reside in the anterior horn of the spinal cord and directly innervate skeletal muscles. - They are not interneurons; rather, they are the target of regulation by interneurons like Renshaw cells.

Question 297: The following skeletal muscle recording shows presence of: (Recent NEET Pattern 2016-17)

A. A = Twitch, B = Summation, C = Incomplete tetanus, D = Complete tetanus (Correct Answer)
B. A = Summation, B = Twitch, C = Complete tetanus, D = Incomplete tetanus
C. A = Twitch, B = Incomplete tetanus, C = Summation, D = Complete tetanus
D. A = Incomplete tetanus, B = Complete tetanus, C = Twitch, D = Summation

Explanation: ***A = Twitch, B = Summation, C = Incomplete tetanus, D = Complete tetanus*** - **A** shows a single muscle contraction and relaxation in response to a single stimulus, which is characteristic of a **twitch**. - **B** shows responses to multiple stimuli delivered before complete relaxation, resulting in increasing tension but still peaks and troughs, indicative of **summation**. - **C** represents **incomplete tetanus** (also known as unfused tetanus), where rapid stimulation causes successive contractions to build upon each other, but the muscle partially relaxes between stimuli, creating a jagged peak. - **D** illustrates **complete tetanus** (or fused tetanus), where very rapid, continuous stimulation prevents any relaxation between stimuli, leading to a sustained, smooth maximal contraction.

Question 298: The fiber marked as X is:

A. Nerve fiber
B. Modified cardiac muscle (Correct Answer)
C. Modified nerve fiber
D. Modified connective tissue

Explanation: ***Modified cardiac muscle*** - The fiber marked as X represents **Purkinje fibers**, which are part of the cardiac conduction system consisting of **specialized cardiac muscle cells** (modified cardiomyocytes). - These cells have **lost most of their contractile elements** and have developed specialized properties for **rapid electrical impulse conduction** throughout the ventricles. - Histologically, they are **larger and paler** than regular cardiac muscle cells, with abundant glycogen and fewer myofibrils, but they retain their cardiac muscle origin and characteristics. - Found in the **subendocardial layer** of the ventricles, they are the terminal component of the cardiac conduction system. *Modified nerve fiber* - This is a **common misconception**. While Purkinje fibers conduct electrical impulses rapidly (similar to nerve fibers), they are **not nerve tissue**. - The cardiac conduction system consists entirely of **modified cardiac muscle cells**, not neurons or nerve fibers. - True nerve fibers (autonomic nervous system) modulate the heart rate but are **separate from the conduction system**. *Nerve fiber* - **Nerve fibers** are axons of neurons and are part of the nervous system. - The cardiac conduction system, including Purkinje fibers, is **not composed of nervous tissue** but rather specialized cardiac muscle. - Autonomic nerve fibers do innervate the heart but are distinct from the conduction system structures. *Modified connective tissue* - **Connective tissue** provides structural support but does not have the ability to generate or conduct electrical impulses. - Purkinje fibers are **specialized cardiac muscle cells**, not connective tissue derivatives.

Question 299: Calculate the tetanizing frequency based on the contraction dynamics of gastrocnemius muscle of frog shown in the image?

A. 10-15 Hz
B. 15-20 Hz
C. 20-25 Hz (Correct Answer)
D. 30-35 Hz

Explanation: **20-25 Hz** - Tetanizing frequency (or fusion frequency) is the stimulation rate at which individual muscle twitches fuse to produce a **smooth, sustained contraction** (tetanus). - For the **frog gastrocnemius muscle**, a common model in physiology, this frequency typically falls within the **20-25 Hz range**. *10-15 Hz* - At this lower frequency, the muscle would likely exhibit **incomplete tetanus** or summation, where individual twitches are still discernible, but tension is increasing. - This range is generally insufficient to achieve a **smooth, fused tetanic contraction** in the frog gastrocnemius. *15-20 Hz* - This range might produce **treppe** or early stages of incomplete tetanus, where successive contractions are slightly stronger, but the relaxation phase is still partially visible between stimuli. - While closer to the tetanizing frequency, it's generally not high enough to achieve **complete fusion** for the frog gastrocnemius. *30-35 Hz* - While this frequency would certainly result in a **fused tetanic contraction**, it's higher than the minimum required for the frog gastrocnemius, which means the muscle is already in complete tetanus at a lower frequency. - Using excessively high frequencies beyond the fusion frequency does not significantly increase tension and can lead to **faster fatigue**.

Question 300: In the process shown below stretch stimulus is mediated by which of the following receptors?

A. Merkel's disc
B. Meissner's corpuscle
C. Pacinian corpuscle
D. Muscle spindle (Correct Answer)

Explanation: ***Muscle spindle*** - The image depicts a **muscle**, and the "Steady stretch" stimulus clearly shows an increase in muscle tension followed by sustained neural firing, characteristic of a **stretch reflex**. - **Muscle spindles** are proprioceptors located within skeletal muscles that detect changes in muscle length and the rate of change of length, playing a crucial role in the stretch reflex. *Merkel's disc* - **Merkel's discs** are mechanoreceptors located in the basal layer of the epidermis, primarily responsible for detecting sustained light touch and pressure. - They are not involved in sensing muscle stretch. *Meissner's corpuscle* - **Meissner's corpuscles** are rapidly adapting mechanoreceptors found in the dermal papillae, specialized for detecting light touch and low-frequency vibration. - They are cutaneous receptors and do not mediate muscle stretch. *Pacinian corpuscle* - **Pacinian corpuscles** are rapidly adapting mechanoreceptors located deep in the dermis and subcutaneous tissue, sensitive to deep pressure and high-frequency vibration. - They are not responsible for detecting muscle stretch.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

Practice Questions

Skeletal Muscle Contraction

Practice Questions

Smooth Muscle Physiology

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Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

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Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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