Nerve and Muscle Physiology Practice Questions

Q: 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 > B > C. ***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 > 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 fibers > C fibers) to compression injury.

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

Efflux of K ions. ***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.

Q: What best describes step 3 in the given diagram?

Efflux of K ions. ***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.

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

I length decrease. ***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.

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

Renshaw cell. ***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.

Q: The fiber marked as X is:

Modified cardiac muscle. ***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.

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

20-25 Hz. **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**.

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

Muscle spindle. ***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.

Question 1

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?

Accepted Answer

A > B > C

Answer

C < B < A

Answer

A < B < C

Answer

C > B > A

Question 2

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

Accepted Answer

Efflux of K ions

Answer

Influx of Na ions

Answer

Efflux of Na ions

Answer

Resting membrane potential is maintained by the Na-K pump

Question 3

What best describes step 3 in the given diagram?

Accepted Answer

Efflux of K ions

Answer

Efflux of Na ions

Answer

Influx of Na ions

Answer

Influx of K ions

Question 4

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

Accepted Answer

I length decrease

Answer

M length increase

Answer

M length increase and I decrease

Answer

A length decrease

Question 5

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

Accepted Answer

25 Hz

Answer

10 Hz

Answer

20 Hz

Answer

40 Hz

Question 6

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

Accepted Answer

Renshaw cell

Answer

Basket cell

Answer

Purkinje cell

Answer

Anterior horn cell

Question 7

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

Accepted Answer

A = Twitch, B = Summation, C = Incomplete tetanus, D = Complete tetanus

Answer

A = Summation, B = Twitch, C = Complete tetanus, D = Incomplete tetanus

Answer

A = Twitch, B = Incomplete tetanus, C = Summation, D = Complete tetanus

Answer

A = Incomplete tetanus, B = Complete tetanus, C = Twitch, D = Summation

Question 8

The fiber marked as X is:

Accepted Answer

Modified cardiac muscle

Answer

Nerve fiber

Answer

Modified nerve fiber

Answer

Modified connective tissue

Question 9

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

Accepted Answer

20-25 Hz

Answer

10-15 Hz

Answer

15-20 Hz

Answer

30-35 Hz

Question 10

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

Accepted Answer

Muscle spindle

Answer

Merkel's disc

Answer

Meissner's corpuscle

Answer

Pacinian corpuscle

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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