Nerve and Muscle Physiology Practice Questions

Q: A Strength-Duration curve is plotted for a skeletal muscle. Identify the parameters labeled in the correct order to successfully determine the tissue excitability.

A: Chronaxie; B: Rheobase; D: Utilization Time. ***A: Chronaxie; B: Rheobase; D: Utilization Time*** - The **Strength-Duration curve** shows the inverse relationship between the **stimulus strength** and the **stimulus duration** required to elicit a response. - **Chronaxie** (A) is defined as the minimum duration of stimulus required to excite a tissue at twice the **rheobase** strength, while **rheobase** (B) is the minimal stimulus strength required to excite the tissue with a long duration, and **utilization time** (D) is the minimum duration required to excite with a stimulus of precisely rheobase strength. *A: Utilization Time; B: Rheobase; D: Chronaxie* - **Utilization time** is significantly longer than **chronaxie**; mislabeling these confuses the fundamental principles of **excitability threshold**. - Placing **chronaxie** at the end of the duration axis is anatomically incorrect as it is a specific, shorter duration compared to the total **utilization time**. *A: Rheobase; B: Chronaxie; D: Utilization Time* - **Rheobase** represents a measure of **stimulus voltage** (y-axis), not a point on the duration axis. - Assigning rheobase to the duration parameters ignores the distinction between **intensity** and **time constants**. *A: Chronaxie; B: Utilization Time; D: Rheobase* - **Utilization time** is the minimum stimulus duration at **rheobase** intensity, making it distinct from **chronaxie**. - Swapping these values disrupts the physiological definition of **tissue excitability** markers.

Q: Why is no new impulse generated during the depolarization phase of an action potential?

Voltage-gated sodium channels are inactivated. ***Voltage-gated sodium channels are inactivated*** - During the depolarization and early repolarization phases, **voltage-gated sodium channels** enter an **inactivated state** (the ball-and-chain mechanism), preventing further influx of sodium. - This state creates the **absolute refractory period**, meaning no new action potential can be triggered regardless of the strength of the stimulus. *Potassium channels are closed* - Voltage-gated potassium channels are actually **opening** during late depolarization and repolarization, not closing. - Closing these channels would promote further depolarization, not prevent the generation of a new impulse. *Sodium channels are closed* - This refers to the **resting state** of sodium channels, where they are closed but capable of opening, not the inactivated state. - If sodium channels were merely in the closed

Q: Based on the provided image of a nerve action potential, at which marked point is the combined transport of Na+ and K+ ions at its minimum?

Point E. ***Point E*** - This represents the **after-hyperpolarization phase** where voltage-gated **Na+ channels are inactivated** and **delayed rectifier K+ channels are closing**, resulting in minimal ion movement. - The membrane potential approaches the **K+ equilibrium potential**, and only the **Na+/K+-ATPase pump** operates at baseline levels to maintain ionic gradients. *Point A* - This is the **resting membrane potential** with continuous activity of **leak channels** allowing steady Na+ influx and K+ efflux. - Despite appearing stable, there is significant **background ion transport** through leak channels that maintains the resting potential. *Point B* - This marks the **threshold/depolarization phase** with massive opening of **voltage-gated Na+ channels** causing rapid Na+ influx. - This represents the **highest combined ion transport** phase, directly opposite to the minimum transport asked in the question. *Point C* - This corresponds to the **repolarization phase** with opening of **voltage-gated K+ channels** causing significant K+ efflux. - High **K+ conductance** results in substantial outward potassium transport to restore the membrane potential.

Q: Intercalated disc is composed of: 1. Cadherins 2. Desmosomes 3. Gap junctions 4. Tight junctions

1, 2 and 3. ***1, 2 and 3*** - Intercalated discs contain **fascia adherens** (containing **cadherins**) and **desmosomes** which provide strong mechanical adhesion, preventing cells from separating during contraction. - **Gap junctions** are essential components of the intercalated disc, allowing for the rapid passage of ions and electrical impulses, which facilitates **functional syncytium** behavior of cardiac muscle. *2, 3 and 4* - While desmosomes and gap junctions are correct, **tight junctions** are not typically components of the intercalated disc. - Tight junctions function primarily to create permeability barriers in epithelial and endothelial tissues, not in contractile cardiac muscle. *1, 3 and 4* - While cadherins and gap junctions are correct, **tight junctions** are not present in cardiac intercalated discs. - Tight junctions are characteristic of the **blood-brain barrier** or intestinal lining, meant for regulating paracellular transport. *1, 2, 3 and 4* - Including **tight junctions** makes this option incorrect, as they are not structural components of the cardiac intercalated disc. - The intercalated disc is specialized for mechanical cohesion and electrical coupling, not for maintaining epithelial-like tight barrier functions.

Q: What is the relationship between nerve thickness and conduction velocity of myelinated nerves?

Linear. **Explanation:** The relationship between the diameter of a myelinated nerve fiber and its conduction velocity is **linear**. According to the **Hursh transformation**, the conduction velocity (V) in meters per second is approximately 6 times the external diameter (D) in micrometers ($V \approx 6 \times D$). In myelinated nerves, conduction occurs via **saltatory conduction**. As the fiber diameter increases, internal axial resistance decreases significantly, and the distance between the Nodes of Ranvier (internodal length) increases. This allows the action potential to "jump" further and faster, resulting in a direct proportionality between thickness and speed. **Analysis of Options:** * **B. Parabolic & C. Hyperbolic:** These are incorrect because the increase in velocity does not follow a squared or inverse curve. While unmyelinated fibers show a relationship where velocity is proportional to the *square root* of the diameter (making it non-linear), myelinated fibers strictly follow a linear progression. * **D. No relation:** This is incorrect as fiber diameter is the primary anatomical determinant of conduction speed. **High-Yield Facts for NEET-PG:** * **Erlanger-Gasser Classification:** Type A fibers (thickest, myelinated) have the highest velocity, while Type C fibers (thinnest, unmyelinated) have the slowest. * **Myelination Effect:** Myelin increases conduction velocity by increasing membrane resistance and decreasing membrane capacitance. * **Clinical Correlation:** In demyelinating diseases like **Guillain-Barré Syndrome (GBS)** or **Multiple Sclerosis**, the loss of myelin disrupts this linear relationship, leading to significantly slowed conduction or conduction block.

Q: TTX-resistant sodium channels are caused by the involvement of which of the following?

Nav 1.8. **Explanation:** The correct answer is **Nav 1.8**. Voltage-gated sodium channels ($Na_v$) are classified based on their sensitivity to **Tetrodotoxin (TTX)**, a potent neurotoxin derived from pufferfish that blocks the extracellular pore of the channel. 1. **Why Nav 1.8 is correct:** Most voltage-gated sodium channels (Nav 1.1–1.4, 1.6, 1.7) are **TTX-sensitive**, meaning they are inhibited by nanomolar concentrations of the toxin. However, **Nav 1.8** and **Nav 1.9** (primarily found in dorsal root ganglion nociceptors) and **Nav 1.5** (found in cardiac muscle) possess a structural variation—specifically a substitution of a cysteine or serine for phenylalanine/tyrosine at a critical binding site—making them **TTX-resistant**. Nav 1.8 plays a crucial role in the transmission of inflammatory and mechanical pain. 2. **Why other options are incorrect:** * **Nav 1.1:** This is a classic **TTX-sensitive** channel found primarily in the Central Nervous System (CNS). Mutations in this channel are associated with Dravet syndrome (epilepsy). * **TRPI (TRPV1):** This refers to Transient Receptor Potential channels. These are non-selective cation channels (not specific sodium channels) activated by heat and capsaicin. * **NMD4:** This appears to be a distractor. **NMDA** receptors are glutamate-gated ion channels involved in synaptic plasticity, not voltage-gated sodium channels. **High-Yield Facts for NEET-PG:** * **TTX-Resistant Channels:** Nav 1.5 (Heart), Nav 1.8, and Nav 1.9 (Nociceptors). * **Nav 1.5 Clinical Link:** It is the primary sodium channel in the heart; its resistance to TTX explains why pufferfish poisoning causes neurological symptoms and paralysis but often spares cardiac conduction until late stages. * **Mechanism of TTX:** It binds to the **P-loop** of the alpha subunit, physically plugging the channel pore.

Question 1

A Strength-Duration curve is plotted for a skeletal muscle. Identify the parameters labeled in the correct order to successfully determine the tissue excitability.

Accepted Answer

A: Chronaxie; B: Rheobase; D: Utilization Time

Answer

A: Utilization Time; B: Rheobase; D: Chronaxie

Answer

A: Rheobase; B: Chronaxie; D: Utilization Time

Answer

A: Chronaxie; B: Utilization Time; D: Rheobase

Question 2

Why is no new impulse generated during the depolarization phase of an action potential?

Accepted Answer

Voltage-gated sodium channels are inactivated

Answer

Potassium channels are closed

Answer

Sodium channels are closed

Answer

The neuron is hyperpolarized

Question 3

Based on the provided image of a nerve action potential, at which marked point is the combined transport of Na+ and K+ ions at its minimum?

Accepted Answer

Point E

Answer

Point A

Answer

Point B

Answer

Point C

Question 4

Intercalated disc is composed of:

1. Cadherins
2. Desmosomes
3. Gap junctions
4. Tight junctions

Accepted Answer

1, 2 and 3

Answer

2, 3 and 4

Answer

1, 3 and 4

Answer

1,2, 3 and 4

Question 5

During a neurophysiology practical, a student applies two successive electrical stimuli to an isolated frog sciatic nerve preparation. The first stimulus is suprathreshold. The second stimulus, applied 4 milliseconds after the first (corresponding to the relative refractory period in this preparation), is stronger than the original suprathreshold stimulus, and is delivered at point C shown on the waveform. The recorded intracellular waveform is displayed on the oscilloscope (Image 1). Which of the following best explains the membrane state at the time the second stimulus was delivered?

Accepted Answer

B. Voltage-gated sodium channels are partially recovered from inactivation, allowing a suprathreshold stimulus to generate a reduced-amplitude action potential

Answer

A. Voltage-gated sodium channels are completely inactivated and cannot be reopened regardless of stimulus strength

Answer

C. Voltage-gated potassium channels are fully closed and the membrane potential has returned exactly to resting level

Answer

D. The membrane is in a state of hyperpolarisation due to electrogenic sodium-potassium ATPase activity, preventing any action potential generation

Question 6

Type I muscle fibers are rich in myosin heavy chain. What is their characteristic property?

Accepted Answer

Slow contracting, resistant to fatigue

Answer

Fast contracting, susceptible to fatigue

Answer

Slow contracting, susceptible to fatigue

Answer

Fast contracting, resistant to fatigue

Question 7

The number of muscle fibers innervated by a single motor axon is smallest in which of the following?

Accepted Answer

Orbicularis oculi

Answer

Gastrocnemius

Answer

Single unit smooth muscle

Answer

Soleus

Question 8

What is the relationship between nerve thickness and conduction velocity of myelinated nerves?

Accepted Answer

Linear

Answer

Parabolic

Answer

Hyperbolic

Answer

No relation

Question 9

In tetany, what causes the increased membrane excitability?

Accepted Answer

Depolarization of the nerve and muscle membranes

Answer

Decreased release of inhibitory neurotransmitter from nerve terminals

Answer

Spontaneous release of calcium from the sarcoplasmic reticulum

Answer

Activation of sodium channels at more negative membrane potentials

Question 10

TTX-resistant sodium channels are caused by the involvement of which of the following?

Accepted Answer

Nav 1.8

Answer

Nav 1.1

Answer

TRPI

Answer

NMD4

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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