Nerve and Muscle Physiology Practice Questions

Q: As a neuron’s diameter increases, what effect manifests?

Conduction velocity increases. ***Conduction velocity increases*** - Increasing the diameter of an axon decreases the **internal (axial) resistance** ($R_i$) to passive current flow longitudinal to the axon. - Reduced internal resistance allows local current loops to spread further and faster, significantly increasing the **length constant**, thereby increasing conduction velocity. *Conduction velocity decreases* - This is incorrect, as larger diameter decreases internal resistance, leading directly to a **faster electrotonic spread** of depolarization and a higher conduction speed. - Decreased conduction velocity is typically observed in **small-diameter** or **demyelinated** axons where internal resistance is higher or membrane capacitance is altered. *Membrane resistance increases* - Membrane resistance ($R_m$) is determined by the density and activity of **leak ion channels** within the cell membrane, which is independent of the overall axon diameter. - While the total membrane area increases, the **specific membrane resistance** (resistance per unit area) does not change with diameter. *Action potential amplitude increases* - Action potential (AP) conduction is an **all-or-none** phenomenon, meaning the amplitude is fixed and determined by the electrochemical gradient of **voltage-gated sodium channels**. - Changes in axon diameter influence the **speed** of propagation (conduction velocity), but they do not alter the required peak voltage (amplitude) of the action potential.

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.

Question 1

The thin filament of muscle consists of all the following except:

Accepted Answer

Myosin

Answer

Actin

Answer

Troponin

Answer

Tropomyosin

Question 2

What does the nerve impulse leading to smooth muscle contraction initiate?

Accepted Answer

Opening of calcium channels, leading to increased intracellular Ca+2 concentration.

Answer

Depolarization of both the plasma membrane and T-tubules.

Answer

Inhibition of Na+ entry into the sarcomere.

Answer

Binding of acetylcholine to receptors in the sarcoplasmic reticulum.

Question 3

As a neuron’s diameter increases, what effect manifests?

Accepted Answer

Conduction velocity increases

Answer

Conduction velocity decreases

Answer

Membrane resistance increases

Answer

Action potential amplitude increases

Question 4

In malignant hyperthermia, a genetic mutation in the ryanodine receptor (RYR1) leads to a life-threatening reaction to certain anesthetic agents. What is the primary ionic abnormality caused by this receptor defect?

Accepted Answer

Increased calcium release from the sarcoplasmic reticulum

Answer

Increased sodium influx into the muscle cell

Answer

Increased potassium uptake into the sarcoplasmic reticulum

Answer

Increased chloride efflux from the muscle cell

Question 5

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 6

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 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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