Nerve and Muscle Physiology Practice Questions

Q: Repolarization is due to the opening of which ion channels?

Potassium (K+). **Explanation:** The Action Potential (AP) consists of two primary phases: depolarization and repolarization. **Repolarization** is the process by which the cell membrane potential returns to its negative resting state after depolarization. **Why Potassium (K+) is correct:** During the peak of an action potential, voltage-gated Na+ channels close (inactivate) and **voltage-gated K+ channels open**. Because the concentration of K+ is much higher inside the cell than outside, K+ ions rapidly exit the cell (efflux) down their electrochemical gradient. This loss of positive charge from the intracellular compartment restores the negative resting membrane potential. **Analysis of Incorrect Options:** * **Sodium (Na+):** The opening of voltage-gated Na+ channels leads to Na+ influx, which causes **Depolarization** (making the cell interior more positive). * **Bicarbonate (HCO3-):** This ion is primarily involved in acid-base balance and CO2 transport; it does not play a primary role in the rapid phases of the neuronal or muscular action potential. * **Chloride (Cl-):** The entry of Cl- into a cell typically causes **Hyperpolarization** (Inhibitory Post-Synaptic Potential - IPSP), making the cell less likely to fire, rather than driving the standard repolarization phase. **High-Yield Facts for NEET-PG:** * **Hyperkalemia:** Increases membrane excitability initially (brings RMP closer to threshold) but can eventually lead to paralysis due to persistent inactivation of Na+ channels. * **After-hyperpolarization:** Occurs because voltage-gated K+ channels are slow to close, allowing the membrane potential to briefly become more negative than the resting level. * **Cardiac Repolarization:** In cardiac ventricular muscle, Phase 2 (Plateau) is due to Ca2+ influx, while **Phase 3 (Rapid Repolarization)** is due to K+ efflux.

Q: What is the resting membrane potential in a nerve fiber?

Depends upon K+ equilibrium. ### Explanation The Resting Membrane Potential (RMP) of a nerve fiber (typically **-70 mV**) is primarily determined by the selective permeability of the cell membrane to specific ions. **Why the correct answer is right:** According to the **Goldman-Hodgkin-Katz equation**, the RMP is closest to the equilibrium potential of the ion to which the membrane is most permeable. At rest, the nerve membrane is significantly more permeable to **Potassium (K+)** than to Sodium (Na+) due to the presence of "leaky" K+ channels. Therefore, the RMP is primarily a reflection of the **K+ equilibrium potential** (calculated by the Nernst equation as approximately -94 mV). While the Na+-K+ ATPase pump maintains the gradient, the actual potential value depends on K+ efflux. **Analysis of Incorrect Options:** * **Option A:** Ventricular muscle fibers have a much more negative RMP (approx. **-90 mV**) compared to nerve fibers (-70 mV) due to higher resting K+ conductance. * **Option B:** RMP is a transmembrane potential (difference between inside and outside). It must be measured using **intracellular microelectrodes**; surface electrodes (like ECG/EEG) only measure extracellular field potentials. * **Option C:** If extracellular K+ increases (hyperkalemia), the concentration gradient for K+ to leave the cell decreases. This causes the RMP to become **less negative (depolarization)**, which is technically a **decrease** in the magnitude of the potential. **High-Yield Facts for NEET-PG:** * **Nernst Potential for K+:** -94 mV | **Na+:** +61 mV | **Cl-:** -70 mV. * **Na+-K+ ATPase:** It is electrogenic, contributing about -4 to -5 mV directly to the RMP. * **Clinical Correlation:** Hypokalemia hyperpolarizes the membrane (makes RMP more negative), making it harder to fire an action potential, leading to muscle weakness and paralysis.

Q: Which of the following statements about sensory fibers is NOT true?

III - A gamma. This question tests your knowledge of the **Erlanger-Gasser classification** (alphabetical) and the **Lloyd-Hunt classification** (numerical) of nerve fibers. ### **Explanation of the Correct Answer** **Option C is NOT true.** Type **III** sensory fibers correspond to **A-delta (Aδ)** fibers, not A-gamma. * **Type III (Aδ):** These are thin, myelinated fibers that carry sensations of fast pain, cold, and touch. * **A-gamma (Aγ):** These are **motor** fibers (not sensory) that innervate the contractile ends of the muscle spindle (intrafusal fibers). ### **Analysis of Other Options** * **A. Ia - Alpha:** Correct. Type Ia fibers are large, myelinated fibers that originate from the primary endings of muscle spindles (annulospiral endings). They are the fastest conducting sensory fibers. * **B. II - A beta:** Correct. Type II fibers correspond to A-beta fibers. They carry information from secondary muscle spindle endings (flower-spray) and cutaneous tactile receptors. * **D. IV - C:** Correct. Type IV fibers are unmyelinated **C fibers**. They are the smallest and slowest, carrying sensations of slow/chronic pain, warmth, and itch. ### **High-Yield NEET-PG Pearls** * **Numerical Classification (I-IV):** Used exclusively for **sensory** (afferent) fibers. * **Alphabetical Classification (A-C):** Used for both **sensory and motor** fibers. * **Fastest vs. Slowest:** Type Ia (Alpha) is the fastest; Type IV (C) is the slowest. * **Muscle Spindles:** Primary endings are **Ia** (velocity/length); Secondary endings are **II** (length only). * **Golgi Tendon Organ (GTO):** Uses **Ib** fibers to monitor muscle tension. * **Susceptibility:** **B fibers** are most sensitive to local anesthetics; **A fibers** are most sensitive to pressure; **C fibers** are most sensitive to hypoxia.

Q: Which of the following is referred to as a 'relaxation protein'?

Tropomyosin. ### Explanation **Correct Option: C. Tropomyosin** **Why Tropomyosin is the 'Relaxation Protein':** In a resting muscle fiber, the interaction between actin and myosin is physically blocked. **Tropomyosin** is a long, filamentous protein that wraps around the actin filament, covering the active myosin-binding sites. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing the power stroke and keeping the muscle in a state of relaxation. Contraction only occurs when Calcium binds to Troponin C, causing a conformational change that pulls tropomyosin away from these binding sites. Therefore, because its primary role is to **inhibit contraction**, it is classically referred to as the relaxation protein. **Analysis of Incorrect Options:** * **A. Actin:** Known as the **thin filament**. It is a contractile protein that provides the binding sites for myosin; it does not facilitate relaxation. * **B. Myosin:** Known as the **thick filament**. It is the primary contractile protein (motor protein) that utilizes ATP to generate force. * **D. Dystrophin:** This is a **structural protein** that links the actin cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. Its clinical significance lies in Duchenne Muscular Dystrophy (DMD). **High-Yield NEET-PG Pearls:** * **Troponin Complex:** Consists of three subunits: **Troponin I** (Inhibitory), **Troponin T** (Tropomyosin-binding), and **Troponin C** (Calcium-binding). * **The "Fenn Effect":** The increased energy consumption (ATP) that occurs when a muscle performs work. * **Rigor Mortis:** Occurs due to the depletion of ATP; without ATP, the myosin head cannot detach from actin, leading to permanent cross-bridge formation. * **Calsequestrin:** A protein in the Sarcoplasmic Reticulum that binds calcium, allowing for high-capacity storage.

Q: Which of the following structures contains Group B nerve fibers?

Autonomic preganglionic fibers. ### Explanation The classification of nerve fibers is a high-yield topic for NEET-PG, primarily based on the **Erlanger-Gasser classification**, which categorizes fibers according to diameter, myelination, and conduction velocity. **Why Option C is Correct:** **Group B fibers** are characterized as medium-diameter, **lightly myelinated** fibers with a moderate conduction velocity (3–15 m/s). The classic anatomical location for Group B fibers is the **autonomic preganglionic neurons** (both sympathetic and parasympathetic). Their light myelination allows for efficient signal transmission to autonomic ganglia before the signal transitions to unmyelinated Group C postganglionic fibers. **Analysis of Incorrect Options:** * **Option A (Intrafusal fibers):** These are innervated by **A-gamma (Aγ)** motor neurons. These are myelinated fibers responsible for maintaining muscle spindle sensitivity. * **Option B (Golgi tendon apparatus):** These are associated with **Group Ib** sensory fibers. These are large, heavily myelinated fibers with the fastest conduction velocities. * **Option D (Spinothalamic tracts):** These tracts primarily carry pain and temperature sensations via **A-delta (Aδ)** (fast pain) and **Group C** (slow pain) fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Hypoxia:** Type A > Type B > Type C (Large fibers suffer first). * **Pressure:** Type A > Type B > Type C (e.g., Saturday Night Palsy). * **Local Anesthetics:** Type C > Type B > Type A (Small, unmyelinated fibers are blocked first). * **Fastest vs. Slowest:** Type **A-alpha (Aα)** are the fastest and thickest; Type **C** are the slowest, thinnest, and the only **unmyelinated** fibers. * **Postganglionic Autonomic Fibers:** Always remember these are **Type C** fibers.

Q: What does the 'Lloyds classification' primarily categorize?

Sensory nerve fibers. **Explanation:** The **Lloyd-Hunt classification** (often referred to as Lloyds classification) is a system specifically designed to categorize **sensory (afferent) nerve fibers** based on their diameter and conduction velocity. This classification uses Roman numerals (I, II, III, and IV) and is primarily used to describe sensory input from muscles and joints (proprioception and mechanoreception). * **Why Option A is correct:** Unlike the Erlanger-Gasser classification (which uses letters like A, B, and C), the Lloyd classification applies **exclusively to sensory fibers**. For example, Type Ia fibers originate from muscle spindle primary endings, and Type Ib fibers originate from Golgi tendon organs. * **Why Options B and C are incorrect:** Motor (efferent) fibers are categorized only under the **Erlanger-Gasser classification** (e.g., Alpha-motor neurons are Aα; Gamma-motor neurons are Aγ). There is no "Type I" or "Type II" motor neuron in the Lloyd system. * **Why Option D is incorrect:** The classification is a fundamental physiological tool used to describe the speed and size of sensory neurons. **High-Yield Facts for NEET-PG:** 1. **Correlation Table:** * **Type Ia/Ib:** Corresponds to **Aα** (Fastest, largest, myelinated). * **Type II:** Corresponds to **Aβ** (Touch, pressure). * **Type III:** Corresponds to **Aδ** (Fast pain, temperature). * **Type IV:** Corresponds to **C fibers** (Slow pain, unmyelinated, smallest). 2. **Clinical Pearl:** Type IV (C fibers) are the only unmyelinated fibers in the body and are the first to be blocked by local anesthetics but the last to be affected by pressure/hypoxia. 3. **Memory Aid:** **L**loyd is for **L**earning what we feel (**Sensory**).

Q: Pricking pain, cold, and crude touch are conducted by which type of nerve fibers?

A delta fibers. ### Explanation The correct answer is **D. A delta fibers**. **1. Why A delta fibers are correct:** Nerve fibers are classified based on diameter and myelination (Erlanger-Gasser classification). **A delta (Aδ) fibers** are thin, myelinated fibers that conduct impulses at a moderate velocity (5–30 m/s). They are responsible for "fast pain" (sharp, pricking sensation), cold temperature, and crude touch. Because they are myelinated, they allow for the rapid localization of a stimulus, unlike the slow, unmyelinated C fibers which carry "slow pain" (dull, aching). **2. Why the other options are incorrect:** * **A alpha (Aα) fibers:** These are the thickest and fastest myelinated fibers. they primarily carry **proprioception** (muscle spindles and Golgi tendon organs) and somatic motor signals. * **A beta (Aβ) fibers:** These are large, myelinated fibers that carry **fine touch**, pressure, and vibration. They are associated with mechanoreceptors like Meissner’s and Pacinian corpuscles. * **A gamma (Aγ) fibers:** These are medium-sized myelinated fibers that specifically innervate the **intrafusal fibers** of the muscle spindle, regulating muscle tone. **3. NEET-PG High-Yield Pearls:** * **Pain Dualism:** Remember that pain has two components: **Aδ** (Fast, pricking, localized) and **C fibers** (Slow, burning/aching, poorly localized). * **Sensitivity to Anesthesia:** Local anesthetics block small, unmyelinated fibers first. The order of loss is: **Pain > Temperature > Touch > Pressure**. * **Susceptibility:** **Type A** fibers are most susceptible to **pressure/hypoxia**, while **Type C** fibers are most susceptible to **local anesthetics**. * **Cold vs. Warm:** Cold is carried by **Aδ**, while warmth is carried by **C fibers**.

Question 1

Repolarization is due to the opening of which ion channels?

Accepted Answer

Potassium (K+)

Answer

Sodium (Na+)

Answer

Bicarbonate (HCO3-)

Answer

Chloride (Cl-)

Question 2

Fibrillation of skeletal muscle is associated with all of the following conditions except?

Accepted Answer

Requires a strong stimulus

Answer

Occurs a long time after denervation

Answer

Hypersensitivity to acetylcholine

Answer

Receptors spread over the entire muscle cell membrane

Question 3

What is the resting membrane potential in a nerve fiber?

Accepted Answer

Depends upon K+ equilibrium

Answer

Equal to the potential of ventricular muscle fiber

Answer

Can be measured by surface electrodes

Answer

Increases as extracellular K+ increases

Question 4

Which of the following statements about sensory fibers is NOT true?

Accepted Answer

III - A gamma

Answer

Ia - Alpha

Answer

II - A beta

Answer

IV - C

Question 5

What is true about end-plate potential (EPP)?

Accepted Answer

It occurs as a result of altered permeability for Na+ and K+.

Answer

It is a self-regenerative change.

Answer

It results from the release of a single transmitter vesicle.

Answer

It exhibits all-or-none properties.

Question 6

A travelling nerve impulse does not depolarize the area immediately behind it, because:

Accepted Answer

It is refractory

Answer

It is hyperpolarized

Answer

It is not self-propagating

Answer

The condition is always orthodromic

Question 7

Which of the following is referred to as a 'relaxation protein'?

Accepted Answer

Tropomyosin

Answer

Actin

Answer

Myosin

Answer

Dystrophin

Question 8

Which of the following structures contains Group B nerve fibers?

Accepted Answer

Autonomic preganglionic fibers

Answer

Intrafusal fibers of the muscle spindle

Answer

Golgi tendon apparatus

Answer

Spinothalamic tracts

Question 9

What does the 'Lloyds classification' primarily categorize?

Accepted Answer

Sensory nerve fibers

Answer

Motor nerve fibers

Answer

Both sensory and motor nerve fibers

Answer

Neither sensory nor motor nerve fibers

Question 10

Pricking pain, cold, and crude touch are conducted by which type of nerve fibers?

Accepted Answer

A delta fibers

Answer

A alpha fibers

Answer

A beta fibers

Answer

A gamma fibers

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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