Nerve and Muscle Physiology Practice Questions

Q: Wallerian degeneration involves:

Successive fragmentation of the axon.. **Explanation:** **Wallerian Degeneration** refers to the sequence of events that occur distal to the site of a nerve injury (axotomy). [1] **1. Why Option C is Correct:** When an axon is severed, the distal segment loses its connection to the metabolic center (the cell body). Within 24–48 hours, the cytoskeleton breaks down, leading to the **successive fragmentation of the axon** and its myelin sheath. [1] This debris is subsequently cleared by macrophages and Schwann cells to prepare a path for potential regeneration. **2. Analysis of Incorrect Options:** * **Options A & D (Chromatolysis and Swelling of the cell body):** These describe the **Retrograde reaction** occurring in the *proximal* segment and the cell body (soma), not the distal segment. * **Option B (Only the CNS):** Wallerian degeneration occurs in both the Peripheral Nervous System (PNS) and the Central Nervous System (CNS). However, it is more efficient in the PNS due to Schwann cells; in the CNS, the process is slower and often inhibited by glial scarring. [2] **3. High-Yield Clinical Pearls for NEET-PG:** * **Timeline:** Degeneration begins within 24 hours; myelin sheath breakdown follows by day 3–5. * **Bands of Büngner:** These are columns of proliferating Schwann cells in the distal stump that guide the regenerating axon. [1] * **Nissl Substance:** Composed of Rough Endoplasmic Reticulum (RER); its disappearance (chromatolysis) is a hallmark of the cell body's response to injury. * **Regeneration Rate:** In the PNS, axons typically regrow at a rate of approximately **1–3 mm/day**.

Q: Which of the following statements is NOT true for smooth muscle?

The actin to myosin ratio is approximately 2:1.. ### Explanation **Why Option B is the Correct Answer (The False Statement):** In smooth muscle, the ratio of actin (thin filaments) to myosin (thick filaments) is significantly higher than in skeletal muscle. While skeletal muscle has a ratio of approximately 2:1 or 3:1, **smooth muscle has a ratio of about 10:1 to 15:1**. This high density of actin allows smooth muscle to generate significant force despite having less myosin. **Analysis of Other Options:** * **A. No Z-discs are present:** This is **true**. Smooth muscle lacks the organized sarcomere structure of striated muscle. Instead of Z-discs, thin filaments are anchored to **Dense Bodies** (composed of $\alpha$-actinin), which are distributed throughout the sarcoplasm and attached to the cell membrane. * **C. Contraction is primarily initiated by the thick filament:** This is **true**. Unlike skeletal muscle (thin-filament regulated via Troponin), smooth muscle is **thick-filament regulated**. Contraction begins when Calcium-Calmodulin activates **Myosin Light Chain Kinase (MLCK)**, which phosphorylates the myosin head. * **D. A 'latch bridge' phenomenon is observed:** This is **true**. This unique mechanism allows smooth muscle to maintain high tension for long periods with very low ATP consumption. It occurs when myosin is dephosphorylated while still attached to actin, slowing the detachment rate. **High-Yield NEET-PG Pearls:** * **Calmodulin** is the functional counterpart of Troponin C in smooth muscle. * **Troponin is absent** in smooth muscle; instead, **Caldesmon** and **Calponin** inhibit the actin-myosin interaction. * Smooth muscle can shorten to a much greater degree than skeletal muscle (up to 80% of its length). * **Caveolae** serve as rudimentary analogs to the T-tubule system.

Q: A 56-year-old woman with severe muscle weakness is hospitalized. The only abnormality in her laboratory values is an elevated serum K+ concentration. The elevated serum K+ causes muscle weakness because?

Na+ channels are closed by depolarization. ### Explanation **1. Why the Correct Answer is Right (Na+ channels are closed by depolarization)** In hyperkalemia, the increased extracellular $K^+$ concentration decreases the concentration gradient across the cell membrane. According to the **Nernst equation**, this causes the resting membrane potential (RMP) to become **less negative (depolarized)**. Initially, this brings the RMP closer to the threshold, making cells more excitable. However, **sustained depolarization** leads to the **inactivation of voltage-gated $Na^+$ channels**. These channels enter a "closed-and-not-ready" state (the inactivation gate or 'h' gate closes). Without functional $Na^+$ channels, the cell cannot generate an action potential, leading to muscle inexcitability and clinical weakness or paralysis. **2. Why the Other Options are Wrong** * **Option A:** The RMP is **depolarized** (becomes more positive), not hyperpolarized. Hyperpolarization would occur in hypokalemia. * **Option B:** The $K^+$ equilibrium potential ($E_K$) becomes **less negative** (e.g., moving from -90mV toward -70mV). This is a depolarizing shift, not hyperpolarization. * **Option C:** The $Na^+$ equilibrium potential is determined by $Na^+$ concentrations, which remain largely unaffected by changes in serum $K^+$. **3. Clinical Pearls for NEET-PG** * **ECG Changes in Hyperkalemia:** Tall peaked T-waves (earliest sign) $\rightarrow$ PR prolongation $\rightarrow$ Loss of P-wave $\rightarrow$ Widening of QRS (Sine wave pattern) $\rightarrow$ V-fib/Asystole. * **Management:** "C BIG K" (Calcium gluconate for membrane stabilization, Bicarbonate/Beta-agonists, Insulin + Glucose, Kayexalate/Dialysis). * **Membrane Stabilization:** Calcium gluconate does *not* lower $K^+$ levels; it antagonizes the membrane effects of hyperkalemia by shifting the threshold potential, restoring excitability.

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.

Question 1

Wallerian degeneration involves:

Accepted Answer

Successive fragmentation of the axon.

Answer

Chromatolysis.

Answer

Only the central nervous system.

Answer

Swelling of the cell body.

Question 2

Which of the following statements is NOT true for smooth muscle?

Accepted Answer

The actin to myosin ratio is approximately 2:1.

Answer

No Z-discs are present.

Answer

Contraction is primarily initiated by the thick filament.

Answer

A 'latch bridge' phenomenon is observed.

Question 3

Arrange the following sequentially in the ascending order of occurrence with regard to signal transmission at the neuromuscular junction: Neurotransmitter degradation, Exocytosis, Calcium influx, End plate potential?

Accepted Answer

Calcium influx - Exocytosis - End plate potential - Neurotransmitter degradation

Answer

Neurotransmitter degradation - Exocytosis - Calcium influx - End plate potential

Answer

Calcium influx - Neurotransmitter degradation - Exocytosis - End plate potential

Answer

Exocytosis - End plate potential - Neurotransmitter degradation - Calcium influx

Question 4

A 56-year-old woman with severe muscle weakness is hospitalized. The only abnormality in her laboratory values is an elevated serum K+ concentration. The elevated serum K+ causes muscle weakness because?

Accepted Answer

Na+ channels are closed by depolarization

Answer

The resting membrane potential is hyperpolarized

Answer

The K+ equilibrium potential is hyperpolarized

Answer

The Na+ equilibrium potential is hyperpolarized

Question 5

Repolarization is due to the opening of which ion channels?

Accepted Answer

Potassium (K+)

Answer

Sodium (Na+)

Answer

Bicarbonate (HCO3-)

Answer

Chloride (Cl-)

Question 6

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 7

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 8

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 9

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 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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