Nerve and Muscle Physiology Practice Questions

Q: What is true about an action potential?

A threshold stimulus is required for its generation.. ### Explanation **Correct Option: D. A threshold stimulus is required for its generation.** An action potential (AP) is an "all-or-none" electrical event. For an AP to occur, the membrane potential must be depolarized to a specific level known as the **threshold potential** (typically -55 mV in neurons). Once this threshold is reached, voltage-gated sodium channels open rapidly, triggering a self-propagating depolarization. If the stimulus is sub-threshold, no AP is generated. **Analysis of Incorrect Options:** * **A. It is a decremental phenomenon:** This is incorrect. Action potentials are **non-decremental**; they maintain a constant amplitude and shape as they propagate along the axon. Local potentials (like EPSPs or IPSPs), however, are decremental. * **B. It does not obey the all-or-none phenomenon:** This is incorrect. The AP strictly follows the **All-or-None Law**. If the threshold is met, a full-strength AP occurs; if not, nothing happens. Increasing the stimulus strength beyond the threshold does not increase the AP's amplitude. * **C. Potassium ions move from ECF to ICF:** This is incorrect. During the repolarization phase of an AP, voltage-gated **potassium channels open**, causing $K^+$ to move **out of the cell** (from ICF to ECF) down its electrochemical gradient. **High-Yield NEET-PG Pearls:** * **Depolarization phase:** Primarily due to $Na^+$ influx. * **Repolarization phase:** Primarily due to $K^+$ efflux. * **Absolute Refractory Period:** Occurs during the peak of AP when $Na^+$ channels are in an inactivated state; no second AP can be fired regardless of stimulus strength. * **Myelination:** Increases conduction velocity via **Saltatory Conduction** (jumping between Nodes of Ranvier).

Q: Z-lines are anchored to the sarcolemma by:

Desmin. **Explanation:** The correct answer is **Desmin**. In skeletal muscle, the structural integrity and spatial alignment of myofibrils are maintained by a complex network of intermediate filaments. **Desmin** is the primary intermediate filament that encircles the Z-lines of adjacent myofibrils, linking them to each other and anchoring the entire Z-disk complex to the sarcolemma (cell membrane) via costameres. This ensures that all myofibrils contract in synchrony and maintains the lateral registration of the sarcomeres. **Analysis of Incorrect Options:** * **Titin (Option A):** This is the largest known protein. It acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the muscle and centering the myosin filaments. * **Dystrophin (Option B):** While it also links the cytoskeleton to the sarcolemma, it specifically connects **F-actin** (not the Z-line directly) to the dystroglycan complex in the membrane. Its deficiency leads to Duchenne Muscular Dystrophy. * **Actinin (Option D):** Specifically **α-actinin**, this protein is located *within* the Z-line. Its primary role is to anchor the plus ends of actin (thin) filaments to the Z-disk, rather than anchoring the Z-disk to the sarcolemma. **High-Yield Clinical Pearls for NEET-PG:** * **Costameres:** These are the functional units (containing desmin, vinculin, and dystrophin) that couple the sarcomere to the extracellular matrix. * **Nebulin:** Acts as a "molecular ruler" to regulate the length of actin filaments. * **Desmin-related Myopathy:** Mutations in the desmin gene lead to myofibrillar myopathies characterized by muscle weakness and cardiac conduction blocks.

Q: Which of the following are regulator proteins?

Both Troponin and Tropomyosin. In skeletal and cardiac muscle contraction, proteins are categorized based on their function into contractile, regulatory, and structural proteins. [2] **Understanding the Concept:** The correct answer is **Both Troponin and Tropomyosin** because these two proteins work together to regulate the interaction between actin and myosin. [2] * **Tropomyosin:** A long, rod-like protein that winds around the actin filament. In a resting state, it physically covers the myosin-binding sites on actin, preventing contraction. * **Troponin:** A complex of three subunits (TnT, TnI, and TnC). When calcium levels rise, calcium binds to **Troponin C**, causing a conformational change that pulls tropomyosin away from the binding sites, allowing the "power stroke" to occur. **Analysis of Options:** * **Option A (Troponin):** While correct, it is incomplete on its own as tropomyosin also serves a regulatory role. * **Option B (Tropomyosin):** Similarly, it is a regulatory protein but does not function independently of the troponin complex. * **Option D (None):** Incorrect, as both proteins are the primary regulators of the sliding filament mechanism. [2] **High-Yield NEET-PG Pearls:** 1. **Contractile Proteins:** Actin (thin filament) and Myosin (thick filament). [2] 2. **Structural Proteins:** Titin (largest protein, provides elasticity), Nebulin, and Dystrophin (defective in Duchenne Muscular Dystrophy). [1] 3. **Clinical Marker:** **Troponin I and T** are highly specific cardiac biomarkers used in the diagnosis of Myocardial Infarction (MI). 4. **Calcium Source:** In skeletal muscle, calcium is released from the Sarcoplasmic Reticulum; in cardiac muscle, it involves Calcium-Induced Calcium Release (CICR). [1]

Q: Increase in cytosolic calcium from intracellular storage during smooth muscle contraction is due to?

Inositol trisphosphate (IP3) and diacylglycerol (DAG). **Explanation:** The correct answer is **D. Inositol trisphosphate (IP3) and diacylglycerol (DAG)**. In smooth muscle, contraction is initiated by an increase in cytosolic calcium ($Ca^{2+}$). While some calcium enters via voltage-gated channels, a significant portion is released from the **Sarcoplasmic Reticulum (SR)**—the primary intracellular storage site. This process is mediated by the **Gq-protein coupled receptor (GPCR) pathway**: 1. A ligand (e.g., Norepinephrine, Acetylcholine) binds to a Gq-coupled receptor. 2. This activates **Phospholipase C (PLC)**, which cleaves membrane-bound PIP2 into **IP3** and **DAG**. 3. **IP3** binds to specific IP3-gated calcium release channels on the SR membrane, triggering the efflux of $Ca^{2+}$ into the cytosol. 4. **DAG** activates Protein Kinase C (PKC), which further modulates contractile proteins. **Why the other options are incorrect:** * **A. cAMP:** Generally promotes **smooth muscle relaxation** (e.g., via $\beta_2$ receptors) by inhibiting Myosin Light Chain Kinase (MLCK) and stimulating calcium sequestration back into the SR. * **B. cGMP:** Acts as a second messenger for Nitric Oxide (NO) and Atrial Natriuretic Peptide (ANP). It causes **vasodilation/relaxation** by activating Protein Kinase G (PKG), which dephosphorylates Myosin Light Chains. * **C. cCMP:** This is a minor signaling molecule and does not play a primary role in the standard excitation-contraction coupling of smooth muscle. **High-Yield NEET-PG Pearls:** * **Calmodulin:** Unlike skeletal muscle (which uses Troponin C), smooth muscle $Ca^{2+}$ binds to **Calmodulin** to activate MLCK. * **L-type Calcium Channels:** These are the targets of Calcium Channel Blockers (CCBs) like Nifedipine. * **Pharmacomechanical Coupling:** The ability of IP3 to cause contraction without a change in membrane potential is a unique feature of smooth muscle.

Q: What is the term for a decrease in reflex response after repetitive stimulation?

Fatigue. **Explanation:** The correct answer is **Fatigue**. In the context of reflex activity, repetitive stimulation of the afferent nerve leads to a gradual decrease and eventual disappearance of the reflex response. This occurs primarily due to the **exhaustion of neurotransmitter stores** at the synaptic level within the reflex arc. Unlike nerve fibers, which are relatively indefatigable, the synapse is the most vulnerable site for fatigue in the neural pathway. **Analysis of Options:** * **A. Summation:** This refers to the cumulative effect of multiple stimuli. It can be *temporal* (repeated stimuli from one neuron) or *spatial* (simultaneous stimuli from multiple neurons) that combine to reach the threshold for an action potential. It increases rather than decreases the response. * **C. Irradiation:** This occurs when the strength of a stimulus is increased, causing the impulse to spread to more neurons in the spinal cord, involving more muscle groups (e.g., a strong painful stimulus causing withdrawal of the entire limb instead of just a finger). * **D. Occlusion:** This is a phenomenon where the response to simultaneous stimulation of two afferent nerves is *less* than the sum of their individual responses because they share a common pool of postsynaptic neurons. While it involves a "decrease" in expected output, it is not caused by repetitive stimulation. **High-Yield Facts for NEET-PG:** * **Site of Fatigue:** In a nerve-muscle preparation, the **synapse** (neuromuscular junction) fatigues first, followed by the muscle, while the nerve fiber is the last to fatigue. * **Synaptic Delay:** The time taken for the neurotransmitter to release and act on the postsynaptic membrane (approx. 0.5 ms) is the reason for the delay in reflex action. * **One-way conduction:** Reflexes follow the **Bell-Magendie Law**, stating that impulses pass only from the axon to the dendrite/soma across a synapse.

Q: Which ion has the lowest electrochemical driving force in a typical neuron with a resting membrane potential of -65 millivolts?

Chloride. ### Explanation The **electrochemical driving force** is the net force acting on an ion, determined by the difference between the actual membrane potential ($V_m$) and the ion's equilibrium potential ($E_{ion}$). The formula is: **Driving Force = $V_m - E_{ion}$** **1. Why Chloride (A) is Correct:** In a typical neuron, the resting membrane potential ($V_m$) is approximately **-65 to -70 mV**. The equilibrium potential for Chloride ($E_{Cl}$) is also approximately **-65 to -70 mV**. Because $V_m$ is almost equal to $E_{Cl}$, the net driving force is near zero. Consequently, there is very little net movement of chloride ions across the resting membrane. **2. Why the Other Options are Incorrect:** * **Potassium (B):** $E_K$ is typically around **-90 mV**. The driving force is $|-65 - (-90)| = 25\text{ mV}$. While small, it is significantly higher than that of Chloride. * **Sodium (C):** $E_{Na}$ is approximately **+60 mV**. The driving force is massive: $|-65 - (+60)| = 125\text{ mV}$. This creates a strong inward pressure for $Na^+$ to enter the cell. * **Calcium (D):** $E_{Ca}$ is very positive (approx. **+120 mV**). Combined with a very low intracellular concentration, the driving force is the highest among common ions ($>180\text{ mV}$). **3. High-Yield Facts for NEET-PG:** * **Nernst Equation:** Used to calculate the equilibrium potential for a single ion. * **Goldman-Hodgkin-Katz (GHK) Equation:** Used to calculate the RMP by considering the permeability of all ions. * **RMP Determinant:** The RMP is closest to the equilibrium potential of the ion with the **highest permeability** (which is Potassium at rest). * **Chloride Paradox:** In some neurons, $Cl^-$ is passively distributed, making its $E_{Cl}$ exactly equal to RMP, resulting in zero driving force.

Question 1

What is true about an action potential?

Accepted Answer

A threshold stimulus is required for its generation.

Answer

It is a decremental phenomenon.

Answer

It does not obey the all-or-none phenomenon.

Answer

Potassium ions move from the extracellular fluid to the intracellular fluid.

Question 2

Z-lines are anchored to the sarcolemma by:

Accepted Answer

Desmin

Answer

Titin

Answer

Dystrophin

Answer

Actinin

Question 3

What is an auxotonic contraction of a muscle?

Accepted Answer

A contraction where the muscle length changes and the tension changes throughout the contraction.

Answer

A contraction where the tension developed changes when the length of the muscle changes.

Answer

A contraction where the muscle length is constant but tension changes.

Answer

A contraction where the muscle length changes but tension remains constant.

Question 4

Maximum number of sodium channels per square micrometer is present in which location?

Accepted Answer

Nodes of Ranvier

Answer

Cell body

Answer

Axon terminal

Answer

Surface of myelin

Question 5

Which of the following statements is true about myoglobin?

Accepted Answer

Myoglobin has a higher O2 affinity compared to that of hemoglobin.

Answer

Myoglobin binds to 4 moles of O2 per mole.

Answer

The myoglobin dissociation curve has a parabolic profile.

Answer

The myoglobin dissociation curve shows the Bohr effect.

Question 6

Which of the following are regulator proteins?

Accepted Answer

Both Troponin and Tropomyosin

Answer

Troponin

Answer

Tropomyosin

Answer

None of the above

Question 7

Increase in cytosolic calcium from intracellular storage during smooth muscle contraction is due to?

Accepted Answer

Inositol trisphosphate (IP3) and diacylglycerol (DAG)

Answer

Cyclic adenosine monophosphate (cAMP)

Answer

Cyclic guanosine monophosphate (cGMP)

Answer

Cyclic cytidine monophosphate (cCMP)

Question 8

What is the term for a decrease in reflex response after repetitive stimulation?

Accepted Answer

Fatigue

Answer

Summation

Answer

Irradiation

Answer

Occlusion

Question 9

Which ion has the lowest electrochemical driving force in a typical neuron with a resting membrane potential of -65 millivolts?

Accepted Answer

Chloride

Answer

Potassium

Answer

Sodium

Answer

Calcium

Question 10

In a nerve, the magnitude of the action potential overshoot is normally a function of which of the following?

Accepted Answer

Extracellular sodium concentration

Answer

Magnitude of the stimulus

Answer

Intracellular potassium concentration

Answer

Resting membrane potential

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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