Nerve and Muscle Physiology Practice Questions

Q: What is an action potential?

A threshold stimulus is required. ### Explanation An **Action Potential (AP)** is a rapid, temporary change in the membrane potential of an excitable cell (nerve or muscle) that propagates along the membrane. **Why the correct answer is right:** For an action potential to occur, the membrane must be depolarized to a specific level known as the **threshold potential** (typically -55 mV in neurons). A stimulus must be strong enough to reach this threshold to trigger the rapid opening of voltage-gated sodium channels. If the stimulus is sub-threshold, no action potential is generated. **Analysis of Incorrect Options:** * **A. A decremental phenomenon:** This is incorrect. Action potentials are **non-decremental**, meaning they maintain a constant amplitude and shape as they propagate. Local potentials (like EPSPs), however, are decremental. * **B. Does not obey the all-or-none phenomenon:** This is incorrect. APs strictly follow the **All-or-None Law**. Once the threshold is reached, an AP of maximal magnitude occurs; if not reached, nothing happens. * **C. Potassium ions move from ECF to ICF:** This is incorrect. During the **repolarization** phase of an AP, voltage-gated potassium channels open, causing $K^+$ ions to move **from the ICF to the ECF** (efflux) down their electrochemical gradient. **High-Yield Facts for NEET-PG:** * **Depolarization phase:** Primarily due to $Na^+$ influx. * **Repolarization phase:** Primarily due to $K^+$ efflux. * **Absolute Refractory Period:** Occurs during the firing and early repolarization phase; no second AP can be fired regardless of stimulus strength (due to inactivation of $Na^+$ channels). * **Overshoot:** The portion of the AP where the membrane potential becomes positive (above 0 mV).

Q: Which ion is primarily responsible for the resting membrane potential in neurons?

Potassium. **Explanation:** The **Resting Membrane Potential (RMP)** of a neuron, typically around **-70 mV**, is primarily determined by the permeability of the cell membrane to specific ions and their concentration gradients. 1. **Why Potassium (K⁺) is correct:** At rest, the neuronal membrane is **significantly more permeable to K⁺** than to any other ion (about 50–100 times more than to Na⁺). This is due to the presence of numerous **"leak channels"** that are open at rest. According to the **Nernst Equation**, the equilibrium potential for K⁺ is approximately -90 mV. Because the membrane is most permeable to K⁺, the RMP stays very close to this value. The Na⁺/K⁺ ATPase pump further maintains this gradient by pumping 3 Na⁺ out and 2 K⁺ in. 2. **Why other options are incorrect:** * **Sodium (Na⁺):** While Na⁺ has a strong electrochemical gradient to enter the cell, the membrane has very low permeability to it at rest. It is primarily responsible for the **depolarization** phase of the action potential. * **Chloride (Cl⁻):** Cl⁻ ions contribute to the RMP in some cells, but their role is secondary to K⁺. In neurons, Cl⁻ levels are often passively distributed. * **Calcium (Ca²⁺):** Ca²⁺ is crucial for neurotransmitter release and muscle contraction, but its resting permeability is negligible; thus, it does not significantly influence the RMP. **High-Yield NEET-PG Pearls:** * **Goldman-Hodgkin-Katz Equation:** Used to calculate RMP by considering the permeability and concentration of all major ions (K⁺, Na⁺, Cl⁻). * **Clinical Correlation:** Changes in extracellular K⁺ (Hyperkalemia/Hypokalemia) have the most profound effect on RMP. **Hyperkalemia** partially depolarizes the membrane (making it less negative), bringing it closer to the threshold and increasing excitability initially.

Q: Regulation of smooth muscle tone is persistently influenced by:

Latch bridge mechanism. **Explanation:** The **Latch bridge mechanism** is the physiological basis for the sustained, energy-efficient contraction (tone) characteristic of smooth muscle. Unlike skeletal muscle, smooth muscle can maintain high tension for long periods with very low ATP consumption. This occurs when **myosin light chain phosphatase (MLCP)** dephosphorylates the myosin head while it is still attached to actin. This "latches" the cross-bridge in place, significantly slowing the detachment rate. This allows the muscle to maintain **persistent tone** without requiring continuous high-frequency stimulation or massive ATP hydrolysis. **Analysis of Incorrect Options:** * **A. Calcium release from SR:** While calcium is essential for initiating contraction (via binding to Calmodulin), it is not the mechanism for *persistent* tone. In fact, smooth muscle relies heavily on extracellular calcium entry through voltage-gated channels rather than just SR release. * **B. Beta 1 receptor:** These receptors are primarily located in the **heart** (increasing heart rate and contractility). Smooth muscle tone in the bronchioles and blood vessels is more typically regulated by **Beta 2** (relaxation) or **Alpha 1** (contraction) receptors. * **C. Troponin:** This is a **high-yield negative fact**. Smooth muscle **lacks troponin**. Instead, it uses **Calmodulin** and **Caldesmon/Calponin** to regulate the interaction between actin and myosin. **NEET-PG High-Yield Pearls:** * **Calmodulin** is the functional analog of Troponin C in smooth muscle. * **Myosin Light Chain Kinase (MLCK)**: Phosphorylates myosin to *initiate* contraction. * **Myosin Light Chain Phosphatase (MLCP)**: Dephosphorylates myosin to *initiate* the latch state or relaxation. * Smooth muscle has the **slowest** cycling of cross-bridges but the **greatest** force of contraction per unit area compared to skeletal muscle.

Q: What is true about summation?

Subthreshold stimuli are used.. **Explanation:** Summation is the process by which individual graded potentials (post-synaptic potentials) are added together to reach the threshold required to trigger an action potential. **1. Why Option C is Correct:** Summation specifically involves **subthreshold stimuli**. A single subthreshold stimulus is insufficient to reach the firing level (threshold) of a neuron. However, if multiple subthreshold stimuli are applied in rapid succession or at different locations simultaneously, their cumulative effect can depolarize the membrane to the threshold, resulting in an action potential. If a stimulus were already suprathreshold, it would trigger an action potential on its own, making summation unnecessary. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Spatial Summation**, not Temporal. Temporal summation occurs when a single presynaptic terminal fires **repeatedly in rapid succession** (one after another). * **Option B:** This describes **Temporal Summation**. Spatial summation occurs when **multiple different presynaptic terminals** fire simultaneously (together) at different locations on the same postsynaptic neuron. **NEET-PG High-Yield Pearls:** * **EPSP vs. IPSP:** Summation can be excitatory (EPSP) or inhibitory (IPSP). The net change in membrane potential depends on the algebraic sum of all inputs. * **Location:** Summation typically occurs at the **axon hillock**, which has the highest density of voltage-gated Na+ channels and the lowest threshold for firing. * **Refractory Period:** Temporal summation is possible because the postsynaptic potential lasts longer than the refractory period of the presynaptic action potential.

Q: Which troponin subunit binds to tropomyosin to form the troponin-tropomyosin complex?

Troponin T. **Explanation:** The troponin complex is a heterotrimeric protein located on the thin (actin) filaments of muscle fibers. It plays a crucial role in regulating muscle contraction by mediating the interaction between actin and myosin. **1. Why Troponin T is Correct:** * **Troponin T (T for Tropomyosin):** This subunit is responsible for binding the troponin complex to **tropomyosin**. It anchors the entire complex and helps position tropomyosin over the myosin-binding sites on the actin filament during the resting state. **2. Analysis of Incorrect Options:** * **Troponin I (I for Inhibitory):** This subunit binds to actin and inhibits the ATPase activity of the actomyosin complex, effectively preventing contraction by blocking the interaction between actin and myosin. * **Troponin C (C for Calcium):** This subunit contains binding sites for calcium ions ($Ca^{2+}$). When calcium binds to Troponin C, it undergoes a conformational change that pulls the troponin-tropomyosin complex away from the binding sites, initiating contraction. * **Troponin M:** This is a distractor; there is no such subunit in the troponin complex. **3. NEET-PG High-Yield Pearls:** * **Cardiac Biomarkers:** Cardiac-specific isoforms of **Troponin I and T** are the "Gold Standard" biomarkers for diagnosing Myocardial Infarction (MI) because they are released into the bloodstream when myocardial cells are damaged. * **Skeletal vs. Cardiac:** While Troponin C is identical in both slow-twitch skeletal and cardiac muscle, Troponin I and T have unique cardiac isoforms, making them highly specific for heart injury. * **Mechanism:** Remember the mnemonic: **T** (Tropomyosin), **I** (Inhibitory), **C** (Calcium).

Q: The hyperpolarization phase of the action potential is due to which of the following?

An outward potassium current. ### Explanation The hyperpolarization phase (also known as the "undershoot") occurs because voltage-gated potassium ($K^+$) channels are slow to close. **1. Why the Correct Answer is Right:** During the repolarization phase, voltage-gated $K^+$ channels open, allowing $K^+$ to flow out of the cell down its electrochemical gradient (**outward potassium current**). Unlike sodium channels, which inactivate rapidly, $K^+$ channels remain open for a prolonged period. This causes the membrane potential to move past the resting membrane potential (RMP) toward the **equilibrium potential of Potassium (approx. -94 mV)**. This transient state where the interior of the cell becomes more negative than the RMP is called hyperpolarization. **2. Why Incorrect Options are Wrong:** * **Option A & D:** Outward currents of positive ions (like $Ca^{2+}$ or $Na^+$) would technically cause repolarization, but these ions primarily move **inward** during an action potential due to their concentration gradients. There is no significant "outward" $Na^+$ or $Ca^{2+}$ current contributing to the hyperpolarization phase. * **Option B:** While an inward chloride ($Cl^-$) current would make the cell more negative (hyperpolarize it), $Cl^-$ channels are not the primary drivers of the action potential phases in standard nerve and muscle cells. **3. NEET-PG High-Yield Pearls:** * **Resting Membrane Potential (RMP):** Primarily determined by $K^+$ leak channels (not voltage-gated channels). * **Depolarization:** Due to $Na^+$ influx (inward current). * **Repolarization/Hyperpolarization:** Due to $K^+$ efflux (outward current). * **Na+-K+ ATPase:** Does NOT cause the phases of the action potential; it restores the ionic gradients *after* the event. * **Tetrodotoxin (TTX):** Blocks voltage-gated $Na^+$ channels, preventing depolarization.

Q: What is true about synaptic vesicles?

Small clear vesicles contain ACh and GABA.. ### Explanation Synaptic vesicles are specialized organelles that store and release neurotransmitters. They are broadly classified into two types based on their size and electron-microscopic appearance: **Small Clear Vesicles** and **Large Dense-Core Vesicles (LDCVs).** **1. Why Option A is Correct:** Small clear vesicles (approx. 40–60 nm) typically contain **small-molecule neurotransmitters** that are synthesized in the nerve terminal and mediate rapid synaptic transmission. Common examples include **Acetylcholine (ACh), GABA, Glutamate, and Glycine.** Therefore, the statement that small clear vesicles contain ACh and GABA is physiologically accurate. **2. Analysis of Incorrect Options:** * **Option B & D:** Neuropeptides (e.g., Substance P, Enkephalins) are synthesized in the cell body and transported to the terminal. They are exclusively stored in **Large Dense-Core Vesicles**, not small vesicles. * **Option C:** Glutamate is a classic small-molecule excitatory neurotransmitter and is stored in **small clear vesicles**, not large dense-core vesicles. Large vesicles typically contain catecholamines (like Norepinephrine) or neuropeptides. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **V-SNAREs (Synaptobrevin):** Proteins on the vesicle membrane that interact with T-SNAREs (Syntaxin, SNAP-25) on the presynaptic membrane to facilitate docking and fusion. * **Calcium Trigger:** The entry of $Ca^{2+}$ via voltage-gated channels is the essential trigger for exocytosis. * **Toxins:** *Clostridium botulinum* and *Clostridium tetani* toxins act by proteolytically cleaving SNARE proteins, thereby inhibiting neurotransmitter release. * **Recycling:** After exocytosis, vesicle membranes are recycled via **clathrin-mediated endocytosis**.

Question 1

What characterizes the inverse stretch reflex?

Accepted Answer

It is a disynaptic reflex activated on change in muscle tension.

Answer

It occurs when muscle length changes.

Answer

It is a monosynaptic reflex initiated by activation of the Golgi tendon organ.

Answer

It uses type II afferent fibers from the Golgi tendon organ.

Question 2

What is an action potential?

Accepted Answer

A threshold stimulus is required

Answer

A decremental phenomenon

Answer

Does not obey the all-or-none phenomenon

Answer

Potassium ions move from extracellular fluid to intracellular fluid

Question 3

Which ion is primarily responsible for the resting membrane potential in neurons?

Accepted Answer

Potassium

Answer

Calcium

Answer

Chloride

Answer

Sodium

Question 4

Regulation of smooth muscle tone is persistently influenced by:

Accepted Answer

Latch bridge mechanism

Answer

Calcium release from sarcoplasmic reticulum

Answer

Beta 1 receptor

Answer

Troponin

Question 5

Which of the following statements is NOT true regarding action potential recording?

Accepted Answer

Compound action potential of a mixed nerve is recorded by cathode ray oscilloscope.

Answer

Biphasic action potential is recorded by a surface electrode.

Answer

Compound action potential is an algebraic summation of all the monophasic recordings from a mixed nerve.

Answer

A typical action potential is recorded by a microelectrode placed inside the nerve fiber.

Question 6

What is true about summation?

Accepted Answer

Subthreshold stimuli are used.

Answer

Temporal summation is the application of two stimuli together.

Answer

Spatial summation is the application of two stimuli one after another.

Answer

All are true.

Question 7

Which troponin subunit binds to tropomyosin to form the troponin-tropomyosin complex?

Accepted Answer

Troponin T

Answer

Troponin I

Answer

Troponin C

Answer

Troponin M

Question 8

The hyperpolarization phase of the action potential is due to which of the following?

Accepted Answer

An outward potassium current

Answer

An outward calcium current

Answer

An inward chloride current

Answer

An outward sodium current

Question 9

Rhomboid major muscle is supplied by which type of neuron?

Accepted Answer

Multipolar

Answer

Unipolar

Answer

Pseudounipolar

Answer

Bipolar

Question 10

What is true about synaptic vesicles?

Accepted Answer

Small clear vesicles contain ACh and GABA.

Answer

Small vesicles contain neuropeptides.

Answer

Large vesicles contain a dense core with glutamate.

Answer

Small clear vesicles contain neuropeptides.

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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