Nerve and Muscle Physiology Practice Questions

Q: What is the resting membrane potential (RMP) of smooth muscle?

-50 mV. **Explanation:** The resting membrane potential (RMP) of **smooth muscle** is typically less negative than that of skeletal muscle or neurons, generally ranging between **-50 mV and -60 mV**. The correct option is **-50 mV**. **Why -50 mV is correct:** Smooth muscle cells have a higher permeability to sodium ($Na^+$) and calcium ($Ca^{2+}$) ions at rest compared to skeletal muscles. Additionally, the $Na^+$-$K^+$ pump in smooth muscle is less active in maintaining a high gradient. This results in a "less negative" or more depolarized RMP, which allows these muscles to be more easily excited by hormonal or mechanical stimuli. **Analysis of Incorrect Options:** * **-75 mV:** This is closer to the RMP of **cardiac atrial cells** or certain large nerve fibers. * **-90 mV (Option C):** This is the characteristic RMP for **skeletal muscle fibers** and **ventricular cardiomyocytes**. It is highly negative due to high resting permeability to potassium ($K^+$). * **-35 mV (Option D):** This is too depolarized for a stable RMP; however, it may be reached during the peak of "slow wave" potentials before an action potential is triggered. **NEET-PG High-Yield Pearls:** 1. **Instability:** Unlike skeletal muscle, the RMP of smooth muscle is often unstable (e.g., **Slow Waves** or Pacemaker potentials in the gut). 2. **Action Potential:** In most smooth muscles, the upstroke of the action potential is caused by the influx of **Calcium ($Ca^{2+}$)** rather than Sodium. 3. **L-type Ca channels:** These are the primary channels involved in smooth muscle contraction and are the targets for Calcium Channel Blockers (CCBs). 4. **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle cells are connected by **gap junctions**, allowing them to contract as a single unit (syncytium).

Q: Which sensory fibers are least susceptible to hypoxia?

Type IV. **Explanation:** The susceptibility of nerve fibers to various insults depends on their physiological characteristics. The sensitivity of nerve fibers to hypoxia, pressure, and local anesthetics follows a specific order, often tested in NEET-PG. **Why Type IV is correct:** Type IV fibers (Erlanger-Gasser **Type C fibers**) are the least susceptible to hypoxia. These are small-diameter, unmyelinated fibers that conduct slow pain and temperature sensations. Because they have a lower metabolic rate and lack the complex machinery of thick myelin sheaths, they can withstand oxygen deprivation longer than larger, myelinated fibers. **Why the other options are incorrect:** * **Type I (A-alpha):** These are the largest, most heavily myelinated fibers (proprioception, somatic motor). They have the highest metabolic demand and are the **most susceptible to pressure**, but they are highly sensitive to hypoxia compared to Type IV. * **Type II (A-beta):** These are medium-sized myelinated fibers (touch, pressure). Like Type I, they are more sensitive to hypoxia than the unmyelinated Type IV fibers. * **Type III (A-delta):** These are small, thinly myelinated fibers (fast pain, cold). While more resistant than Type I, they are still more sensitive to hypoxia than Type IV. **High-Yield Clinical Pearls for NEET-PG:** To remember the sensitivity of nerve fibers, use the following hierarchy (from Most Sensitive to Least Sensitive): 1. **Hypoxia:** B > A > C (Type IV is C, thus least sensitive). 2. **Pressure:** A > B > C (Large fibers are compressed first; C is least sensitive). 3. **Local Anesthetics:** C > B > A (Small, unmyelinated fibers are blocked first). *Note: Type IV fibers are the only unmyelinated sensory fibers; Types I, II, and III are all subtypes of myelinated Type A fibers.*

Q: The phase of after depolarization in cardiac action potential is characterized by which of the following?

Slower net exit of K+. ### Explanation In the cardiac action potential (specifically the ventricular muscle fiber), the **Phase 3 (Rapid Repolarization)** is primarily driven by the efflux of $K^+$ ions through voltage-gated potassium channels. **Why Option D is Correct:** As repolarization nears completion, the membrane potential approaches the resting level. During the terminal portion of this phase, there is a **slower net exit of $K^+$**. This occurs because the driving force for potassium decreases as the membrane potential gets closer to the equilibrium potential for $K^+$. This deceleration in $K^+$ efflux leads to a "tailing off" effect known as **after-depolarization** (or the terminal phase of repolarization), before the membrane stabilizes at Phase 4. **Analysis of Incorrect Options:** * **Option A & B:** Sodium ($Na^+$) influx is the hallmark of **Phase 0 (Depolarization)**. By the time the cell reaches the after-depolarization stage, $Na^+$ channels are inactivated (refractory period). $Na^+$ influx above zero potential specifically refers to the "overshoot" during Phase 0. * **Option C:** While $K^+$ channels eventually close to return to the resting state, the specific phenomenon of after-depolarization is defined by the **rate of ion movement (flux)** rather than the mechanical closure speed of the channels themselves. ### High-Yield Clinical Pearls for NEET-PG: * **Phase 2 (Plateau Phase):** Unique to cardiac muscle; caused by a balance between $Ca^{2+}$ influx (L-type channels) and $K^+$ efflux. * **Refractory Period:** The absolute refractory period (ARP) in cardiac muscle lasts until mid-Phase 3, preventing tetany and allowing the heart to act as a pump. * **Early After-Depolarizations (EADs):** Occur during Phase 2 or 3; if they reach threshold, they can trigger "Torsades de Pointes," especially in Long QT Syndrome.

Q: Tetany may be present in all the following conditions except?

Hyperkalemia. Tetany is a state of increased neuromuscular excitability characterized by carpopedal spasms, paresthesia, and laryngospasm. It is primarily caused by a decrease in the threshold for nerve depolarization. **Why Hyperkalemia is the Correct Answer:** Hyperkalemia (Option C) actually **decreases** neuromuscular excitability by causing persistent depolarization of the cell membrane, which eventually leads to a state of inactivation of sodium channels (refractoriness) [1]. This typically results in muscle weakness or paralysis, not tetany. In contrast, **Hypokalemia** can occasionally be associated with tetany, particularly when it coexists with metabolic alkalosis. **Analysis of Incorrect Options:** * **Acute Pancreatitis (Option A):** This condition often leads to **hypocalcemia** due to the saponification of calcium in necrotic fat (calcium soaps) [2]. Low ionized calcium levels reduce the threshold for action potentials, leading to tetany. * **Hysterical Hyperventilation (Option B):** Hyperventilation causes respiratory alkalosis. The increase in blood pH promotes the binding of ionized calcium to albumin, leading to **decreased ionized calcium** levels (despite normal total calcium), which triggers tetany [2]. * **Hypomagnesemia (Option D):** Magnesium is a cofactor for PTH secretion and action. Severe magnesium deficiency leads to secondary hypocalcemia and can also directly increase neuromuscular excitability, both of which cause tetany [2]. **NEET-PG High-Yield Pearls:** * **Trousseau’s Sign:** Induction of carpal spasm by inflating a BP cuff above systolic pressure for 3 minutes (more sensitive than Chvostek). * **Chvostek’s Sign:** Tapping the facial nerve leads to twitching of facial muscles. * **The "Ionized" Rule:** Tetany is always a function of **ionized calcium** levels, not total calcium [2]. * **Metabolic Alkalosis:** Always consider tetany in alkalotic states due to increased calcium-albumin binding [2].

Q: Following repolarization, a neuron may become hyperpolarized. How does the membrane potential subsequently return to the resting membrane potential (RMP)?

Inward leakage of Na+ ions. **Explanation:** The return of the membrane potential to the Resting Membrane Potential (RMP) after hyperpolarization is a passive process primarily driven by the **inward leakage of Na+ ions**. 1. **Why Option B is Correct:** During the undershoot (hyperpolarization), the membrane potential is closer to the equilibrium potential of K+ (-94 mV) than the RMP (-70 mV). At this stage, the membrane is highly permeable to K+ but also possesses **non-gated "leak" channels**. Because the electrochemical gradient for Na+ is very high (high concentration outside, negative charge inside), Na+ ions slowly leak into the cell. This influx of positive charge gradually depolarizes the membrane back to its steady-state RMP. 2. **Why Other Options are Incorrect:** * **Option A (Na+/K+-ATPase):** While this pump is essential for maintaining long-term concentration gradients, it is **electrogenic** (pumping 3 Na+ out for 2 K+ in). Its net effect is to make the interior *more* negative. Therefore, it helps maintain or establish RMP but is not the primary mechanism that "pulls" the potential up from a hyperpolarized state. * **Option C (Decrease in K+ efflux):** While the closing of voltage-gated K+ channels stops further hyperpolarization, it does not actively return the potential to RMP; it merely prevents it from getting more negative. **High-Yield NEET-PG Pearls:** * **RMP** is primarily determined by **K+ efflux** through leak channels (K+ permeability is 50–100x higher than Na+ at rest). * **Hyperpolarization** occurs because voltage-gated K+ channels are slow to close. * **The Goldmann-Hodgkin-Katz equation** is used to calculate RMP considering multiple ions, whereas the **Nernst equation** is for a single ion.

Q: End plate potential follows which law?

Depolarization. **Explanation:** The **End Plate Potential (EPP)** is a localized, non-propagated potential change that occurs at the motor end plate of the neuromuscular junction (NMJ). **1. Why Depolarization is Correct:** When an action potential reaches the motor nerve terminal, it triggers the release of Acetylcholine (ACh). ACh binds to nicotinic receptors (nAChR) on the motor end plate, opening ligand-gated cation channels. This leads to a massive influx of **Na⁺ ions** (and a smaller efflux of K⁺), causing the resting membrane potential to shift toward a more positive value. This process is, by definition, **Depolarization**. If the EPP reaches a certain threshold, it triggers a propagated action potential in the muscle fiber. **2. Why other options are incorrect:** * **All or none law:** Unlike an action potential, the EPP is a **graded potential**. Its magnitude depends on the amount of ACh released; it does not follow the all-or-none law. * **Hyperpolarization:** This involves the membrane potential becoming more negative. EPP is excitatory and always involves a shift toward positivity. * **Propagation:** EPP is a **local potential**. It does not travel down the muscle fiber; instead, it triggers an action potential which then propagates. **High-Yield Clinical Pearls for NEET-PG:** * **Miniature End Plate Potential (MEPP):** The smallest possible EPP caused by the spontaneous release of a single vesicle (quantum) of ACh. * **Myasthenia Gravis:** Antibodies against nAChR reduce the amplitude of the EPP, making it difficult to reach the threshold for muscle contraction. * **Lambert-Eaton Syndrome:** Antibodies against voltage-gated Ca²⁺ channels reduce ACh release, also resulting in a decreased EPP. * **Curare:** Competitively inhibits nAChR, decreasing the EPP amplitude and causing paralysis.

Q: Small nerve fibers carry all except:

Position sense. **Explanation:** The classification of nerve fibers is based on diameter and conduction velocity (Erlanger-Gasser classification). The core principle is that **fiber diameter is directly proportional to conduction velocity and the degree of myelination.** **1. Why "Position Sense" is the correct answer:** Position sense (proprioception) is carried by the largest, most heavily myelinated fibers in the body, specifically **Type Ia** (from muscle spindles) and **Type Ib** (from Golgi tendon organs) fibers, or **Type A-alpha** fibers. These fibers require rapid conduction velocities (70–120 m/s) to provide the brain with real-time feedback on body posture and movement. Therefore, they are the opposite of "small" fibers. **2. Why the other options are incorrect:** * **Pain and Cold (A & D):** These are primarily carried by **Type A-delta** fibers. These are small, thinly myelinated fibers that transmit "fast" pain and cold sensations. * **Heat and Slow Pain (B):** These are carried by **Type C** fibers. These are the smallest nerve fibers and are completely **unmyelinated**, resulting in slow conduction velocities (0.5–2 m/s). **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Local Anesthetics:** Block **Small** fibers first (Type C > B > A). Clinically, pain is lost before touch and pressure. * **Hypoxia:** Blocks **Large** fibers first (Type A > B > C). * **Pressure:** Blocks **Large** fibers first (Type A > B > C). This is why your limb "falls asleep" (loss of touch/motor) before you lose the ability to feel pain. * **Type B fibers** are preganglionic autonomic fibers and are the most sensitive to hypoxia. * **Type C fibers** are the only unmyelinated fibers and carry the majority of "slow/chronic" pain.

Question 1

True about nerve impulse is:

Accepted Answer

Travels in one direction at a synapse

Answer

Travels in one direction along an axon

Answer

If the current is increased too slowly, the nerve responds faster

Answer

Travels with the speed of electric current

Question 2

What is the resting membrane potential (RMP) of smooth muscle?

Accepted Answer

-50 mV

Answer

-75 mV

Answer

-90 mV

Answer

-35 mV

Question 3

Which sensory fibers are least susceptible to hypoxia?

Accepted Answer

Type IV

Answer

Type I

Answer

Type II

Answer

Type III

Question 4

The phase of after depolarization in cardiac action potential is characterized by which of the following?

Accepted Answer

Slower net exit of K+

Answer

Delayed influx of Na+

Answer

Na+ influx above zero potential

Answer

Slow closure of K+ channels

Question 5

Which of the following statements is FALSE regarding the stretch reflex?

Accepted Answer

Relaxation of the muscle is the response.

Answer

Stretch of the muscle initiates the reflex.

Answer

The muscle spindle is the sense organ.

Answer

Impulses are conducted by fast sensory fibers.

Question 6

If a supramaximal stimulus is applied to an excitable tissue like a nerve or a muscle and it elicits a response, then the tissue is said to be in which state?

Accepted Answer

Relative refractory period

Answer

Absolute refractory period

Answer

Latent period

Answer

After-depolarization

Question 7

Tetany may be present in all the following conditions except?

Accepted Answer

Hyperkalemia

Answer

Acute pancreatitis

Answer

Hysterical hyperventilation

Answer

Hypomagnesemia

Question 8

Following repolarization, a neuron may become hyperpolarized. How does the membrane potential subsequently return to the resting membrane potential (RMP)?

Accepted Answer

Inward leakage of Na+ ions

Answer

Action of the Na+/K+-ATPase pump

Answer

Decrease in K+ ion efflux

Answer

None of the above mechanisms

Question 9

End plate potential follows which law?

Accepted Answer

Depolarization

Answer

All or none law

Answer

Hyperpolarization

Answer

Propagation

Question 10

Small nerve fibers carry all except:

Accepted Answer

Position sense

Answer

Cold

Answer

Heat

Answer

Pain

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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