Nerve and Muscle Physiology Practice Questions

Q: Which mechanism maintains the action potential across the cell membrane?

Na+/K+ pump. **Explanation:** The **Na+/K+ ATPase pump** is the primary mechanism for maintaining the ionic gradients necessary for the resting membrane potential (RMP) and the long-term restoration of the action potential. It is an active transport mechanism that pumps **3 Na+ ions out** of the cell and **2 K+ ions into** the cell against their concentration gradients, using energy from ATP hydrolysis. This process is "electrogenic," as it creates a net loss of positive charges from the intracellular compartment, contributing to the negative internal environment. Without this pump, the concentration gradients would dissipate due to "leak" channels, rendering the cell unable to fire subsequent action potentials. **Why other options are incorrect:** * **cAMP:** This is a second messenger involved in intracellular signal transduction (e.g., GPCR pathways). It regulates cellular metabolism and protein kinase activation but does not directly maintain the membrane potential. * **Ca++:** While calcium is crucial for the plateau phase of the cardiac action potential and neurotransmitter release, it is not the primary ion responsible for maintaining the baseline resting potential or the Na+/K+ balance. * **Phosphodiesterase:** This is an enzyme that breaks down cyclic nucleotides (cAMP/cGMP). It is a target for drugs (like Sildenafil or Caffeine) but has no direct role in maintaining ionic gradients across the membrane. **High-Yield Clinical Pearls for NEET-PG:** * **Stoichiometry:** 3 Na+ Out / 2 K+ In. * **Inhibitor:** **Ouabain** and **Digitalis (Digoxin)** inhibit the Na+/K+ pump, leading to increased intracellular Na+, which subsequently increases intracellular Ca++ via the Na+/Ca++ exchanger (positive inotropic effect). * The Na+/K+ pump accounts for approximately **60-70% of the total energy consumption** in neurons.

Q: True about the relative refractory period of an action potential?

It is due to the opening of potassium channels.. ### Explanation The **Relative Refractory Period (RRP)** is the interval immediately following the Absolute Refractory Period (ARP) during which a second action potential can be evoked, but only by a stimulus that is stronger than the normal threshold. **1. Why Option C is Correct:** The RRP coincides with the period of **delayed outward potassium ($K^+$) conductance**. During repolarization, voltage-gated $K^+$ channels are wide open, causing $K^+$ to leave the cell. This creates a "hyperpolarizing" force that opposes depolarization. To trigger a new action potential, a stimulus must be strong enough to overcome this outward $K^+$ flow and open enough "recovered" $Na^+$ channels to reach the threshold. **2. Why Other Options are Incorrect:** * **Option A:** This describes the **Absolute Refractory Period (ARP)**. In ARP, no stimulus, regardless of strength, can excite the nerve because $Na^+$ channels are either already open or in an inactivated state. * **Option B:** The closure of inactivation gates ($h$-gates) of $Na^+$ channels is the hallmark of the **Absolute Refractory Period**. The transition from ARP to RRP occurs when these inactivation gates begin to reopen (and activation gates close), making the channels "excitable" again. **3. High-Yield Facts for NEET-PG:** * **ARP vs. RRP Timing:** ARP corresponds to the period from the start of the upstroke until roughly one-third of repolarization is complete. RRP lasts from the end of ARP until the membrane returns to its resting potential. * **Accommodation:** If a nerve is subjected to a slowly rising current, the threshold for firing increases. This is due to the slow inactivation of $Na^+$ channels and the opening of $K^+$ channels, similar to the ionic environment of the RRP. * **Clinical Significance:** The refractory period ensures the **unidirectional propagation** of action potentials and limits the maximum frequency of nerve impulses.

Q: What type of fibers are postganglionic fibers?

Type C fibers. **Explanation:** The classification of nerve fibers is based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, myelination, and conduction velocity. **1. Why Type C fibers is the correct answer:** Postganglionic autonomic fibers (both sympathetic and parasympathetic) are **Type C fibers**. These are the smallest in diameter (0.4–1.2 μm) and are **unmyelinated**. Due to the lack of myelin and small size, they have the slowest conduction velocity (0.5–2.0 m/s). Their primary function is to carry autonomic signals to effector organs (smooth muscle, cardiac muscle, and glands) and transmit slow pain/temperature sensations. **2. Why other options are incorrect:** * **Type B fibers:** These are small, **myelinated** fibers. In the autonomic nervous system, **preganglionic fibers** are Type B. They conduct impulses faster than Type C fibers but slower than Type A. * **Type A fibers:** These are large, myelinated fibers subdivided into Alpha (proprioception/somatic motor), Beta (touch/pressure), Gamma (muscle spindles), and Delta (fast pain/cold). They are not involved in postganglionic autonomic transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Myelination Rule:** Preganglionic = Myelinated (Type B); Postganglionic = Unmyelinated (Type C). * **Sensitivity to Local Anesthetics:** Type C fibers are highly sensitive to local anesthetics (like Lidocaine), which is why pain and autonomic function are blocked before motor function (Type A). * **Sensitivity to Pressure/Hypoxia:** Type A fibers are most sensitive to pressure; Type B fibers are most sensitive to hypoxia. * **Fast vs. Slow Pain:** Type A-delta fibers carry "fast/sharp" pain, while Type C fibers carry "slow/dull/aching" pain.

Q: Which of the following is an anchoring protein?

Actinin. **Explanation:** The correct answer is **Actinin**. **1. Why Actinin is the Correct Answer:** Anchoring proteins (also known as linking or scaffolding proteins) are responsible for tethering the cytoskeleton to specific structures within the cell. **$\alpha$-Actinin** is a major cross-linking protein found in the **Z-disk** of skeletal muscle. Its primary function is to anchor the ends of the thin (actin) filaments to the Z-disk, ensuring the structural stability of the sarcomere during muscle contraction. **2. Analysis of Incorrect Options:** * **Myosin (Option A):** This is a **contractile protein**. It forms the thick filaments and possesses ATPase activity to generate force via the sliding filament mechanism. * **Troponin (Option C):** This is a **regulatory protein** complex (consisting of subunits T, I, and C). It binds calcium and moves tropomyosin to uncover myosin-binding sites on actin. * **Tropomyosin (Option D):** This is also a **regulatory protein**. It winds around the actin helix and blocks the active sites in a resting muscle to prevent contraction. **3. NEET-PG High-Yield Clinical Pearls:** * **Dystrophin:** Another critical anchoring protein. It links the actin cytoskeleton to the extracellular matrix. Mutations in the *DMD* gene lead to **Duchenne Muscular Dystrophy**. * **Titin:** The largest known protein; it acts as a molecular spring, anchoring the thick (myosin) filaments to the Z-disk and providing passive elasticity. * **Desmin:** An intermediate filament that anchors Z-disks of adjacent myofibrils to each other and to the plasma membrane (sarcolemma). * **Z-Disk Composition:** Remember that the Z-disk marks the boundary of a sarcomere and contains $\alpha$-actinin, desmin, and vimentin.

Q: What is true about neuropraxia?

Physiological block. **Explanation:** **Neuropraxia** is the mildest form of nerve injury according to **Seddon’s classification**. It is characterized by a **physiological block** of nerve conduction without any structural damage to the axon or the connective tissue sheaths (epineurium, perineurium, and endoneurium). 1. **Why the correct answer is right:** In neuropraxia, the nerve remains anatomically intact, but conduction is temporarily halted, usually due to focal demyelination or ischemia caused by sustained pressure (e.g., Saturday Night Palsy). Since the axon is preserved, there is **no Wallerian degeneration** distal to the site of injury. Recovery is typically spontaneous and complete within days to weeks once the pressure is relieved. 2. **Why the incorrect options are wrong:** * **Blockage of axon:** This is misleading. While conduction is blocked, the physical continuity of the axon is not interrupted. * **Incomplete/Complete transection:** These describe higher grades of injury. **Axonotmesis** involves axonal disruption with preservation of the sheath, while **Neurotmesis** (Option D) involves complete transection of both the axon and the connective tissue, requiring surgical intervention. **High-Yield Facts for NEET-PG:** * **Seddon’s Classification:** Neuropraxia (Mildest) → Axonotmesis → Neurotmesis (Most severe). * **Sunderland’s Classification:** Neuropraxia corresponds to **First-degree** injury. * **Clinical Feature:** Motor fibers are more susceptible than sensory fibers; autonomic functions are usually preserved. * **EMG Finding:** No denervation potentials (fibrillations) are seen because the axon is intact.

Q: During the repolarization phase of the action potential of a neuron, which of the following occurs?

Increased permeability to K+ ion. **Explanation:** The action potential is a rapid change in membrane potential characterized by sequential changes in ion permeability. **Repolarization** is the phase where the membrane potential returns toward the resting level after peak depolarization. **Why Option A is Correct:** Repolarization is primarily driven by two simultaneous events: 1. **Inactivation of Voltage-Gated Na+ Channels:** The "h-gates" (inactivation gates) close, stopping the influx of sodium. 2. **Activation of Voltage-Gated K+ Channels:** These channels open (though more slowly than Na+ channels), leading to a marked **increase in K+ permeability**. Since the concentration of K+ is higher inside the cell, K+ ions exit the cell (efflux), carrying positive charges out and restoring the negative resting membrane potential. **Why Other Options are Incorrect:** * **Option B:** Decreased K+ permeability would prevent the cell from returning to its resting state, leading to prolonged depolarization. * **Option C:** Increased Ca2+ permeability is characteristic of the **plateau phase** in cardiac muscle action potentials, not the standard neuronal repolarization. * **Option D:** Increased Na+ permeability is the hallmark of the **Depolarization phase**. **High-Yield NEET-PG Pearls:** * **Hyperpolarization:** Occurs because K+ channels are slow to close, allowing the membrane potential to become more negative than the resting membrane potential (RMP). * **Absolute Refractory Period:** Corresponds to the period from the threshold to the early part of repolarization (due to Na+ channel inactivation). * **Tetraethylammonium (TEA):** A pharmacological tool that blocks voltage-gated K+ channels, thereby abolishing the repolarization phase.

Question 1

Following a tetanizing stimulus to a muscle, there is a constant contraction. This is due to:

Accepted Answer

At high stimulus frequency all muscle fibers are contracting

Answer

Recruitment phenomenon

Answer

Contraction of different muscle fibers at different times

Answer

All of the above

Question 2

Which mechanism maintains the action potential across the cell membrane?

Accepted Answer

Na+/K+ pump

Answer

cAMP

Answer

Ca++

Answer

Phosphodiesterase

Question 3

True about the relative refractory period of an action potential?

Accepted Answer

It is due to the opening of potassium channels.

Answer

An action potential cannot be generated at all.

Answer

It is due to the closure of inactive gates of sodium channels.

Answer

All of the above.

Question 4

What type of fibers are postganglionic fibers?

Accepted Answer

Type C fibers

Answer

Type B fibers

Answer

Both

Answer

None

Question 5

Increase in threshold level on applying a subthreshold, slowly rising stimulus is known as?

Accepted Answer

Accommodation

Answer

Adaptation

Answer

Refractoriness

Answer

Electrotonus

Question 6

Sudden decrease in serum calcium is associated with which of the following?

Accepted Answer

Increased excitability of muscle and nerve

Answer

Increased thyroxine and parathyroid hormone secretion

Answer

Increased phosphate

Answer

Cardiac conduction abnormalities

Question 7

Which of the following is an anchoring protein?

Accepted Answer

Actinin

Answer

Myosin

Answer

Troponin

Answer

Tropomyosin

Question 8

Repolarization of a nerve action potential is due to which of the following?

Accepted Answer

Outward potassium current and inward sodium current

Answer

Inward sodium current

Answer

Inward potassium current

Answer

Inward potassium current and outward sodium current

Question 9

What is true about neuropraxia?

Accepted Answer

Physiological block

Answer

Blockage of axon

Answer

Incomplete transection of nerve

Answer

Complete transection of nerve

Question 10

During the repolarization phase of the action potential of a neuron, which of the following occurs?

Accepted Answer

Increased permeability to K+ ion

Answer

Decreased permeability to K+ ion

Answer

Increased permeability to Ca2+ ion

Answer

Increased permeability to Na+ ion

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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