Nerve and Muscle Physiology Practice Questions

Q: What happens when sodium enters cells?

There is a spike in action potential.. ### Explanation The correct answer is **A. There is a spike in action potential.** **1. Why Option A is correct:** The generation of an action potential is primarily driven by the movement of ions across the cell membrane. When a stimulus reaches the threshold potential, **voltage-gated sodium (Na⁺) channels** open rapidly. Since Na⁺ concentration is much higher extracellularly, it rushes into the cell following its electrochemical gradient. This influx of positive charge causes **depolarization**, making the membrane potential more positive. This rapid upstroke is known as the **"spike"** of the action potential. **2. Why other options are incorrect:** * **B. Plateau in action potential:** This is characteristic of cardiac ventricular muscle cells (Phase 2), caused by the balanced influx of **Calcium (Ca²⁺)** through L-type channels and the efflux of Potassium (K⁺). * **C. Repolarization:** This occurs when the membrane potential returns to its resting state, primarily due to the **efflux of Potassium (K⁺)** out of the cell and the closure of Na⁺ channels. * **D. Hyperpolarization:** This happens when the membrane potential becomes more negative than the resting membrane potential, usually due to an **excessive efflux of K⁺** or an **influx of Chloride (Cl⁻)**. **3. NEET-PG High-Yield Pearls:** * **Tetrodotoxin (from Pufferfish) and Saxitoxin:** Block voltage-gated Na⁺ channels, preventing the spike/depolarization. * **Local Anesthetics (e.g., Lidocaine):** Work by blocking voltage-gated Na⁺ channels from the inside, inhibiting signal conduction. * **Overshoot:** The portion of the action potential where the membrane potential is positive (above 0 mV) is called the overshoot. * **Na⁺-K⁺ ATPase:** This pump does not create the action potential but maintains the ionic gradients necessary for it to occur (3 Na⁺ out, 2 K⁺ in).

Q: The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

Tropomyosin. ### Explanation **Correct Answer: A. Tropomyosin** In skeletal muscle, the interaction between actin and myosin is the fundamental step for contraction. In a resting (relaxed) state, the **active sites on the F-actin strand**—where myosin heads must attach to form cross-bridges—are physically blocked. This masking is performed by **Tropomyosin**, a long, rod-like protein that wraps around the actin filament. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing contraction. **Why other options are incorrect:** * **B. Troponin:** This is a complex of three subunits (I, T, and C). While it is attached to tropomyosin, its role is to act as a "switch." When calcium binds to Troponin C, it undergoes a conformational change that pulls the tropomyosin away from the binding sites. It does not cover the sites itself. * **C. Calcium ions:** Calcium is the trigger for contraction. It binds to Troponin C to *uncover* the binding sites; it does not cover them. * **D. Calmodulin:** This is the calcium-binding protein used in **smooth muscle** contraction (where troponin is absent). It does not play a structural role in masking actin sites in skeletal muscle. **High-Yield NEET-PG Pearls:** * **The Troponin Complex:** * **Troponin T:** Binds to **T**ropomyosin. * **Troponin I:** **I**nhibits the actin-myosin interaction. * **Troponin C:** Binds **C**alcium ions (up to 4 ions per molecule). * **The "Walk-along" Theory:** Contraction occurs when tropomyosin moves into the "groove" of the actin helix, exposing the sites. * **Clinical Correlation:** Cardiac Troponins (I and T) are highly specific biomarkers for myocardial infarction because they are released into the blood when cardiac myocytes are damaged.

Q: During skeletal muscle contraction, which of the following occurs?

Both H and I bands shorten. ### Explanation The fundamental mechanism of skeletal muscle contraction is explained by the **Sliding Filament Theory**. According to this theory, contraction occurs when thin (actin) filaments slide over thick (myosin) filaments toward the center of the sarcomere. **Why Option B is Correct:** * **I Band (Isotropic):** This band contains only thin filaments. As actin filaments slide toward the M-line, the distance between thick filaments of adjacent sarcomeres decreases, causing the I band to **shorten**. * **H Zone (Heller):** This is the central part of the A band containing only thick filaments. As thin filaments slide inward, they overlap more of the thick filaments, causing the H zone to **shorten** or even disappear. * **Sarcomere:** The distance between two Z-lines also decreases, leading to overall shortening of the muscle fiber. **Why Other Options are Incorrect:** * **A Band (Anisotropic):** This band represents the entire length of the thick (myosin) filament. Since the thick filaments themselves do not change length or move, the **A band remains constant** during contraction. * Options A, C, and D are incorrect because they all suggest that the A band shortens. **High-Yield NEET-PG Pearls:** 1. **Mnemonic:** During contraction, **"HI"** disappears (H and I bands shorten), but the **"A"** stays the same (A band is constant). 2. **Z-lines:** Move closer together during contraction. 3. **ATP Role:** ATP binding is required for the **detachment** of the myosin head from actin; hydrolysis of ATP "cocks" the myosin head into a high-energy state. 4. **Calcium:** Binds to **Troponin C**, causing a conformational change in Tropomyosin to uncover the active sites on actin.

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.

Question 1

What happens when sodium enters cells?

Accepted Answer

There is a spike in action potential.

Answer

There is a plateau in action potential.

Answer

There is repolarization.

Answer

There is hyperpolarization.

Question 2

The sites where myosin heads bind to actin in skeletal muscles are covered by which of the following?

Accepted Answer

Tropomyosin

Answer

Troponin

Answer

Calcium ions

Answer

Calmodulin

Question 3

During skeletal muscle contraction, which of the following occurs?

Accepted Answer

Both H and I bands shorten

Answer

A band shortens

Answer

Both A and I bands shorten

Answer

Both A and H bands shorten

Question 4

When a muscle is not contracting, by which mechanism are actin and myosin prevented from interacting?

Accepted Answer

Troponin-tropomyosin complex

Answer

Phosphocreatine

Answer

Heavy meromyosin

Answer

Acetylcholinesterase

Question 5

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 6

Which mechanism maintains the action potential across the cell membrane?

Accepted Answer

Na+/K+ pump

Answer

cAMP

Answer

Ca++

Answer

Phosphodiesterase

Question 7

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 8

What type of fibers are postganglionic fibers?

Accepted Answer

Type C fibers

Answer

Type B fibers

Answer

Both

Answer

None

Question 9

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

Accepted Answer

Accommodation

Answer

Adaptation

Answer

Refractoriness

Answer

Electrotonus

Question 10

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

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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