Nerve and Muscle Physiology Practice Questions

Q: Tetrodotoxin blocks which of the following during the action potential?

Na+ channels. **Explanation:** **Mechanism of Action:** Tetrodotoxin (TTX) is a potent neurotoxin that specifically and reversibly binds to the extracellular pore of **voltage-gated Na+ channels**. By blocking these channels, it prevents the rapid influx of sodium ions required for the **depolarization phase** of the action potential. This leads to the failure of nerve impulse conduction and muscle paralysis. **Analysis of Options:** * **Option A (Correct):** TTX blocks voltage-gated Na+ channels, specifically inhibiting the upstroke of the action potential. * **Option B (Incorrect):** K+ channels are responsible for the repolarization phase. These are blocked by agents like **Tetraethylammonium (TEA)**, not TTX. * **Options C & D (Incorrect):** The resting membrane potential is primarily maintained by **K+ leak channels** and the Na+-K+ ATPase pump. TTX specifically targets the voltage-gated channels that open during an active impulse, rather than the channels responsible for the resting state. **High-Yield Clinical Pearls for NEET-PG:** 1. **Source:** TTX is found in the **Pufferfish (Fugu)**, blue-ringed octopus, and certain newts. 2. **Symptoms:** Ingestion leads to paresthesia, flaccid paralysis, and potentially fatal respiratory failure (due to diaphragm paralysis), while the patient remains conscious. 3. **Saxitoxin:** Produced by dinoflagellates (Red Tide), it has a similar mechanism of action to TTX (blocks Na+ channels). 4. **Batrachotoxin:** Found in poison dart frogs; unlike TTX, it keeps Na+ channels **permanently open**, causing persistent depolarization. 5. **Dendrotoxin:** A snake toxin (Mamba) that blocks voltage-gated **K+ channels**.

Q: The gamma efferent system is involved in which of the following?

All of the above. The **gamma efferent system** consists of small motor neurons in the anterior horn of the spinal cord that innervate the **intrafusal muscle fibers** of the muscle spindle. Its primary role is to regulate the sensitivity of the spindle to stretch. ### Why "All of the Above" is Correct: The gamma system is the physiological basis for the **Gamma Loop**, which maintains muscle spindle sensitivity even during muscle contraction. 1. **Muscle Tone (Option C):** This is the fundamental function of the gamma system. By causing contraction of the ends of intrafusal fibers, gamma efferents stretch the central sensory portion of the spindle. This triggers alpha motor neurons via the stretch reflex, maintaining a continuous state of partial muscle contraction (basal muscle tone). 2. **Tendon Reflex (Option A):** Deep tendon reflexes (e.g., knee jerk) are monosynaptic stretch reflexes. The gamma system sets the "gain" or sensitivity of these reflexes. High gamma discharge leads to brisk reflexes, while low discharge results in sluggish reflexes. 3. **Clonus (Option B):** Clonus is a series of involuntary, rhythmic muscle contractions. It occurs due to a hyperactive stretch reflex, typically seen in upper motor neuron (UMN) lesions. This hyperactivity is driven by **gamma motor neuron overactivity**, which makes the spindles hypersensitive to even slight stretches. ### High-Yield NEET-PG Pearls: * **Alpha-Gamma Co-activation:** During voluntary movement, both alpha and gamma motor neurons fire simultaneously. This prevents the muscle spindle from going "slack" during contraction, allowing the brain to monitor muscle length continuously. * **Anxiety & Gamma Discharge:** Anxiety increases gamma efferent discharge, which is why anxious patients often exhibit hyperactive tendon reflexes. * **Jendrassik Maneuver:** This clinical technique increases gamma efferent activity, reinforcing reflexes that are otherwise difficult to elicit.

Q: What is true about tropomyosin?

Lies on top of actin. **Explanation:** **1. Why the Correct Answer is Right:** Tropomyosin is a long, rod-shaped protein that consists of two polypeptide chains coiled around each other. In a resting muscle fiber, tropomyosin molecules lie in the **grooves of the actin filament**, physically covering the active sites (myosin-binding sites) on the actin molecules. This prevents the interaction between actin and myosin heads, thereby maintaining the muscle in a relaxed state. **2. Why the Other Options are Incorrect:** * **Option A:** Tropomyosin does not lie on top of troponin. Instead, the **Troponin complex** (consisting of subunits T, I, and C) sits at regular intervals *on top* of the tropomyosin molecule. Troponin T specifically functions to bind the troponin complex to tropomyosin. * **Option C:** ATP does not bind to tropomyosin. ATP binds to the **Myosin head** (specifically at the ATPase site) to provide the energy required for the power stroke and to facilitate the detachment of the cross-bridge. * **Option D:** Calcium does not bind to tropomyosin. Calcium binds to **Troponin C**. This binding causes a conformational change in the troponin-tropomyosin complex, pulling tropomyosin away from the active sites on actin to initiate contraction. **High-Yield NEET-PG Pearls:** * **Regulatory Proteins:** Tropomyosin and Troponin are known as regulatory proteins, while Actin and Myosin are contractile proteins. * **The "Lock":** Think of tropomyosin as the "safety lock" on actin. * **Length:** One tropomyosin molecule covers approximately **seven G-actin residues**. * **Clinical Correlation:** Mutations in the genes encoding tropomyosin (TPM1) or troponin are common causes of **Familial Hypertrophic Cardiomyopathy (HCM)**.

Q: Maximum conduction occurs in which type of nerve fibers?

A alpha fibers. **Explanation:** The velocity of nerve impulse conduction is primarily determined by two factors: **fiber diameter** and the **presence of myelin**. According to the Erlanger-Gasser classification, nerve fibers are categorized based on these properties. **1. Why A alpha fibers are correct:** A alpha fibers are the **thickest** (12–20 µm) and most **heavily myelinated** nerve fibers in the body. According to the principles of cable theory, a larger diameter reduces internal resistance to current flow, while myelin provides saltatory conduction. This combination allows A alpha fibers to achieve the maximum conduction velocity (approximately **70–120 m/sec**). They primarily carry motor impulses to skeletal muscles and sensory information from proprioceptors (muscle spindles and Golgi tendon organs). **2. Why the other options are incorrect:** * **B fibers:** These are preganglionic autonomic fibers. While myelinated, they have a much smaller diameter ( <3 µm) than A fibers, resulting in slower conduction (3–15 m/sec). * **C fibers:** These are the slowest of all nerve fibers (0.5–2 m/sec) because they are **unmyelinated** and have the smallest diameter. They carry slow pain, temperature, and postganglionic autonomic impulses. * **Sympathetic fibers:** Most postganglionic sympathetic fibers are C fibers (unmyelinated), making them among the slowest conductors. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Hypoxia:** B fibers are most sensitive (B > A > C). * **Pressure:** A fibers are most sensitive (A > B > C). * **Local Anesthetics:** C fibers are most sensitive (C > B > A). * **Fastest to Slowest:** Aα > Aβ > Aγ > Aδ > B > C. * **A delta (Aδ) fibers** carry "fast pain" (sharp/localized), while **C fibers** carry "slow pain" (dull/aching).

Q: Hyperpolarization is due to which of the following?

Influx of chloride ions. **Explanation:** Hyperpolarization refers to a change in a cell's membrane potential that makes it **more negative** than the resting membrane potential (RMP). This increases the threshold required to generate an action potential, thereby inhibiting the neuron. 1. **Why Option A is Correct:** Chloride ($Cl^-$) is a negatively charged anion with a higher concentration in the extracellular fluid. When chloride channels open (e.g., via GABA-A receptors), $Cl^-$ flows **into** the cell (influx). Adding negative charges to the interior of the cell makes the membrane potential more negative (e.g., moving from -70mV to -80mV), resulting in **hyperpolarization** [1]. 2. **Why Other Options are Incorrect:** * **Option B (Influx of Potassium):** Potassium ($K^+$) has a much higher concentration inside the cell. Therefore, $K^+$ does not naturally flow into the cell; it flows **out** (efflux) [3]. **Efflux** of $K^+$ causes hyperpolarization, but **influx** would cause depolarization. * **Option C (Influx of Sodium):** Sodium ($Na^+$) is the primary extracellular cation. Its influx into the cell adds positive charges, making the interior less negative [2]. This leads to **depolarization**, which is the basis for the rising phase of an action potential. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitory Postsynaptic Potential (IPSP):** Hyperpolarization is the mechanism behind IPSPs [1]. The most common inhibitory neurotransmitters in the CNS are **GABA** (brain) and **Glycine** (spinal cord). * **Mechanism of Action:** Benzodiazepines and Barbiturates work by increasing the frequency or duration of $Cl^-$ channel opening at the GABA-A receptor, leading to hyperpolarization. * **After-hyperpolarization:** This occurs at the end of an action potential due to the delayed closure of voltage-gated $K^+$ channels, allowing continued $K^+$ efflux [3].

Q: Which of the proteins connect Z lines to M lines?

Titin. **Explanation:** **1. Why Titin is the Correct Answer:** Titin (also known as connectin) is the largest known protein in the human body. It acts as a molecular spring that extends from the **Z-disk to the M-line** within the sarcomere. Its primary functions are to provide structural scaffolding, maintain the central position of the thick (myosin) filaments during contraction, and contribute to the passive elasticity of the muscle. By anchoring the thick filaments to the Z-lines, it ensures the sarcomere returns to its original length after being stretched. **2. Why Other Options are Incorrect:** * **Nebulin:** This is a large protein that wraps around the **thin (actin) filaments**. It acts as a "molecular ruler" to regulate the length of actin filaments during assembly but does not reach the M-line. * **Actin:** These are the primary components of the **thin filaments**. They are anchored to the Z-lines (via alpha-actinin) and extend toward the center of the sarcomere but do not connect to the M-line. * **Myosin:** These are the primary components of the **thick filaments**. While they are centered at the M-line, they do not directly attach to the Z-lines; they rely on Titin for that connection. **3. NEET-PG High-Yield Pearls:** * **Dystrophin:** Connects the cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. Deficiency leads to Duchenne Muscular Dystrophy. * **Alpha-actinin:** The protein that anchors actin filaments to the Z-line. * **Desmin:** An intermediate filament that links Z-disks of adjacent myofibrils together, ensuring synchronized contraction. * **M-line proteins:** Include **Myomesin** and **M-protein**, which hold the thick filaments in a hexagonal lattice.

Question 1

Tetrodotoxin blocks which of the following during the action potential?

Accepted Answer

Na+ channels

Answer

K+ channels

Answer

Na+ channels during the resting state

Answer

K+ channels during the resting state

Question 2

The gamma efferent system is involved in which of the following?

Accepted Answer

All of the above

Answer

Tendon reflex

Answer

Clonus

Answer

Muscle tone

Question 3

The sodium gradient across the nerve cell membrane is:

Accepted Answer

Used as a source of energy for the transport of other ions

Answer

A result of the Donnan equilibrium

Answer

Significantly changed during an action potential

Answer

An important determinant of the resting membrane potential

Question 4

What is true about tropomyosin?

Accepted Answer

Lies on top of actin

Answer

Lies on top of troponin

Answer

ATP binds to it

Answer

Calcium binds to it

Question 5

Maximum conduction occurs in which type of nerve fibers?

Accepted Answer

A alpha fibers

Answer

B fibers

Answer

Sympathetic fibers

Answer

C fibers

Question 6

What potential would develop from the movement and eventual equilibrium of Na+, K+, and Cl- across the muscle membrane?

Accepted Answer

- 86 mV

Answer

- 94 mV

Answer

- 89 mV

Answer

+ 61 mV

Question 7

Which of the following is a true statement about the latch bridge mechanism?

Accepted Answer

Sustained contraction of smooth muscle with low energy consumption

Answer

Binding of tropomyosin to actin

Answer

Variability of tension at a particular length

Answer

None of the above

Question 8

Hyperpolarization is due to which of the following?

Accepted Answer

Influx of chloride ions

Answer

Influx of potassium ions

Answer

Influx of sodium ions

Answer

All of the above

Question 9

The repolarization phase of an action potential is due to which of the following ionic changes?

Accepted Answer

Decreasing sodium permeability and a delayed increase in potassium permeability

Answer

Increasing sodium permeability and increasing potassium permeability

Answer

Increasing sodium permeability and decreasing potassium permeability

Answer

Decreasing sodium permeability and an immediate increasing potassium permeability

Question 10

Which of the proteins connect Z lines to M lines?

Accepted Answer

Titin

Answer

Nebulin

Answer

Actin

Answer

Myosin

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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