Nerve and Muscle Physiology Practice Questions

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.

Q: Which of the following channelopathies is associated with a calcium channel disorder of muscles?

Hypokalemic periodic paralysis. **Explanation:** **Hypokalemic Periodic Paralysis (HypoPP)** is the correct answer because it is primarily caused by mutations in the **CACNA1S gene**, which encodes the alpha-1 subunit of the **L-type voltage-gated calcium channel** (dihydropyridine receptor) in skeletal muscle [1]. This defect leads to muscle inexcitability and episodic flaccid paralysis, typically triggered by high carbohydrate meals or strenuous exercise which lower serum potassium levels [1]. **Analysis of Incorrect Options:** * **Episodic Ataxia Type 1 (EA1):** This is a potassium channelopathy involving the **KCNA1 gene** (Kv1.1 voltage-gated K+ channel). It presents with brief episodes of ataxia and myokymia. * **Hyperkalemic Periodic Paralysis (HyperPP):** This is a sodium channelopathy caused by mutations in the **SCN4A gene** [1]. It is characterized by attacks of weakness triggered by fasting or potassium intake. * **Spinocerebellar Ataxia Type 1 (SCA1):** This is not a primary channelopathy; it is a **trinucleotide repeat (CAG) disorder** resulting in polyglutamine expansion, leading to progressive cerebellar degeneration. **High-Yield NEET-PG Pearls:** * **HypoPP Type 1:** Calcium channel (CACNA1S) - Most common. * **HypoPP Type 2:** Sodium channel (SCN4A) - Less common [1]. * **Lambert-Eaton Myasthenic Syndrome (LEMS):** Associated with P/Q-type calcium channel antibodies (Presynaptic) [2]. * **Treatment Tip:** Acetazolamide is used for prophylaxis in both HypoPP and HyperPP, but acute attacks of HypoPP are managed with potassium replacement.

Q: Which muscle fiber type has a high glycogen content?

White fibers. **Explanation:** The classification of muscle fibers is based on their metabolic profile and contraction speed. **White fibers (Type IIb/IIx)** are designed for short bursts of high-intensity activity (anaerobic power). Because they have fewer mitochondria and less myoglobin, they rely primarily on **anaerobic glycolysis** for ATP production. To support this rapid energy demand, they store significantly **high amounts of glycogen** and have high glycolytic enzyme activity. **Analysis of Options:** * **White fibers (Correct):** These are "Fast-twitch" fibers. They have high glycogen content, high myosin ATPase activity, and fatigue rapidly. * **Red fibers / Type 1 fibers (Incorrect):** These are "Slow-twitch" fibers. They are rich in myoglobin and mitochondria, relying on aerobic oxidative phosphorylation. They have high triglyceride (fat) stores rather than high glycogen stores and are resistant to fatigue. * **Tonic fibers (Incorrect):** These are specialized fibers (like those in extraocular muscles) that do not follow the "all-or-none" law and have multiple nerve endings. They are not characterized by high glycogen content. **High-Yield NEET-PG Pearls:** 1. **Myoglobin Content:** Type 1 (Red) = High; Type 2 (White) = Low. 2. **Mitochondria/Capillaries:** Type 1 = High (for oxygen utilization); Type 2 = Low. 3. **ATPase Activity:** Type 2 fibers have high myosin ATPase activity, leading to faster contraction. 4. **Fatigability:** Type 1 is fatigue-resistant (postural muscles); Type 2 is easily fatigued (sprinter's muscles). 5. **Intermediate Fibers:** Type IIa fibers are "Fast-Oxidative Glycolytic," sharing characteristics of both types.

Question 1

Maximum conduction occurs in which type of nerve fibers?

Accepted Answer

A alpha fibers

Answer

B fibers

Answer

Sympathetic fibers

Answer

C fibers

Question 2

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 3

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 4

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 5

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 6

Which of the proteins connect Z lines to M lines?

Accepted Answer

Titin

Answer

Nebulin

Answer

Actin

Answer

Myosin

Question 7

What is meant by resting muscle length?

Accepted Answer

The length of the muscle fiber at the onset of contraction, which is required to attain maximum active tension.

Answer

The length of actin and myosin filaments before contraction.

Answer

Approximately 4 micrometers.

Answer

The length at which the muscle has minimal tension.

Question 8

Which of the following channelopathies is associated with a calcium channel disorder of muscles?

Accepted Answer

Hypokalemic periodic paralysis

Answer

Episodic ataxia type 1

Answer

Hyperkalemic periodic paralysis

Answer

Spinocerebellar ataxia type 1

Question 9

Which muscle fiber type has a high glycogen content?

Accepted Answer

White fibers

Answer

Red fibers

Answer

Type 1 fibers

Answer

Tonic fibers

Question 10

Presynaptic facilitation is caused by?

Accepted Answer

Prolonged opening of calcium channels

Answer

Prolonged opening of chloride channels

Answer

Prolonged opening of sodium channels

Answer

Prolonged closure of potassium channels

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?