Nerve and Muscle Physiology Practice Questions

Q: Who postulated that nerve terminals release chemicals?

Loewi. **Explanation:** The correct answer is **Otto Loewi**. In 1921, Loewi conducted a landmark experiment using two frog hearts to prove that synaptic transmission was chemical rather than purely electrical. By stimulating the vagus nerve of one heart and transferring the surrounding fluid to a second heart, he observed the second heart slow down. He termed the released substance *"Vagusstoff,"* which was later identified as **Acetylcholine**. For this discovery, he shared the Nobel Prize in 1936. **Analysis of Options:** * **Dale (Sir Henry Dale):** While he worked closely with Loewi and identified Acetylcholine as a neurotransmitter, he is best known for **Dale’s Principle**, which originally suggested that a neuron releases the same neurotransmitter at all its synapses. * **Withering (William Withering):** A British physician famous for discovering the clinical use of **Digitalis** (from the foxglove plant) in treating dropsy (heart failure). * **Domagk (Gerhard Domagk):** A pathologist credited with the discovery of **Prontosil**, the first commercially available sulfonamide antibiotic, which revolutionized the treatment of bacterial infections. **High-Yield NEET-PG Pearls:** * **Vagusstoff:** The original name for Acetylcholine (ACh). * **Acceleransstoff:** The name given to the sympathetic substance (Norepinephrine) discovered in similar experiments. * **Chemical Synapse:** The most common type of synapse in the human CNS; it involves a **synaptic delay** (approx. 0.5 ms), unlike electrical synapses. * **Calcium ions:** Essential for the docking and release of neurotransmitter vesicles from the presynaptic terminal.

Q: The injured nerve regenerates at what rate?

0.1 cm/day. **Explanation:** The correct answer is **0.1 cm/day**. Nerve regeneration occurs after Wallerian degeneration has taken place in the distal segment of a transected nerve. The process is driven by axonal sprouting from the proximal stump, which then traverses the Schwann cell columns (Bungner bands). In humans, the average rate of axonal regrowth is approximately **1 mm per day**. Since 1 mm is equal to 0.1 cm, option A is the mathematically correct representation of this physiological process. **Analysis of Options:** * **A. 0.1 cm/day:** Correct. This is equivalent to 1 mm/day, the standard physiological rate of regeneration. * **B. 1 cm/day:** Incorrect. This is 10 times faster than the actual rate; such rapid growth is not seen in human peripheral nerves. * **C. 0.1 mm/day:** Incorrect. This rate is too slow (only 3 mm per month), which would result in permanent muscle atrophy before reinnervation could occur. * **D. 1 mm/hour:** Incorrect. This is an impossibly high rate for protein synthesis and axonal transport required for structural regrowth. **Clinical Pearls for NEET-PG:** 1. **Wallerian Degeneration:** This process begins 24–36 hours after injury in the distal segment. 2. **Tinel’s Sign:** A clinical test used to track regeneration. A "pins and needles" sensation elicited by percussing over the nerve indicates the site of the regenerating axonal tips. 3. **Factors Affecting Rate:** Regeneration is faster in crushed nerves (Neuropraxia/Axonotmesis) compared to completely severed nerves (Neurotmesis) because the endoneurial tube remains intact. 4. **Proximal vs. Distal:** Regeneration generally occurs faster in proximal segments of the limb compared to distal segments.

Q: What is the resting membrane potential in a nerve fiber?

Depends upon K+ equilibrium. ### Explanation The Resting Membrane Potential (RMP) of a nerve fiber (typically **-70 mV**) is primarily determined by the selective permeability of the cell membrane to specific ions. **Why the correct answer is right:** According to the **Goldman-Hodgkin-Katz equation**, the RMP is closest to the equilibrium potential of the ion to which the membrane is most permeable. At rest, the nerve membrane is significantly more permeable to **Potassium (K+)** than to Sodium (Na+) due to the presence of "leaky" K+ channels. Therefore, the RMP is primarily a reflection of the **K+ equilibrium potential** (calculated by the Nernst equation as approximately -94 mV). While the Na+-K+ ATPase pump maintains the gradient, the actual potential value depends on K+ efflux. **Analysis of Incorrect Options:** * **Option A:** Ventricular muscle fibers have a much more negative RMP (approx. **-90 mV**) compared to nerve fibers (-70 mV) due to higher resting K+ conductance. * **Option B:** RMP is a transmembrane potential (difference between inside and outside). It must be measured using **intracellular microelectrodes**; surface electrodes (like ECG/EEG) only measure extracellular field potentials. * **Option C:** If extracellular K+ increases (hyperkalemia), the concentration gradient for K+ to leave the cell decreases. This causes the RMP to become **less negative (depolarization)**, which is technically a **decrease** in the magnitude of the potential. **High-Yield Facts for NEET-PG:** * **Nernst Potential for K+:** -94 mV | **Na+:** +61 mV | **Cl-:** -70 mV. * **Na+-K+ ATPase:** It is electrogenic, contributing about -4 to -5 mV directly to the RMP. * **Clinical Correlation:** Hypokalemia hyperpolarizes the membrane (makes RMP more negative), making it harder to fire an action potential, leading to muscle weakness and paralysis.

Q: Which of the following is referred to as a 'relaxation protein'?

Tropomyosin. ### Explanation **Correct Option: C. Tropomyosin** **Why Tropomyosin is the 'Relaxation Protein':** In a resting muscle fiber, the interaction between actin and myosin is physically blocked. **Tropomyosin** is a long, filamentous protein that wraps around the actin filament, covering the active myosin-binding sites. As long as tropomyosin remains in this position, the myosin heads cannot bind to actin, preventing the power stroke and keeping the muscle in a state of relaxation. Contraction only occurs when Calcium binds to Troponin C, causing a conformational change that pulls tropomyosin away from these binding sites. Therefore, because its primary role is to **inhibit contraction**, it is classically referred to as the relaxation protein. **Analysis of Incorrect Options:** * **A. Actin:** Known as the **thin filament**. It is a contractile protein that provides the binding sites for myosin; it does not facilitate relaxation. * **B. Myosin:** Known as the **thick filament**. It is the primary contractile protein (motor protein) that utilizes ATP to generate force. * **D. Dystrophin:** This is a **structural protein** that links the actin cytoskeleton of a muscle fiber to the surrounding extracellular matrix through the cell membrane. Its clinical significance lies in Duchenne Muscular Dystrophy (DMD). **High-Yield NEET-PG Pearls:** * **Troponin Complex:** Consists of three subunits: **Troponin I** (Inhibitory), **Troponin T** (Tropomyosin-binding), and **Troponin C** (Calcium-binding). * **The "Fenn Effect":** The increased energy consumption (ATP) that occurs when a muscle performs work. * **Rigor Mortis:** Occurs due to the depletion of ATP; without ATP, the myosin head cannot detach from actin, leading to permanent cross-bridge formation. * **Calsequestrin:** A protein in the Sarcoplasmic Reticulum that binds calcium, allowing for high-capacity storage.

Q: Which of the following structures contains Group B nerve fibers?

Autonomic preganglionic fibers. ### Explanation The classification of nerve fibers is a high-yield topic for NEET-PG, primarily based on the **Erlanger-Gasser classification**, which categorizes fibers according to diameter, myelination, and conduction velocity. **Why Option C is Correct:** **Group B fibers** are characterized as medium-diameter, **lightly myelinated** fibers with a moderate conduction velocity (3–15 m/s). The classic anatomical location for Group B fibers is the **autonomic preganglionic neurons** (both sympathetic and parasympathetic). Their light myelination allows for efficient signal transmission to autonomic ganglia before the signal transitions to unmyelinated Group C postganglionic fibers. **Analysis of Incorrect Options:** * **Option A (Intrafusal fibers):** These are innervated by **A-gamma (Aγ)** motor neurons. These are myelinated fibers responsible for maintaining muscle spindle sensitivity. * **Option B (Golgi tendon apparatus):** These are associated with **Group Ib** sensory fibers. These are large, heavily myelinated fibers with the fastest conduction velocities. * **Option D (Spinothalamic tracts):** These tracts primarily carry pain and temperature sensations via **A-delta (Aδ)** (fast pain) and **Group C** (slow pain) fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Order of Susceptibility:** * **Hypoxia:** Type A > Type B > Type C (Large fibers suffer first). * **Pressure:** Type A > Type B > Type C (e.g., Saturday Night Palsy). * **Local Anesthetics:** Type C > Type B > Type A (Small, unmyelinated fibers are blocked first). * **Fastest vs. Slowest:** Type **A-alpha (Aα)** are the fastest and thickest; Type **C** are the slowest, thinnest, and the only **unmyelinated** fibers. * **Postganglionic Autonomic Fibers:** Always remember these are **Type C** fibers.

Question 1

Arrange the following sequentially in the ascending order of occurrence with regard to signal transmission at the neuromuscular junction: Neurotransmitter degradation, Exocytosis, Calcium influx, End plate potential?

Accepted Answer

Calcium influx - Exocytosis - End plate potential - Neurotransmitter degradation

Answer

Neurotransmitter degradation - Exocytosis - Calcium influx - End plate potential

Answer

Calcium influx - Neurotransmitter degradation - Exocytosis - End plate potential

Answer

Exocytosis - End plate potential - Neurotransmitter degradation - Calcium influx

Question 2

Who postulated that nerve terminals release chemicals?

Accepted Answer

Loewi

Answer

Dale

Answer

Withering

Answer

Domagk

Question 3

Fibrillation of skeletal muscle is associated with all of the following conditions except?

Accepted Answer

Requires a strong stimulus

Answer

Occurs a long time after denervation

Answer

Hypersensitivity to acetylcholine

Answer

Receptors spread over the entire muscle cell membrane

Question 4

Which of the following is true regarding the action potential of skeletal muscle?

Accepted Answer

It spreads inward to all parts of the muscle via the T-tubules

Answer

It has a prolonged plateau phase

Answer

It causes the immediate uptake of Ca2+ into the lateral sacs of the sarcoplasmic reticulum

Answer

It is longer than the action potential of cardiac muscle

Question 5

The injured nerve regenerates at what rate?

Accepted Answer

0.1 cm/day

Answer

1 cm/day

Answer

0.1 mm/day

Answer

1 mm/hour

Question 6

What is the resting membrane potential in a nerve fiber?

Accepted Answer

Depends upon K+ equilibrium

Answer

Equal to the potential of ventricular muscle fiber

Answer

Can be measured by surface electrodes

Answer

Increases as extracellular K+ increases

Question 7

What is true about end-plate potential (EPP)?

Accepted Answer

It occurs as a result of altered permeability for Na+ and K+.

Answer

It is a self-regenerative change.

Answer

It results from the release of a single transmitter vesicle.

Answer

It exhibits all-or-none properties.

Question 8

Which of the following is NOT true about cardiac muscles?

Accepted Answer

Cardiac muscle cells are branched and interconnected.

Answer

Possesses spontaneous and rhythmic contraction.

Answer

Cardiac muscle exhibits cross-striations.

Answer

Cardiac muscle is supplied by autonomic nerve fibers.

Question 9

Which of the following is referred to as a 'relaxation protein'?

Accepted Answer

Tropomyosin

Answer

Actin

Answer

Myosin

Answer

Dystrophin

Question 10

Which of the following structures contains Group B nerve fibers?

Accepted Answer

Autonomic preganglionic fibers

Answer

Intrafusal fibers of the muscle spindle

Answer

Golgi tendon apparatus

Answer

Spinothalamic tracts

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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