Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 51: 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?

A. Neurotransmitter degradation - Exocytosis - Calcium influx - End plate potential
B. Calcium influx - Exocytosis - End plate potential - Neurotransmitter degradation (Correct Answer)
C. Calcium influx - Neurotransmitter degradation - Exocytosis - End plate potential
D. Exocytosis - End plate potential - Neurotransmitter degradation - Calcium influx

Explanation: ### Explanation The transmission of a signal at the Neuromuscular Junction (NMJ) follows a precise electrochemical sequence to ensure the conversion of an electrical impulse into a mechanical contraction. **1. Why the Correct Answer is Right:** The sequence begins when an action potential reaches the presynaptic terminal, causing depolarization. * **Calcium influx:** Voltage-gated $Ca^{2+}$ channels open, allowing calcium to enter the nerve terminal. * **Exocytosis:** The rise in intracellular $Ca^{2+}$ triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing Acetylcholine (ACh) into the synaptic cleft. * **End plate potential (EPP):** ACh binds to nicotinic receptors ($N_m$) on the motor end plate, increasing permeability to $Na^+$ and $K^+$. This creates a local depolarization known as the EPP. * **Neurotransmitter degradation:** Finally, Acetylcholinesterase (AChE) hydrolyzes ACh into choline and acetate to terminate the signal and prevent continuous muscle contraction. **2. Why Other Options are Wrong:** * **Option A & D:** These are incorrect because **Calcium influx** must precede exocytosis; without calcium, the SNARE proteins cannot facilitate vesicle fusion. * **Option C:** This is incorrect because **Neurotransmitter degradation** is the final step. If degradation occurred before exocytosis or EPP, no signal would ever reach the muscle. **3. Clinical Pearls for NEET-PG:** * **Lambert-Eaton Syndrome:** Antibodies against presynaptic voltage-gated $Ca^{2+}$ channels (affects **Calcium influx**). * **Botulinum Toxin:** Cleaves SNARE proteins, preventing vesicle fusion (affects **Exocytosis**). * **Myasthenia Gravis:** Antibodies against post-synaptic $N_m$ receptors (reduces the magnitude of **EPP**). * **Organophosphate Poisoning:** Inhibits AChE, leading to a failure of **Neurotransmitter degradation** and resulting in a "cholinergic crisis."

Question 52: Who postulated that nerve terminals release chemicals?

A. Dale
B. Withering
C. Domagk
D. Loewi (Correct Answer)

Explanation: **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.

Question 53: Fibrillation of skeletal muscle is associated with all of the following conditions except?

A. Occurs a long time after denervation
B. Hypersensitivity to acetylcholine
C. Receptors spread over the entire muscle cell membrane
D. Requires a strong stimulus (Correct Answer)

Explanation: **Explanation:** **Fibrillation** refers to the spontaneous, repetitive contractions of individual muscle fibers that occur following the loss of lower motor neuron (LMN) innervation. **Why Option D is the correct answer:** Fibrillations are **not** triggered by external stimuli; they are **spontaneous** electrical activities. They occur because the denervated muscle membrane becomes unstable and develops rhythmic, low-voltage potentials. Unlike fasciculations (which are visible to the naked eye), fibrillations are invisible and can only be detected via Electromyography (EMG). Therefore, they do not "require a strong stimulus." **Analysis of other options:** * **Option A (Occurs after denervation):** Fibrillation typically begins 1–3 weeks after a muscle loses its nerve supply (denervation). It is a classic sign of LMN lesions. * **Option B & C (Hypersensitivity and Receptor Spread):** This is known as **Denervation Supersensitivity**. Normally, Acetylcholine (ACh) receptors are concentrated at the neuromuscular junction (NMJ). After denervation, the muscle compensates by synthesizing new nicotinic receptors that **spread over the entire surface** of the muscle membrane. This makes the fiber exquisitely sensitive to even minute amounts of circulating acetylcholine. **High-Yield Clinical Pearls for NEET-PG:** * **Fibrillation vs. Fasciculation:** Fibrillations involve single muscle fibers (invisible); Fasciculations involve entire motor units (visible twitches). * **EMG Finding:** Fibrillations are characterized by "biphasic spikes" or "positive sharp waves" on EMG. * **Receptor Type:** The extrajunctional receptors that develop during denervation are the **fetal isoform** (containing the gamma subunit instead of the epsilon subunit). * **Clinical Significance:** Fibrillation is a hallmark of **Lower Motor Neuron (LMN)** disorders (e.g., Polio, ALS, peripheral nerve injury) and is absent in Upper Motor Neuron (UMN) lesions.

Question 54: Which of the following is true regarding the action potential of skeletal muscle?

A. It spreads inward to all parts of the muscle via the T-tubules (Correct Answer)
B. It has a prolonged plateau phase
C. It causes the immediate uptake of Ca2+ into the lateral sacs of the sarcoplasmic reticulum
D. It is longer than the action potential of cardiac muscle

Explanation: ### Explanation **1. Why Option A is Correct:** Skeletal muscle fibers have a large diameter, making it impossible for a surface membrane action potential (AP) to reach deep-seated myofibrils by simple diffusion. To solve this, the sarcolemma invaginates to form **T-tubules (Transverse tubules)**. These tubules carry the depolarization inward to the center of the muscle fiber. This ensures that the AP reaches the **Dihydropyridine (DHP) receptors**, which are voltage-gated sensors that mechanically trigger the release of $Ca^{2+}$ from the sarcoplasmic reticulum, ensuring synchronized contraction of the entire fiber. **2. Why the Other Options are Incorrect:** * **Option B:** A prolonged plateau phase is a hallmark of **cardiac muscle** (due to $L$-type $Ca^{2+}$ channels), not skeletal muscle. Skeletal muscle APs are "spike-like" and very brief. * **Option C:** The AP causes the **release** of $Ca^{2+}$ from the lateral sacs (terminal cisternae) into the sarcoplasm to initiate contraction. The **uptake** of $Ca^{2+}$ back into the SR is a process of relaxation mediated by the SERCA pump, occurring *after* depolarization. * **Option D:** Skeletal muscle APs are much shorter (approx. 2–5 ms) compared to cardiac muscle APs (approx. 200–300 ms). **3. High-Yield Clinical Pearls for NEET-PG:** * **Triad:** In skeletal muscle, a triad consists of one T-tubule and two terminal cisternae. It is located at the **A-I junction**. (In cardiac muscle, it is a *diad* located at the **Z-line**). * **Ryanodine Receptor (RyR1):** This is the $Ca^{2+}$ release channel in the SR. Mutations in RyR1 lead to **Malignant Hyperthermia** when exposed to volatile anesthetics (e.g., Halothane). * **Excitation-Contraction (E-C) Coupling:** In skeletal muscle, the DHP-RyR interaction is **mechanical/electromechanical**, whereas in cardiac muscle, it is **calcium-induced calcium release (CICR)**.

Question 55: The injured nerve regenerates at what rate?

A. 0.1 cm/day (Correct Answer)
B. 1 cm/day
C. 0.1 mm/day
D. 1 mm/hour

Explanation: **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.

Question 56: What is the resting membrane potential in a nerve fiber?

A. Equal to the potential of ventricular muscle fiber
B. Can be measured by surface electrodes
C. Increases as extracellular K+ increases
D. Depends upon K+ equilibrium (Correct Answer)

Explanation: ### 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.

Question 57: What is true about end-plate potential (EPP)?

A. It occurs as a result of altered permeability for Na+ and K+. (Correct Answer)
B. It is a self-regenerative change.
C. It results from the release of a single transmitter vesicle.
D. It exhibits all-or-none properties.

Explanation: ### Explanation The **End-Plate Potential (EPP)** is a localized depolarization of the motor end-plate in response to acetylcholine (ACh) release. **1. Why Option A is Correct:** When ACh binds to nicotinic receptors (nAChR) at the neuromuscular junction, it opens ligand-gated ion channels. These channels are **non-selective cation channels** that allow the simultaneous movement of both **Na+ (influx)** and **K+ (efflux)**. Because the electrochemical gradient for Na+ is much stronger, the net result is a large influx of positive charge, leading to depolarization. **2. Why Other Options are Incorrect:** * **Option B & D:** Unlike action potentials, the EPP is a **graded potential**, not a self-regenerative one. It does **not** follow the "all-or-none" law; its magnitude depends on the amount of ACh released. It is a local response that must reach a threshold to trigger a voltage-gated action potential in the adjacent muscle membrane. * **Option C:** A single vesicle of ACh produces a **Miniature End-Plate Potential (MEPP)**, which typically measures about 0.5 mV. A full EPP requires the synchronous release of approximately 125–300 vesicles (quanta). ### High-Yield Clinical Pearls for NEET-PG: * **Safety Factor:** The EPP is normally much larger than required to reach the threshold (approx. 30-40 mV), ensuring every nerve impulse results in a muscle contraction. * **Myasthenia Gravis:** Characterized by antibodies against nAChR, reducing the EPP amplitude below threshold, leading to muscle weakness. * **Lambert-Eaton Syndrome:** Antibodies against presynaptic voltage-gated Ca²+ channels reduce the number of ACh quanta released, also diminishing the EPP. * **Curare:** Competitively blocks nAChR, reducing EPP magnitude and causing paralysis.

Question 58: Which of the following is NOT true about cardiac muscles?

A. Possesses spontaneous and rhythmic contraction.
B. Cardiac muscle exhibits cross-striations.
C. Cardiac muscle cells are branched and interconnected. (Correct Answer)
D. Cardiac muscle is supplied by autonomic nerve fibers.

Explanation: **Explanation:** The question asks for the statement that is **NOT** true regarding cardiac muscle. However, based on physiological principles, Option C is actually a **true** anatomical feature of cardiac muscle. In standard medical literature (e.g., Guyton, Ganong), cardiac muscle is characterized by branching fibers and intercalated discs that form a functional syncytium. *Note: If this question appeared in a competitive exam with Option C as the "correct" answer, it is likely due to a technicality in wording or an error in the question key, as all four options provided are traditionally true characteristics.* **Analysis of Options:** * **Option A (True):** Cardiac muscle possesses **automaticity** and **rhythmicity**. The SA node acts as the primary pacemaker, generating spontaneous action potentials without external nervous stimulation. * **Option B (True):** Like skeletal muscle, cardiac muscle contains organized sarcomeres with actin and myosin filaments, giving it a **striated** appearance under a microscope. * **Option C (True):** Cardiac myocytes are **branched** (unlike the parallel cylinders of skeletal muscle) and are interconnected by **intercalated discs** containing gap junctions, allowing rapid electrical conduction. * **Option D (True):** While the heart initiates its own beat, the **Autonomic Nervous System (ANS)** modulates the rate (chronotropy) and force (inotropy) of contraction via sympathetic and parasympathetic fibers. **High-Yield NEET-PG Pearls:** 1. **Functional Syncytium:** Gap junctions allow the heart to contract as a single unit, though it is not a structural syncytium (unlike skeletal muscle). 2. **Refractory Period:** Cardiac muscle has a very long absolute refractory period (250ms), which prevents **tetanization**. 3. **Calcium Source:** Unlike skeletal muscle, cardiac muscle depends on **Extracellular Calcium** for contraction (Calcium-induced calcium release via RyR2 receptors).

Question 59: Which of the following is referred to as a 'relaxation protein'?

A. Actin
B. Myosin
C. Tropomyosin (Correct Answer)
D. Dystrophin

Explanation: ### 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.

Question 60: Which of the following structures contains Group B nerve fibers?

A. Intrafusal fibers of the muscle spindle
B. Golgi tendon apparatus
C. Autonomic preganglionic fibers (Correct Answer)
D. Spinothalamic tracts

Explanation: ### 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.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

Practice Questions

Neuromuscular Junction

Practice Questions

Skeletal Muscle Contraction

Practice Questions

Smooth Muscle Physiology

Practice Questions

Cardiac Muscle Properties

Practice Questions

Muscle Metabolism and Fatigue

Practice Questions

Motor Unit Function

Practice Questions

Neurotransmitters and Receptors

Practice Questions

Electrophysiological Measurements

Practice Questions

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