Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 211: Action potential is initiated at the axon hillock/initial segment of a neuron because:

A. The threshold for excitation is lowest at this site. (Correct Answer)
B. Neurotransmitter is released at this site.
C. It is an unmyelinated segment.
D. It has the lowest concentration of voltage-gated Na+ channels.

Explanation: ### Explanation **1. Why Option A is Correct:** The axon hillock and the initial segment (collectively known as the **Trigger Zone**) have the **highest density of voltage-gated Na+ channels** in the entire neuron. According to the principles of electrophysiology, a high density of these channels means that a smaller depolarization is required to open enough channels to trigger the regenerative positive-feedback loop of an action potential. Consequently, the **threshold for excitation is lowest** at this site (approximately -45 mV compared to -35 mV in the soma), making it the most excitable part of the neuron. **2. Why Other Options are Incorrect:** * **Option B:** Neurotransmitters are released at the **axon terminals (synaptic knobs)**, not the axon hillock. The hillock is responsible for signal integration and initiation, not transmission across a synapse. * **Option C:** While the initial segment is indeed unmyelinated, this is not the *reason* for action potential initiation. Many parts of a neuron (like the dendrites and soma) are unmyelinated but do not initiate action potentials because they lack the necessary Na+ channel density. * **Option D:** This is factually opposite. The axon hillock has the **highest** concentration of voltage-gated Na+ channels (roughly 100–1000 times higher than the soma). **Clinical Pearls & High-Yield Facts:** * **Spatial and Temporal Summation:** The axon hillock acts as the "calculator" of the neuron, summing up Excitatory Postsynaptic Potentials (EPSPs) and Inhibitory Postsynaptic Potentials (IPSPs). * **Nodes of Ranvier:** In myelinated axons, these are the only sites where action potentials are regenerated (Saltatory Conduction) due to high Na+ channel density, similar to the initial segment. * **Accommodation:** If a neuron is subjected to a slow, constant depolarization, the threshold at the hillock may rise because Na+ channels undergo inactivation before the threshold is reached.

Question 212: Saltatory conduction in myelinated axons results from the fact that?

A. Salt concentration is increased beneath the myelin segments
B. Nongated ion channels are present beneath the segments of myelin
C. Membrane resistance is decreased beneath the segments of myelin
D. Voltage-gated sodium channels are concentrated at the nodes of Ranvier (Correct Answer)

Explanation: **Explanation:** **Why Option D is Correct:** Saltatory conduction (from the Latin *saltare*, meaning "to leap") is the rapid propagation of action potentials along myelinated axons. Myelin acts as an electrical insulator, significantly increasing membrane resistance and decreasing capacitance. Because of this insulation, the ionic current cannot flow through the membrane in myelinated segments. Instead, the action potential "jumps" from one **Node of Ranvier** to the next. This is possible because **voltage-gated Na+ channels** are highly concentrated at these nodes (approximately 2000–12000 per $\mu m^2$), allowing for the rapid depolarization required to regenerate the action potential. **Why Other Options are Incorrect:** * **Option A:** Saltatory conduction is unrelated to the "salt concentration" (sodium/potassium levels) beneath the myelin; the term refers to the "leaping" nature of the impulse. * **Option B:** The area beneath the myelin (internode) is actually characterized by a relative **absence** of ion channels. If channels were present and active there, the insulating property of myelin would be bypassed. * **Option C:** Myelin **increases** membrane resistance ($R_m$). By increasing resistance, it prevents the leakage of current across the axonal membrane, forcing the current to flow longitudinally to the next node. **NEET-PG High-Yield Pearls:** * **Energy Efficiency:** Saltatory conduction is more energy-efficient than continuous conduction because the $Na^+-K^+$ ATPase pump only needs to work at the nodes to restore ionic gradients. * **Velocity:** Conduction velocity in myelinated fibers is directly proportional to the fiber diameter ($V \propto \text{diameter}$), whereas in unmyelinated fibers, it is proportional to the square root of the diameter ($V \propto \sqrt{\text{diameter}}$). * **Clinical Correlation:** In **Multiple Sclerosis** (CNS) and **Guillain-Barré Syndrome** (PNS), demyelination leads to a loss of insulation, causing current leak and "conduction block," as the density of Na+ channels in the formerly myelinated segments is too low to sustain an action potential.

Question 213: A person wakes up with pain, paresthesia, and tingling of the arms after sleeping with their arm below their head. Which nerve fibres are involved?

A. Type A fibres (Correct Answer)
B. Type B fibres
C. Type C fibres (pain)
D. Type C fibres (postganglionic)

Explanation: ### Explanation The clinical scenario describes **"Saturday Night Palsy"** or **"Sleep Palsy,"** which occurs due to prolonged mechanical compression of a peripheral nerve (typically the radial nerve). The susceptibility of nerve fibers to different types of insults follows a specific order (Gasser-Erlanger classification): 1. **Pressure/Compression:** **Type A fibers** are the **most sensitive** to pressure, while Type C fibers are the least sensitive. 2. **Hypoxia:** Type B fibers are the most sensitive. 3. **Local Anesthetics:** Type C fibers are the most sensitive. In this case, mechanical compression leads to the blockage of Type A fibers first. Since Type A-alpha and A-beta fibers carry motor functions and touch/pressure sensations, their involvement results in the "pins and needles" sensation (paresthesia) and temporary weakness. #### Analysis of Options: * **Option A (Correct):** Type A fibers are large, myelinated fibers. Their large diameter makes them highly susceptible to mechanical deformation and ischemia caused by direct pressure. * **Option B (Incorrect):** Type B fibers (preganglionic autonomic) are most sensitive to **hypoxia**, not mechanical pressure. * **Option C & D (Incorrect):** Type C fibers are small, unmyelinated fibers. They are the **most resistant to pressure** but the most sensitive to **local anesthesia**. While Type C fibers do carry slow pain, the initial tingling and paresthesia from "falling asleep" on an arm are classic signs of Type A fiber compromise. #### NEET-PG High-Yield Pearls: * **Order of Sensitivity to Pressure:** A > B > C (Mnemonic: **P**ressure affects **P**rimary/A fibers). * **Order of Sensitivity to Hypoxia:** B > A > C. * **Order of Sensitivity to Local Anesthesia:** C > B > A (Mnemonic: **L**ocal **L**ast/C fibers). * **Neuropraxia:** This scenario is a form of Neuropraxia (Seddon’s classification), where there is a temporary conduction block without axonal degeneration.

Question 214: What is the study of the electrical activity of the muscle called?

A. Electroencephalogram (EEG)
B. Electromyogram (EMG) (Correct Answer)
C. Venn diagram
D. Electrocardiogram (ECG)

Explanation: **Explanation:** The correct answer is **Electromyogram (EMG)**. **1. Why Electromyogram (EMG) is correct:** Electromyography is the diagnostic procedure used to assess the health of muscles and the nerve cells (motor neurons) that control them. It records the **electrical activity of muscle fibers** during rest, slight contraction, and forceful contraction. When a muscle fiber is stimulated by a motor neuron, it generates an action potential; the EMG electrodes detect these electrical signals and translate them into graphs or numerical data. **2. Why the other options are incorrect:** * **Electroencephalogram (EEG):** This records the electrical activity of the **brain** (cerebral cortex) using electrodes placed on the scalp. It is primarily used to diagnose epilepsy and sleep disorders. * **Electrocardiogram (ECG/EKG):** This records the electrical activity of the **heart** over a period of time. It is the gold standard for diagnosing arrhythmias and myocardial infarctions. * **Venn diagram:** This is a **mathematical/logical tool** used to show relationships between sets; it has no application in physiological electrical recording. **3. High-Yield Clinical Pearls for NEET-PG:** * **Motor Unit Action Potential (MUAP):** The fundamental unit studied in EMG. Changes in MUAP morphology help differentiate between **myopathic** (small, short-duration potentials) and **neurogenic** (large, long-duration polyphasic potentials) disorders. * **Fibrillation potentials and Fasciculations:** Spontaneous electrical activity seen in EMG during rest, often indicating denervation or lower motor neuron (LMN) lesions. * **Nerve Conduction Studies (NCS):** Often performed alongside EMG to measure the speed and strength of signals traveling through a nerve, helping to localize peripheral nerve entrapments (like Carpal Tunnel Syndrome).

Question 215: What is the velocity of conduction for A-alpha nerve fibers in m/second?

A. 80
B. 70
C. 90
D. 120 (Correct Answer)

Explanation: **Explanation:** The conduction velocity of a nerve fiber is directly proportional to its diameter and the presence of a myelin sheath. According to the **Erlanger-Gasser classification**, nerve fibers are categorized based on their diameter and velocity. **Why Option D is Correct:** **A-alpha (Aα) fibers** are the thickest and most heavily myelinated fibers in the human body. They have a diameter of **12–20 μm** and a conduction velocity ranging from **70 to 120 m/sec**. In the context of NEET-PG, when a range is provided, the maximum value (120 m/sec) is typically considered the defining characteristic for A-alpha fibers. These fibers are responsible for **proprioception** (muscle spindles and Golgi tendon organs) and **somatic motor function**. **Why Other Options are Incorrect:** * **Options A, B, and C (80, 70, 90 m/sec):** While these values fall within the functional *range* of A-alpha fibers, they do not represent the maximum potential velocity. In competitive exams, the upper limit of 120 m/sec is the standard "textbook" answer used to distinguish A-alpha from A-beta fibers. **High-Yield Clinical Pearls for NEET-PG:** * **Hursh’s Factor:** Conduction velocity (m/s) = Diameter (μm) × 6. This is a quick rule of thumb to estimate velocity. * **Order of Sensitivity to Blockade:** * **Local Anesthetics:** Type C > Type B > Type A (Smallest/unmyelinated are blocked first). * **Pressure:** Type A > Type B > Type C (Largest are blocked first; e.g., "foot falling asleep"). * **Hypoxia:** Type B > Type A > Type C. * **Type C Fibers:** These are the only unmyelinated fibers, have the smallest diameter (0.4–1.2 μm), and the slowest velocity (0.5–2 m/sec). They carry slow pain and temperature.

Question 216: Which of the following is a true statement regarding malignant hyperthermia?

A. Increased Ca++ transport through T-type calcium channels in T-tubules
B. Increased Ca++ transport through T-type calcium channels in L-tubules
C. Increased Ca++ transport through L-type calcium channels in T-tubules (Correct Answer)
D. Increased Ca++ transport through L-type calcium channels in L-tubules

Explanation: **Explanation:** Malignant Hyperthermia (MH) is a life-threatening pharmacogenetic disorder triggered by volatile anesthetics (e.g., Halothane) or depolarizing muscle relaxants (e.g., Succinylcholine). **Why Option C is Correct:** The pathophysiology involves a defect in the **Ryanodine Receptor (RyR1)** on the Sarcoplasmic Reticulum (SR) or, less commonly, the **Dihydropyridine Receptor (DHPR)**. The DHPR is an **L-type calcium channel** located specifically in the **T-tubules** (transverse tubules). In MH, a mutation leads to an abnormal interaction between these two receptors, causing an uncontrolled, massive release of $Ca^{++}$ from the SR into the sarcoplasm. This excess calcium triggers continuous muscle contraction, leading to hypermetabolism, heat production, and rhabdomyolysis. **Why Other Options are Incorrect:** * **Options A & B:** **T-type calcium channels** are low-voltage-activated channels primarily found in the heart (pacemaker cells) and neurons, not the skeletal muscle T-tubule system involved in MH. * **Options B & D:** There is no anatomical structure known as **"L-tubules"** in muscle physiology. The calcium-handling system consists of T-tubules (invaginations of the sarcolemma) and the Sarcoplasmic Reticulum (longitudinal tubules). **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Dominant. * **Most common mutation:** RYR1 gene (Chromosome 19q). * **Clinical Triad:** Muscle rigidity, rapid rise in body temperature (hyperpyrexia), and metabolic acidosis. * **Earliest Sign:** Increase in End-Tidal $CO_2$ ($ETCO_2$). * **Drug of Choice:** **Dantrolene** (mechanism: binds to RyR1 and inhibits $Ca^{++}$ release). * **Associated Conditions:** Central Core Disease and King-Denborough Syndrome.

Question 217: Z-lines are anchored to the sarcolemma by:

A. Titin
B. Dystrophin
C. Desmin (Correct Answer)
D. Actinin

Explanation: **Explanation:** The correct answer is **Desmin**. In skeletal muscle, the structural integrity and spatial alignment of myofibrils are maintained by a complex network of intermediate filaments. **Desmin** is the primary intermediate filament that encircles the Z-lines of adjacent myofibrils, linking them to each other and anchoring the entire Z-disk complex to the sarcolemma (cell membrane) via costameres. This ensures that all myofibrils contract in synchrony and maintains the lateral registration of the sarcomeres. **Analysis of Incorrect Options:** * **Titin (Option A):** This is the largest known protein. It acts as a molecular spring, connecting the Z-line to the M-line, providing passive elasticity to the muscle and centering the myosin filaments. * **Dystrophin (Option B):** While it also links the cytoskeleton to the sarcolemma, it specifically connects **F-actin** (not the Z-line directly) to the dystroglycan complex in the membrane. Its deficiency leads to Duchenne Muscular Dystrophy. * **Actinin (Option D):** Specifically **α-actinin**, this protein is located *within* the Z-line. Its primary role is to anchor the plus ends of actin (thin) filaments to the Z-disk, rather than anchoring the Z-disk to the sarcolemma. **High-Yield Clinical Pearls for NEET-PG:** * **Costameres:** These are the functional units (containing desmin, vinculin, and dystrophin) that couple the sarcomere to the extracellular matrix. * **Nebulin:** Acts as a "molecular ruler" to regulate the length of actin filaments. * **Desmin-related Myopathy:** Mutations in the desmin gene lead to myofibrillar myopathies characterized by muscle weakness and cardiac conduction blocks.

Question 218: What is an auxotonic contraction of a muscle?

A. A contraction where the tension developed changes when the length of the muscle changes.
B. A contraction where the muscle length changes and the tension changes throughout the contraction. (Correct Answer)
C. A contraction where the muscle length is constant but tension changes.
D. A contraction where the muscle length changes but tension remains constant.

Explanation: ### Explanation **1. Why Option B is Correct:** In physiology, muscle contractions are categorized based on changes in length and tension. An **auxotonic contraction** is a dynamic contraction where **both the muscle length and the tension change simultaneously**. The word is derived from the Greek *auxein* (to increase) and *tonos* (tension). A classic example of an auxotonic contraction is stretching a physical spring or a rubber band; as the muscle shortens to pull the spring, the resistance increases, requiring the muscle to generate progressively more tension. In the human body, most natural movements are auxotonic rather than purely isotonic or isometric. **2. Analysis of Incorrect Options:** * **Option A:** While it mentions tension changing with length, it is less precise than Option B, which specifies that both parameters are actively changing *throughout* the duration of the contraction. * **Option C (Isometric):** This describes an **Isometric contraction** (e.g., pushing against a wall). The length remains constant (*iso* = same, *metric* = length), but tension increases. * **Option D (Isotonic):** This describes an **Isotonic contraction** (e.g., lifting a constant free weight). The tension remains constant (*iso* = same, *tonic* = tension) while the muscle length changes. **3. High-Yield NEET-PG Pearls:** * **Isotonic vs. Isometric:** In Isotonic contractions, **work is done** (Work = Force × Distance). In Isometric contractions, **no external work** is done, and all energy is released as heat. * **Isokinetic Contraction:** A contraction where the velocity of shortening remains constant (usually achieved with specialized gym equipment). * **Concentric vs. Eccentric:** These are subtypes of isotonic contractions. Concentric involves shortening (e.g., upward phase of a bicep curl), while eccentric involves lengthening (e.g., lowering the weight). * **Clinical Note:** Most functional activities (like walking or climbing stairs) are **auxotonic**, as joint angles and load leverage change throughout the range of motion.

Question 219: Which of the following statements is true about myoglobin?

A. Myoglobin binds to 4 moles of O2 per mole.
B. The myoglobin dissociation curve has a parabolic profile.
C. Myoglobin has a higher O2 affinity compared to that of hemoglobin. (Correct Answer)
D. The myoglobin dissociation curve shows the Bohr effect.

Explanation: ### Explanation **Correct Answer: C. Myoglobin has a higher O2 affinity compared to that of hemoglobin.** **Underlying Concept:** Myoglobin is a monomeric heme protein found in skeletal and cardiac muscle. Unlike hemoglobin (Hb), which is a tetramer, myoglobin has a much higher affinity for oxygen. This allows it to "pull" oxygen from the blood into the muscle cells. On the oxygen-hemoglobin dissociation curve, the myoglobin curve is shifted significantly to the **left**, meaning it remains saturated at lower partial pressures of oxygen ($PO_2$), only releasing its oxygen when cellular $PO_2$ drops to very low levels (e.g., during intense exercise). **Analysis of Incorrect Options:** * **Option A:** Myoglobin consists of a single polypeptide chain and one heme group; therefore, it binds to **only 1 mole of $O_2$** per mole. Hemoglobin binds 4 moles of $O_2$. * **Option B:** The myoglobin dissociation curve is **hyperbolic**, not parabolic. The hyperbolic shape reflects simple binding kinetics without cooperativity. In contrast, hemoglobin’s curve is **sigmoidal** due to "positive cooperativity." * **Option D:** The **Bohr Effect** (the shift of the curve due to $CO_2$ and $pH$) is a property of hemoglobin. Myoglobin does not show the Bohr effect because it lacks the quaternary structure and inter-chain interactions required for allosteric regulation. **High-Yield Facts for NEET-PG:** * **$P_{50}$ Values:** The $P_{50}$ (partial pressure at which 50% of the protein is saturated) for myoglobin is approximately **2.75 mmHg**, whereas for adult hemoglobin (HbA), it is **26.6 mmHg**. A lower $P_{50}$ indicates a higher affinity. * **Function:** Myoglobin acts as an **oxygen storage** unit, while hemoglobin acts as an **oxygen transporter**. * **Clinical Pearl:** In cases of **Rhabdomyolysis** (muscle breakdown), myoglobin is released into the blood and filtered by the kidneys, leading to "cola-colored" urine and potential acute tubular necrosis.

Question 220: Wallerian degeneration is observed in which part of the neuron after axonal injury?

A. Distal to the injury (Correct Answer)
B. Proximal to the injury
C. At both ends
D. In the cell body (soma)

Explanation: **Explanation:** **Wallerian Degeneration** refers to the sequence of events that occur when an axon is severed from its metabolic source—the cell body (soma). **1. Why Option A is Correct:** The axon depends on the cell body for the synthesis of proteins and organelles, which are transported via axoplasmic flow. When an injury occurs, the segment **distal to the injury** is physically separated from the soma. Deprived of essential nutrients, the distal cytoskeleton and membrane disintegrate within 24–36 hours. This is followed by the infiltration of macrophages to clear myelin debris, creating a path for potential regeneration. **2. Why the Other Options are Incorrect:** * **Option B (Proximal to the injury):** The proximal segment remains attached to the cell body. While it may undergo limited "retrograde degeneration" (up to the nearest Node of Ranvier), the primary process of Wallerian degeneration is a distal phenomenon. * **Option C (Both ends):** Degeneration is asymmetrical. The distal end undergoes complete breakdown, whereas the proximal end prepares for repair. * **Option D (Cell body):** The cell body does not degenerate; instead, it undergoes **Chromatolysis** (swelling of the soma, displacement of the nucleus to the periphery, and dispersal of Nissl bodies) to ramp up protein synthesis for repair. **High-Yield Facts for NEET-PG:** * **Chromatolysis:** The characteristic change in the cell body after axonal injury. * **Rate of Regeneration:** Peripheral nerves typically regrow at a rate of **1–3 mm/day**. * **Schwann Cells:** In the PNS, these cells survive Wallerian degeneration and form **Bungner bands** (tubes) to guide the regenerating axon sprout. * **CNS vs. PNS:** Wallerian degeneration is much slower in the CNS because oligodendrocytes do not provide the same guidance, and myelin debris (which contains inhibitory factors like Nogo-A) persists longer.

Practice by Chapter

Resting Membrane Potential

Practice Questions

Action Potential Generation and Propagation

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Neuromuscular Junction

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Skeletal Muscle Contraction

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Smooth Muscle Physiology

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Cardiac Muscle Properties

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Muscle Metabolism and Fatigue

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Motor Unit Function

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Neurotransmitters and Receptors

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Electrophysiological Measurements

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