Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 141: Which of the following statements regarding nerve conduction is false?

A. It obeys the all-or-none phenomenon.
B. Myelinated nerves conduct impulses faster than unmyelinated nerves.
C. Nerve impulse conduction occurs at a constant velocity and amplitude.
D. None of the above statements are false. (Correct Answer)

Explanation: This question tests the fundamental principles of nerve fiber physiology. The correct answer is **D** because all three preceding statements accurately describe the characteristics of nerve conduction. ### **Detailed Explanation** 1. **All-or-None Phenomenon (Option A):** This principle states that if a stimulus is below the threshold, no action potential is generated. Once the threshold is reached, an action potential of maximum and constant amplitude is produced. It does not increase in size with stronger stimuli; instead, the *frequency* of firing increases. 2. **Myelination and Speed (Option B):** Myelinated nerves utilize **Saltatory Conduction**, where the impulse "jumps" from one Node of Ranvier to the next. This is significantly faster and more energy-efficient than the continuous conduction seen in unmyelinated fibers. 3. **Constant Velocity and Amplitude (Option C):** Once an action potential is initiated in a single nerve fiber, it propagates without decrement (loss of strength). The amplitude remains constant due to the all-or-none law, and the velocity remains constant as long as the fiber diameter and temperature are uniform. ### **Why other options are "Incorrect":** Options A, B, and C are all scientifically accurate statements. Therefore, they cannot be the "false" statement the question is seeking. ### **High-Yield NEET-PG Pearls** * **Conduction Velocity:** Directly proportional to the **fiber diameter** and the presence of **myelin**. * **Erlanger-Gasser Classification:** Type A-alpha fibers are the fastest (proprioception/motor), while Type C fibers are the slowest (pain/temperature, unmyelinated). * **Local Anesthetics:** These block nerve conduction by inhibiting voltage-gated Na+ channels, typically affecting smaller, unmyelinated fibers (Type C) before larger myelinated ones. * **Energy Efficiency:** Saltatory conduction conserves ATP because Na+/K+ pump activity is only required at the Nodes of Ranvier.

Question 142: Where does the initiation of an impulse start?

A. Axon
B. Axon hillock and initial segment (Correct Answer)
C. Cell body
D. Dendritic tree

Explanation: The initiation of a nerve impulse (action potential) occurs at the **Axon Hillock and Initial Segment** (often collectively called the **Trigger Zone**). ### Why Option B is Correct The initiation of an action potential depends on the density of **voltage-gated sodium (Na+) channels**. The initial segment of the axon has a significantly higher concentration of these channels compared to the rest of the neuron. Consequently, this region has the **lowest threshold** for excitation, meaning it requires the least amount of depolarization to trigger an all-or-none action potential. While the axon hillock funnels the graded potentials, the actual spike usually begins in the adjacent initial segment. ### Why Other Options are Incorrect * **Axon (A):** While the action potential *propagates* along the axon, it does not *initiate* there. The distal axon serves as a conduction cable. * **Cell Body / Soma (C):** The soma contains a relatively low density of voltage-gated Na+ channels. It primarily functions to integrate incoming signals (EPSPs and IPSPs) rather than generating the spike. * **Dendritic Tree (D):** Dendrites are the primary sites for receiving inputs. They generate **graded potentials** (local changes in membrane potential) which decay over distance and do not typically initiate an action potential. ### High-Yield NEET-PG Pearls * **Threshold Value:** The threshold at the initial segment is approximately **-35 to -40 mV**, whereas the rest of the soma requires depolarization to about -10 mV. * **Orthodromic vs. Antidromic:** Normal conduction from the soma to the axon terminal is "orthodromic." Experimental stimulation of an axon that travels backward to the soma is "antidromic." * **Myelination:** In myelinated neurons, the action potential "jumps" between **Nodes of Ranvier** (Saltatory conduction), where Na+ channel density is also very high.

Question 143: All are true about local potential except?

A. Does not follow the all-or-none law
B. Can be depolarized or hyperpolarized
C. Is proportional to stimulus strength
D. Summation is not possible (Correct Answer)

Explanation: ### Explanation Local potentials (also known as graded potentials) are sub-threshold changes in membrane potential that occur in response to a stimulus. Unlike Action Potentials (AP), they have distinct physiological characteristics. **Why Option D is the Correct Answer (The Exception):** Summation **is possible** for local potentials. Because local potentials do not have a refractory period, multiple stimuli can be added together. This can occur as **temporal summation** (repeated stimuli at one location) or **spatial summation** (simultaneous stimuli at different locations). If the summated local potential reaches the "threshold" voltage, it triggers an action potential. **Analysis of Incorrect Options:** * **Option A: Does not follow all-or-none law:** This is true. Unlike action potentials, which either occur fully or not at all, local potentials vary in magnitude based on the stimulus. * **Option B: Can be depolarized or hyperpolarized:** This is true. Excitatory Postsynaptic Potentials (EPSP) cause depolarization, while Inhibitory Postsynaptic Potentials (IPSP) cause hyperpolarization. Action potentials, conversely, are always depolarizing. * **Option C: Proportional to stimulus strength:** This is true. They are "graded" potentials; a stronger stimulus opens more ion channels, resulting in a larger change in membrane potential. --- ### High-Yield Comparison for NEET-PG | Feature | Local Potential | Action Potential | | :--- | :--- | :--- | | **Amplitude** | Graded (Proportional to stimulus) | All-or-none (Constant) | | **Summation** | Possible | Not possible (due to refractory period) | | **Propagation** | Decremental (dies out) | Non-decremental (self-propagating) | | **Threshold** | No threshold required | Requires threshold (~ -55mV) | | **Examples** | Receptor potential, EPSP, IPSP | Nerve impulse, Muscle contraction | **Clinical Pearl:** The concept of summation is the basis of **Synaptic Integration** in the CNS. A single neuron may receive thousands of local potentials; the decision to "fire" an action potential depends entirely on the summation of these inputs at the **Axon Hillock** (the area with the lowest threshold for AP generation).

Question 144: What is the approximate number of Golgi tendon organs per 100 muscle fibers?

A. 1-10 (Correct Answer)
B. 200-400
C. 50-60
D. 80-100

Explanation: **Explanation:** The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons (musculotendinous junction). Its primary function is to sense **muscle tension** and protect the muscle from damage due to excessive contraction via the inverse stretch reflex. **Why Option A is Correct:** In human skeletal muscle, GTOs are relatively sparse compared to the total number of muscle fibers. Anatomical studies indicate that approximately **10 to 15 muscle fibers** are usually connected in series with a single Golgi tendon organ. When calculated as a ratio per 100 muscle fibers, the number typically falls within the **1–10 range**. This distribution ensures that the GTO can sample tension from a representative group of motor units without requiring a 1:1 ratio. **Why Other Options are Incorrect:** * **Options B, C, and D:** These values (50–400) are significantly higher than the physiological reality. If there were 50–100 GTOs per 100 fibers, nearly every muscle fiber would have its own dedicated GTO, which is not the case. Such high numbers would also occupy excessive space within the tendon, interfering with structural integrity. **High-Yield Clinical Pearls for NEET-PG:** * **Location:** GTOs are arranged **in series** with muscle fibers (unlike Muscle Spindles, which are **in parallel**). * **Innervation:** GTOs are innervated by **Type Ib sensory fibers** (fast-conducting). * **Function:** They mediate the **Inverse Stretch Reflex** (Autogenic Inhibition), causing the muscle to relax when tension becomes too high. * **Contrast:** While Muscle Spindles detect **length/stretch**, GTOs detect **force/tension**.

Question 145: Salutatory conduction refers to all of the following except?

A. It is a rapid process.
B. It involves the jumping of depolarization from node to node.
C. Current flow through the myelin sheath is negligible.
D. Depolarization travels in opposite directions along the axon. (Correct Answer)

Explanation: ### Explanation **Saltatory conduction** (from the Latin *saltare*, meaning "to leap") is the mechanism by which action potentials propagate along myelinated axons. **1. Why Option D is the Correct Answer (The "Except"):** Depolarization in a physiological setting is **unidirectional** (orthodromic). This is due to the **refractory period** of the sodium channels; once a Node of Ranvier has depolarized, the sodium channels behind it are inactivated, preventing the impulse from traveling backward. Saltatory conduction does not imply bidirectional travel. **2. Analysis of Incorrect Options:** * **Option A (Rapid process):** Myelin increases the length constant and decreases capacitance. This allows the electrical impulse to spread much faster (up to 120 m/s) compared to the slow, continuous conduction in unmyelinated fibers (0.5–2 m/s). * **Option B (Jumping from node to node):** In myelinated fibers, voltage-gated Na+ channels are highly concentrated at the **Nodes of Ranvier**. The myelin sheath acts as an insulator, forcing the current to "jump" from one node to the next. * **Option C (Negligible current flow through myelin):** Myelin has high electrical resistance. It prevents ions from leaking out across the axonal membrane, ensuring that the current flow through the sheath is virtually zero, conserving energy and signal strength. ### NEET-PG High-Yield Pearls * **Energy Efficiency:** Saltatory conduction is energy-efficient because depolarization occurs only at the nodes, requiring less activity from the **Na+-K+ ATPase pump** to restore ionic gradients. * **Myelin Producers:** Myelin is formed by **Oligodendrocytes** in the CNS and **Schwann cells** in the PNS. * **Clinical Correlation:** **Multiple Sclerosis** is a demyelinating disease of the CNS where saltatory conduction is disrupted, leading to slowed nerve impulses or conduction block. * **Node Concentration:** The density of Na+ channels at the Node of Ranvier is approximately 2000–12000/µm².

Question 146: Which examination finding is associated with lower motor neuron lesions?

A. Spasticity
B. Flaccid paralysis (Correct Answer)
C. Hyperactive stretch reflex
D. Muscular incoordination

Explanation: **Explanation:** In clinical neurology, distinguishing between Upper Motor Neuron (UMN) and Lower Motor Neuron (LMN) lesions is a high-yield topic for NEET-PG. **1. Why Flaccid Paralysis is Correct:** A Lower Motor Neuron (LMN) is the "final common pathway" connecting the spinal cord to the muscle. When the LMN (located in the anterior horn of the spinal cord or cranial nerve nuclei) is damaged, the muscle loses all neural input. This results in **flaccid paralysis**, characterized by a complete loss of muscle tone (**hypotonia**) and significant muscle wasting (**atrophy**) due to denervation. **2. Why the Other Options are Incorrect:** * **A. Spasticity:** This is a hallmark of **UMN lesions**. It occurs due to the loss of inhibitory control from higher centers, leading to a velocity-dependent increase in muscle tone. * **C. Hyperactive stretch reflex:** Also known as hyperreflexia, this is seen in **UMN lesions**. In LMN lesions, reflexes are diminished (**hyporeflexia**) or absent (**areflexia**) because the efferent limb of the reflex arc is broken. * **D. Muscular incoordination:** This is typically a sign of **cerebellar lesions** (ataxia) rather than a primary motor neuron lesion. **Clinical Pearls for NEET-PG:** * **LMN Signs (The "Downs"):** Tone is down (hypotonia), Reflexes are down (hyporeflexia), Muscle mass is down (atrophy), and **Fasciculations** (spontaneous muscle twitches) are present. * **UMN Signs (The "Ups"):** Tone is up (spasticity), Reflexes are up (hyperreflexia), and the **Babinski sign** is positive (upgoing toe). * **Classic LMN Examples:** Polio, Bell’s Palsy, and Spinal Muscular Atrophy.

Question 147: Which of the following is not a sarcolemmal protein?

A. Sarcoglycan
B. Dystrophin
C. Dystroglycan
D. Perlecan (Correct Answer)

Explanation: **Explanation:** The sarcolemma is the specialized cell membrane of a muscle fiber, which is reinforced by a complex of proteins known as the **Dystrophin-Glycoprotein Complex (DGC)**. This complex links the internal cytoskeleton (actin) to the external basal lamina, providing structural integrity during muscle contraction. * **Why Perlecan is the correct answer:** Perlecan is a large multidomain heparan sulfate proteoglycan. Crucially, it is **not** a sarcolemmal protein; rather, it is a major component of the **extracellular matrix (basal lamina)**. It interacts with dystroglycan to anchor the sarcolemma to the matrix but remains an external structural element. * **Why the other options are incorrect:** * **Dystrophin:** An essential intracellular protein that links the F-actin cytoskeleton to the sarcolemmal proteins. * **Sarcoglycan:** A group of transmembrane proteins (α, β, γ, δ) that form a sub-complex within the sarcolemma. * **Dystroglycan:** A transmembrane protein consisting of α and β subunits. The β-subunit spans the sarcolemma, while the α-subunit binds to laminin in the matrix. **High-Yield Clinical Pearls for NEET-PG:** * **Duchenne Muscular Dystrophy (DMD):** Caused by the total absence of **Dystrophin** (X-linked recessive). It is the most common and severe dystrophy. * **Becker Muscular Dystrophy (BMD):** Caused by a mutation leading to partially functional or reduced Dystrophin. * **Limb-Girdle Muscular Dystrophy (LGMD):** Often associated with mutations in the **Sarcoglycan** complex. * **Function of DGC:** It acts as a "shock absorber" during contraction; without these proteins, the sarcolemma tears, leading to muscle necrosis.

Question 148: In an isotonic contraction of the muscle, what occurs?

A. The muscle moves a load through a distance. (Correct Answer)
B. The muscle is not able to move the load.
C. The latent period of contraction is shorter than in isometric contraction.
D. The length of the muscle is increased.

Explanation: ### Explanation **1. Why Option A is Correct:** In an **isotonic contraction** (Greek: *iso* = same, *tonos* = tension), the muscle tension remains constant once it exceeds the weight of the load. Because the tension is sufficient to overcome the resistance, the muscle fibers shorten, resulting in the **movement of a load through a distance**. This represents external work being done ($Work = Force \times Distance$). **2. Analysis of Incorrect Options:** * **Option B:** This describes an **isometric contraction**. In isometric contractions, the load is too heavy for the muscle to move; the muscle develops tension, but its length remains constant, and no external work is performed. * **Option C:** In isotonic contraction, the **latent period is longer** than in isometric contraction. This is because, in an isotonic contraction, the muscle must first build enough tension to equal the load (the isometric phase) before shortening can begin. * **Option D:** During an isotonic contraction, the muscle **shortens** (concentric) or lengthens under controlled tension (eccentric). However, the standard definition in this context refers to shortening to move a load. The length never increases during the active contractile phase of a typical isotonic twitch. **3. NEET-PG High-Yield Pearls:** * **Isotonic vs. Isometric:** In isometric contraction, the **Elastic Component (SEC)** is stretched while the **Contractile Component (CC)** shortens, keeping the overall length the same. In isotonic, the CC shortens further after the SEC is stretched. * **Work Done:** Isotonic contraction performs external work; Isometric contraction performs zero external work (all energy is dissipated as heat). * **Velocity:** The velocity of muscle shortening is maximal when the load is zero and decreases as the load increases (**Force-Velocity Relationship**).

Question 149: Tapping the tendon of a muscle may elicit what response?

A. Contraction of the muscle (Correct Answer)
B. Relaxation of the muscle
C. No response
D. Fibrillation of the muscle

Explanation: ### Explanation **Concept: The Stretch Reflex (Myotatic Reflex)** Tapping a muscle tendon (e.g., the patellar tendon) causes a rapid, passive stretch of the muscle fibers. This stretch is detected by **muscle spindles** (the primary sensory receptors). These spindles send afferent impulses via **Type Ia nerve fibers** to the spinal cord, where they synapse directly with **alpha motor neurons**. This monosynaptic pathway results in the contraction of the same muscle that was stretched. This is the physiological basis of all Deep Tendon Reflexes (DTRs). **Analysis of Options:** * **Option A (Correct):** As described above, the stretch reflex leads to an immediate **contraction** of the agonist muscle to counteract the stretch and maintain muscle length. * **Option B (Incorrect):** Relaxation would occur if the **Golgi Tendon Organ (GTO)** were primarily activated by high tension (Inverse Stretch Reflex), but a quick tap specifically triggers the spindle-mediated contraction. * **Option C (Incorrect):** In a healthy individual, a response is always expected. A lack of response (areflexia) indicates a lower motor neuron (LMN) lesion or peripheral nerve damage. * **Option D (Incorrect):** Fibrillations are spontaneous, invisible contractions of individual muscle fibers seen in denervated muscle (LMN lesions) on EMG. They are not a response to mechanical tapping. **High-Yield Clinical Pearls for NEET-PG:** * **Monosynaptic Reflex:** The stretch reflex is the only monosynaptic reflex in the human body. * **Reciprocal Inhibition:** While the agonist muscle contracts, the antagonist muscle is simultaneously inhibited (relaxed) via an inhibitory interneuron. * **Dynamic vs. Static:** Tapping a tendon tests the **dynamic** stretch reflex (Type Ia fibers); muscle tone tests the **static** stretch reflex (Type II fibers). * **Jendrassik Maneuver:** A reinforcement technique used to elicit a reflex when it is difficult to evoke, by increasing the excitatory drive in the spinal cord.

Question 150: Which of the following conditions is characterized by a decremental response on electromyography (EMG)?

A. Myasthenia gravis (Correct Answer)
B. Lambert-Eaton syndrome
C. Duchenne muscular dystrophy
D. Upper motor neuron lesion

Explanation: ### Explanation **Correct Answer: A. Myasthenia Gravis** **Mechanism:** Myasthenia Gravis (MG) is an autoimmune disorder characterized by antibodies against **post-synaptic nicotinic acetylcholine receptors (nAchR)** at the neuromuscular junction (NMJ). This leads to a reduction in the number of available receptors. During **Repetitive Nerve Stimulation (RNS)** at low frequencies (2–3 Hz), the physiological depletion of acetylcholine (ACh) vesicles occurs. In a healthy individual, there are enough receptors to maintain an action potential; however, in MG patients, the reduced receptor pool cannot compensate for the decreasing ACh levels. This results in a progressive decline in the **Compound Muscle Action Potential (CMAP)** amplitude, known as a **decremental response**. **Analysis of Incorrect Options:** * **B. Lambert-Eaton Syndrome (LEMS):** This is a **pre-synaptic** disorder (antibodies against voltage-gated calcium channels). It typically shows an **incremental response** (facilitation) on high-frequency RNS because rapid stimulation allows calcium to accumulate in the nerve terminal, increasing ACh release. * **C. Duchenne Muscular Dystrophy:** This is a primary myopathy due to a lack of dystrophin. EMG typically shows small-amplitude, short-duration, polyphasic motor unit action potentials (MUAPs), but not a decremental response to RNS. * **D. Upper Motor Neuron (UMN) Lesion:** These lesions (e.g., stroke) affect the central nervous system pathways. EMG/RNS findings are generally normal as the peripheral motor unit and NMJ remain intact. **High-Yield Clinical Pearls for NEET-PG:** * **MG Hallmark:** Fatigability (worsens with activity, improves with rest). * **Tensilon Test:** Uses Edrophonium (short-acting acetylcholinesterase inhibitor) for diagnosis. * **Ice Pack Test:** Improvement of ptosis with cold (inhibits acetylcholinesterase). * **Associated Pathology:** Often associated with **Thymic hyperplasia** or **Thymoma**. * **LEMS vs. MG:** LEMS improves with exercise (incremental), whereas MG worsens (decremental).

Practice by Chapter

Resting Membrane Potential

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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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