Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 61: What does the 'Lloyds classification' primarily categorize?

A. Sensory nerve fibers (Correct Answer)
B. Motor nerve fibers
C. Both sensory and motor nerve fibers
D. Neither sensory nor motor nerve fibers

Explanation: **Explanation:** The **Lloyd-Hunt classification** (often referred to as Lloyds classification) is a system specifically designed to categorize **sensory (afferent) nerve fibers** based on their diameter and conduction velocity. This classification uses Roman numerals (I, II, III, and IV) and is primarily used to describe sensory input from muscles and joints (proprioception and mechanoreception). * **Why Option A is correct:** Unlike the Erlanger-Gasser classification (which uses letters like A, B, and C), the Lloyd classification applies **exclusively to sensory fibers**. For example, Type Ia fibers originate from muscle spindle primary endings, and Type Ib fibers originate from Golgi tendon organs. * **Why Options B and C are incorrect:** Motor (efferent) fibers are categorized only under the **Erlanger-Gasser classification** (e.g., Alpha-motor neurons are Aα; Gamma-motor neurons are Aγ). There is no "Type I" or "Type II" motor neuron in the Lloyd system. * **Why Option D is incorrect:** The classification is a fundamental physiological tool used to describe the speed and size of sensory neurons. **High-Yield Facts for NEET-PG:** 1. **Correlation Table:** * **Type Ia/Ib:** Corresponds to **Aα** (Fastest, largest, myelinated). * **Type II:** Corresponds to **Aβ** (Touch, pressure). * **Type III:** Corresponds to **Aδ** (Fast pain, temperature). * **Type IV:** Corresponds to **C fibers** (Slow pain, unmyelinated, smallest). 2. **Clinical Pearl:** Type IV (C fibers) are the only unmyelinated fibers in the body and are the first to be blocked by local anesthetics but the last to be affected by pressure/hypoxia. 3. **Memory Aid:** **L**loyd is for **L**earning what we feel (**Sensory**).

Question 62: The rate of skeletal muscle relaxation is related to which of the following?

A. The rate at which free Ca++ is removed from the sarcoplasm (Correct Answer)
B. The rate at which phosphocreatine is metabolized
C. The rate of ATP hydrolysis
D. The rate of acetylcholine resynthesis

Explanation: **Explanation** The correct answer is **A. The rate at which free Ca++ is removed from the sarcoplasm.** **Why Option A is Correct:** Skeletal muscle relaxation is an active process that begins when motor neuron stimulation ceases. For relaxation to occur, the concentration of free intracellular calcium ($Ca^{2+}$) must decrease. This is primarily achieved by the **SERCA pump** (Sarcoplasmic/Endoplasmic Reticulum $Ca^{2+}$ ATPase), which actively transports $Ca^{2+}$ from the sarcoplasm back into the sarcoplasmic reticulum (SR). As $Ca^{2+}$ levels drop, it dissociates from Troponin C, allowing the tropomyosin-troponin complex to re-cover the myosin-binding sites on actin, thereby terminating the cross-bridge cycle. Therefore, the speed of relaxation is directly proportional to the rate of $Ca^{2+}$ sequestration. **Why the Other Options are Incorrect:** * **Option B:** Phosphocreatine metabolism is involved in the rapid regeneration of ATP during the initial stages of muscle contraction, not the termination of the process. * **Option C:** The rate of ATP hydrolysis (by Myosin ATPase) determines the **velocity of contraction** (shortening), not the rate of relaxation. * **Option D:** Acetylcholine (ACh) is degraded by Acetylcholinesterase in the synaptic cleft to stop the stimulus; its resynthesis occurs in the nerve terminal and does not limit the rate of muscle fiber relaxation. **High-Yield NEET-PG Pearls:** * **Rigor Mortis:** Occurs because ATP is required for the *detachment* of myosin heads from actin. Without ATP, the muscle remains in a rigid, contracted state. * **Calsequestrin:** A protein within the SR that binds to $Ca^{2+}$, allowing the SR to store large amounts of calcium at low osmotic pressure. * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine Receptor (RyR1)**, leading to excessive $Ca^{2+}$ release and sustained muscle contraction.

Question 63: Myelin in the Central Nervous System (CNS) is formed by which type of glial cell?

A. Oligodendrocytes (Correct Answer)
B. Schwann cells
C. Microglia
D. Astrocytes

Explanation: **Explanation:** The correct answer is **Oligodendrocytes**. Myelination is a process where specialized glial cells wrap their plasma membranes around axons to provide electrical insulation, increasing the speed of impulse conduction via saltatory conduction. 1. **Why Oligodendrocytes are correct:** In the **Central Nervous System (CNS)**—which includes the brain and spinal cord—myelin is produced by oligodendrocytes. A key characteristic of these cells is that a single oligodendrocyte can extend multiple processes to myelinate segments of **several different axons** (up to 50). 2. **Why other options are incorrect:** * **Schwann cells:** These cells form myelin in the **Peripheral Nervous System (PNS)**. Unlike oligodendrocytes, one Schwann cell myelinates only a **single segment of one axon**. * **Microglia:** These are the resident macrophages of the CNS. They are derived from the mesoderm and function in immune defense and phagocytosis, not myelination. * **Astrocytes:** These are star-shaped cells that form the **Blood-Brain Barrier (BBB)**, maintain the extracellular ionic environment (especially $K^+$ buffering), and provide structural support. **High-Yield Clinical Pearls for NEET-PG:** * **Multiple Sclerosis (MS):** An autoimmune demyelinating disease specifically affecting the **CNS** (target: Oligodendrocytes). * **Guillain-Barré Syndrome (GBS):** An acute inflammatory demyelinating polyneuropathy affecting the **PNS** (target: Schwann cells). * **Origin:** Most glial cells (Astrocytes, Oligodendrocytes) are **ectodermal** in origin, whereas Microglia are **mesodermal**. * **Friedreich’s Ataxia:** Often involves both central and peripheral demyelination.

Question 64: Which of the following are force-generating proteins?

A. Myosin and myoglobin
B. Dynein and kinesin (Correct Answer)
C. Calmodulin and G protein
D. Troponin

Explanation: **Explanation:** The question focuses on **molecular motors**, which are specialized proteins that convert chemical energy (ATP) into mechanical work to generate force and movement within cells. **1. Why Dynein and Kinesin are correct:** Dynein and kinesin are the primary force-generating motor proteins associated with **microtubules**. * **Kinesin** moves cargo (vesicles/organelles) toward the (+) end of the microtubule (**anterograde transport**). * **Dynein** moves cargo toward the (-) end (**retrograde transport**) and is also responsible for the beating motion of cilia and flagella. * *Note:* While **Myosin** is also a force-generating protein (associated with actin), it is paired incorrectly in Option A. **2. Why other options are incorrect:** * **Option A:** Myosin is a motor protein, but **Myoglobin** is an oxygen-binding protein found in muscle sarcoplasm; it does not generate force. * **Option C:** **Calmodulin** is a calcium-binding messenger protein (crucial for smooth muscle contraction), and **G proteins** are molecular switches involved in signal transduction. Neither acts as a motor protein. * **Option D:** **Troponin** is a regulatory protein complex (consisting of T, I, and C subunits) that controls the interaction between actin and myosin. It does not generate force itself. **High-Yield Clinical Pearls for NEET-PG:** * **Kartagener Syndrome:** Caused by a defect in **Dynein arms**, leading to immotile cilia, bronchiectasis, and situs inversus. * **Axonal Transport:** Fast anterograde transport (kinesin-mediated) is used by viruses like **Herpes Simplex**, while retrograde transport (dynein-mediated) is used by the **Rabies virus** and **Tetanus toxin** to reach the CNS. * **Myosin II** is the specific isoform responsible for skeletal muscle contraction.

Question 65: Resting membrane potential is close to the equilibrium potential of which ion?

A. Sodium (Na+)
B. Chloride (Cl-) (Correct Answer)
C. Potassium (K+)
D. Magnesium (Mg2+)

Explanation: ### Explanation The **Resting Membrane Potential (RMP)** of a cell is determined by the relative permeability of the membrane to specific ions and their respective equilibrium potentials (calculated by the Nernst equation). **Why Chloride (Cl-) is the correct answer:** In most neurons and muscle cells, the RMP typically ranges between **-70 mV to -90 mV**. The equilibrium potential for **Chloride ($E_{Cl}$)** is approximately **-70 mV to -80 mV**. Because the cell membrane at rest is highly permeable to Chloride and its equilibrium potential aligns almost exactly with the measured RMP, it is considered the ion whose equilibrium potential is closest to the RMP. **Analysis of Incorrect Options:** * **Potassium ($K^+$):** While $K^+$ is the *primary* determinant of RMP because the membrane is most permeable to it at rest, its equilibrium potential ($E_K$) is approximately **-94 mV**. While close, $E_{Cl}$ is numerically closer to the actual RMP of -70 mV. * **Sodium ($Na^+$):** The equilibrium potential for Sodium ($E_{Na}$) is approximately **+60 mV**. This is far from the RMP because the membrane has very low permeability to $Na^+$ at rest. * **Magnesium ($Mg^{2+}$):** Magnesium is primarily an intracellular cation and does not play a significant role in determining the RMP of excitable tissues. **High-Yield NEET-PG Pearls:** 1. **Goldman-Hodgkin-Katz Equation:** Unlike the Nernst equation (one ion), this equation calculates RMP by considering the permeability and concentration gradients of all major ions ($Na^+$, $K^+$, and $Cl^-$). 2. **The $Na^+$-$K^+$ Pump:** This pump is "electrogenic"; it contributes about **-4 to -10 mV** directly to the RMP by pumping 3 $Na^+$ out for every 2 $K^+$ in. 3. **Gibbs-Donnan Effect:** The presence of non-diffusible intracellular proteins (anions) influences the distribution of $Cl^-$ and $K^+$, helping establish the RMP.

Question 66: Which of the following nerve fibres serve as preganglionic autonomic fibres?

A. A (A Alpha)
B. A Gamma
C. B (Correct Answer)
D. C

Explanation: **Explanation:** The classification of nerve fibers (Erlanger-Gasser classification) is a high-yield topic for NEET-PG. Nerve fibers are categorized based on their diameter, myelination, and conduction velocity. **Why Option C (B fibers) is correct:** **Type B fibers** are characterized as medium-diameter, **myelinated** axons. In the human body, these fibers specifically serve as the **preganglionic autonomic fibers** (both sympathetic and parasympathetic). They have a slower conduction velocity (3–15 m/s) compared to Type A fibers but are faster than Type C fibers. **Analysis of Incorrect Options:** * **Option A (A-Alpha):** These are the thickest and fastest myelinated fibers. They function as somatic motor fibers (to extrafusal muscle fibers) and carry proprioception (from muscle spindles and Golgi tendon organs). * **Option B (A-Gamma):** These myelinated fibers supply the **intrafusal fibers** of the muscle spindle, regulating muscle tone. * **Option D (C fibers):** These are small-diameter, **unmyelinated** fibers. They serve as **postganglionic autonomic fibers** and also carry slow pain, temperature, and crude touch sensations. **High-Yield Facts for NEET-PG:** * **Preganglionic = B fibers** (Myelinated). * **Postganglionic = C fibers** (Unmyelinated). * **Sensitivity to Local Anesthetics:** Type C fibers are the most sensitive, while Type A-Alpha are the least sensitive. * **Sensitivity to Pressure:** Type A fibers are most sensitive. * **Sensitivity to Hypoxia:** Type B fibers are most sensitive.

Question 67: How is an electrical synapse different from a chemical synapse?

A. There is no cytoplasmic continuity between the presynaptic and postsynaptic neuron.
B. Connexons are involved in electrical synapses. (Correct Answer)
C. The direction of impulse transmission is unidirectional.
D. Electrical synapses have a longer latent period than chemical synapses.

Explanation: ### Explanation **1. Why the correct answer is right:** Electrical synapses are characterized by the presence of **Gap Junctions**, which provide direct **cytoplasmic continuity** between the presynaptic and postsynaptic neurons. These gap junctions are formed by clusters of transmembrane protein channels called **Connexons**. Each connexon is composed of six subunits called **connexins**. This physical bridge allows ions to flow directly from one cell to another, ensuring rapid signal transmission. **2. Why the incorrect options are wrong:** * **Option A:** In electrical synapses, there **is** cytoplasmic continuity via gap junctions. Chemical synapses, however, have a distinct synaptic cleft (20–40 nm) with no physical continuity. * **Option C:** Electrical synapses are typically **bidirectional**, allowing signals to flow in either direction. Chemical synapses are strictly **unidirectional** (one-way) due to the release of neurotransmitters from the presynaptic terminal to the postsynaptic receptors. * **Option D:** Electrical synapses have **virtually no synaptic delay** (latent period), making them much faster than chemical synapses. Chemical synapses have a delay (approx. 0.5 ms) due to the time required for neurotransmitter release, diffusion, and receptor binding. ### NEET-PG High-Yield Pearls: * **Location:** Electrical synapses are common in the **cardiac muscle** (intercalated discs), **smooth muscle** (unitary type), and specific brain regions like the **inferior olive**. * **Function:** They are essential for **synchronizing** the activity of a group of neurons or muscle fibers (e.g., "all-or-none" contraction of the heart). * **Comparison:** Chemical synapses are the most common type in the human CNS and allow for **signal amplification** and **plasticity**, which electrical synapses lack.

Question 68: Which efferent nerve fibers are responsible for regulating muscle tone?

A. Alpha neuron
B. Gamma neuron (Correct Answer)
C. Beta neuron
D. Delta neuron

Explanation: **Explanation:** The regulation of muscle tone is primarily mediated by the **Gamma (γ) motor neurons**. Muscle tone is defined as the continuous and passive partial contraction of the muscles, which is maintained by the **stretch reflex arc**. 1. **Why Gamma neuron is correct:** Gamma motor neurons innervate the **intrafusal muscle fibers** of the muscle spindle. When these neurons fire, they cause the ends of the intrafusal fibers to contract, stretching the central sensory portion. This increases the sensitivity of the muscle spindle to stretch. By adjusting the "set-point" of the spindle, gamma neurons maintain a constant level of alpha motor neuron activity, thereby regulating **muscle tone** and posture. 2. **Why other options are incorrect:** * **Alpha (α) neurons:** These are the largest lower motor neurons that innervate **extrafusal muscle fibers**. They are responsible for actual muscle contraction and voluntary movement, rather than the regulatory "tone." * **Beta (β) neurons:** These are less common fibers that innervate both intrafusal and extrafusal fibers (co-activation), but they are not the primary regulators of tone. * **Delta (δ) neurons:** This term is typically used in the context of sensory fibers (A-delta), which carry fast pain and temperature sensations, not motor efferents. **High-Yield Clinical Pearls for NEET-PG:** * **Alpha-Gamma Co-activation:** During voluntary movement, both neurons fire simultaneously to ensure the muscle spindle remains sensitive even when the muscle shortens. * **Gamma Loop:** Consists of Gamma efferent → Muscle spindle → Ia afferent → Alpha motor neuron → Extrafusal muscle contraction. * **Clinical Correlation:** Lesions of the upper motor neuron (UMN) lead to **hypertonicity** (spasticity) due to overactivity of the gamma motor system.

Question 69: A 12-year-old girl fractures her right radius and ulna. Her arm is placed in a cast extending over the elbow, which nearly eliminates movement of her forearm muscles. Which of the following will occur in her forearm muscles after several weeks in the cast?

A. Decreased capillarity (Correct Answer)
B. Decreased intracellular pO2
C. Decreased myoglobin oxygen saturation
D. Increased metabolic rate

Explanation: ### Explanation The correct answer is **A. Decreased capillarity**. **1. Why it is correct:** This scenario describes **disuse atrophy**. When a limb is immobilized in a cast, the metabolic demand of the skeletal muscles drops significantly. Skeletal muscle is highly plastic; it adjusts its vascular supply based on demand. In the absence of regular contraction, the muscle requires less oxygen and nutrients, leading to a reduction in the number of functional capillaries per muscle fiber (decreased capillarity). This is a physiological adaptation to prevent "luxury perfusion" in inactive tissue. **2. Why the other options are wrong:** * **B & C (Decreased intracellular pO2 and Myoglobin saturation):** These occur during **acute exercise** when oxygen consumption exceeds delivery. In an immobilized limb, the metabolic rate is so low that the remaining blood flow is more than sufficient to maintain normal intracellular $pO_2$ and keep myoglobin fully saturated. * **D (Increased metabolic rate):** Disuse leads to a **decrease** in metabolic rate. The muscle fibers (especially Type I slow-twitch fibers) shrink in size (atrophy), and there is a downregulation of mitochondrial enzymes and glycolytic activity. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Atrophy Mechanism:** Disuse atrophy primarily involves the **Ubiquitin-Proteasome pathway**. Muscle proteins are tagged with ubiquitin and degraded by the 26S proteasome. * **Fiber Type Shift:** Prolonged immobilization often leads to a shift from **Type I (slow-oxidative)** to **Type II (fast-glycolytic)** characteristics, as oxidative capacity drops faster than glycolytic capacity. * **Opposite Effect:** Chronic endurance exercise (aerobic training) leads to the opposite effect: **increased capillarity** and increased mitochondrial density (angiogenesis mediated by VEGF). * **Key Histological Change:** In disuse atrophy, there is a decrease in the cross-sectional area of muscle fibers, but the **number of fibers** usually remains constant initially.

Question 70: The Golgi tendon apparatus conveys information to the central nervous system regarding which parameter?

A. Tension in the muscle (Correct Answer)
B. Length of the muscle
C. Rapidity of muscle contraction
D. Blood supply to the muscle

Explanation: ### Explanation **1. Why Option A is Correct:** The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons, arranged **in series** with the muscle fibers. Because of this anatomical arrangement, any force generated by muscle contraction or passive stretching pulls on the collagen fibers of the GTO. This mechanical deformation stimulates Ib afferent fibers, which transmit information regarding **muscle tension** to the spinal cord. Its primary role is to prevent excessive tension that could damage the muscle or tendon (Autogenic Inhibition). **2. Why Other Options are Incorrect:** * **Option B (Length of the muscle):** This is the primary function of the **Muscle Spindle**, which is arranged **in parallel** with extrafusal fibers. Muscle spindles detect changes in muscle length and the rate of change in length. * **Option C (Rapidity of muscle contraction):** While GTOs can respond to the rate of tension development, the "rapidity of contraction" (velocity of shortening) is primarily sensed by **Nuclear Bag fibers** within the muscle spindle. * **Option D (Blood supply):** Blood supply is monitored by metabolic receptors (chemoreceptors) and baroreceptors, not by specialized mechanoreceptors like the GTO. **3. High-Yield Clinical Pearls for NEET-PG:** * **Arrangement:** Muscle Spindle = Parallel; Golgi Tendon Organ = Series. * **Afferent Nerve Fibers:** Muscle Spindle (Type Ia and II); GTO (Type Ib). * **Inverse Stretch Reflex:** The GTO mediates this reflex. When tension is too high, the GTO inhibits the alpha motor neuron of the agonist muscle and excites the antagonist, causing the muscle to relax. * **Clasp-Knife Response:** In upper motor neuron (UMN) lesions, the sudden relaxation of a spastic muscle under tension is partly attributed to the activation of the Golgi tendon reflex.

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

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Electrophysiological Measurements

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