Nerve and Muscle Physiology Practice Questions

Q: A mutation in the gene coding for ryanodine receptors is associated with malignant hyperthermia. Which of the following statements best explains the increased heat production in malignant hyperthermia?

Increased muscle metabolism due to excess calcium ions. **Explanation** **Why Option A is correct:** Malignant Hyperthermia (MH) is a pharmacogenetic disorder triggered by volatile anesthetics (e.g., halothane) or succinylcholine. The underlying pathology involves a mutation in the **RYR1 gene**, which codes for the **Ryanodine Receptor** on the sarcoplasmic reticulum of skeletal muscle. In MH, the mutated receptor remains stuck in an open state, leading to a massive, uncontrolled release of **calcium ions (Ca²⁺)** into the cytosol. This excess calcium causes: 1. **Persistent muscle contraction (rigidity):** Continuous cross-bridge cycling. 2. **Hypermetabolism:** The plasma membrane and sarcoplasmic reticulum pumps (SERCA) work over-time to sequester the excess calcium, consuming massive amounts of **ATP**. This accelerated metabolic activity generates excessive heat, CO₂, and lactic acid, leading to hyperthermia and metabolic acidosis. **Why the other options are incorrect:** * **Option B:** The thermic effect of food refers to the energy expenditure required for digestion and absorption; it plays no role in anesthetic-induced hyperthermia. * **Option C:** While sympathetic overactivity (tachycardia, hypertension) occurs as a *secondary* response to the metabolic crisis in MH, it is not the primary source of heat production. * **Option D:** Mitochondrial thermogenesis (via uncoupling proteins) is the primary mechanism in **brown adipose tissue** (non-shivering thermogenesis), not the mechanism behind MH. **Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Dominant. * **Earliest Sign:** Increase in end-tidal CO₂ (ETCO₂). * **Classic Sign:** Masseter muscle rigidity. * **Drug of Choice:** **Dantrolene** (acts by binding to the RYR1 receptor and inhibiting calcium release). * **Associated Conditions:** Central Core Disease and King-Denborough Syndrome.

Q: What is the effect of axon thickening on nerve impulse conduction speed?

Increased speed of conduction. **Explanation:** The speed of nerve impulse conduction is directly proportional to the diameter of the axon. When an axon thickens (increases in diameter), the **internal longitudinal resistance** to the flow of ions decreases significantly. According to Ohm’s Law, lower resistance allows local current circuits to flow more rapidly and over longer distances, leading to a faster rate of depolarization in adjacent segments of the axonal membrane. In myelinated fibers, thicker axons also have longer internodal distances, allowing the action potential to "jump" (saltatory conduction) across larger gaps, further increasing velocity. **Analysis of Options:** * **Option A (Correct):** Increased diameter reduces internal resistance, facilitating faster current flow and higher conduction velocity. * **Option B:** Decreased speed is seen in axonal tapering, demyelinating diseases (like Multiple Sclerosis), or cooling of the nerve. * **Option C:** The absolute refractory period is determined by the inactivation gate of voltage-gated sodium channels, not by the physical thickness of the axon. * **Option D:** Unmyelination is a structural state (lack of Schwann cells/Oligodendrocytes) and is not a direct result of axonal thickening; in fact, thicker fibers in the body are typically myelinated. **NEET-PG High-Yield Pearls:** * **Erlanger-Gasser Classification:** Type A-alpha fibers are the thickest (12–20 µm) and fastest (70–120 m/s), while Type C fibers are the thinnest and slowest. * **Relationship:** Conduction velocity (m/s) in myelinated fibers is roughly **6 × diameter (µm)**. * **Factors increasing velocity:** Myelination, increased diameter, and increased temperature.

Q: Dynamic response is due to which type of muscle ending?

Primary ending. **Explanation:** The muscle spindle is a complex sensory organ that monitors muscle length and the rate of change in length. It contains two types of intrafusal fibers: **Nuclear Bag fibers** and **Nuclear Chain fibers**. 1. **Why Primary Ending is Correct:** Primary sensory endings (also known as **Type Ia fibers**) wrap around the central portions of both nuclear bag and nuclear chain fibers. Because nuclear bag fibers are highly sensitive to the *velocity* of stretch, the primary endings provide a **dynamic response**. This means they fire rapidly during the actual movement or change in length, signaling how fast the muscle is being stretched. 2. **Why Other Options are Incorrect:** * **Secondary Ending (Type II fibers):** These are located primarily on nuclear chain fibers. They are responsible for the **static response**, meaning they fire at a constant rate proportional to the *absolute length* of the muscle, rather than the speed of change. * **Tertiary Ending:** This term is not standard in muscle spindle physiology. Sensory innervation is categorized into Primary (Ia) and Secondary (II) endings. * **All of the above:** Incorrect because dynamic and static responses are mediated by distinct fiber types with different physiological properties. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Endings (Ia):** Responsible for the **monosynaptic stretch reflex** (e.g., Knee jerk). They are "velocity sensors." * **Secondary Endings (II):** Responsible for "position sensors." * **Gamma Motor Neurons:** Maintain spindle sensitivity during muscle contraction. **Gamma-dynamic** neurons affect bag fibers (dynamic response), while **Gamma-static** neurons affect chain fibers (static response). * **Golgi Tendon Organ (Ib):** Unlike spindles, these sense **muscle tension** and are arranged in series with extrafusal fibers.

Q: Where are calcium ions stored in skeletal muscles?

Terminal cistern. **Explanation:** In skeletal muscle, the **Sarcoplasmic Reticulum (SR)** is a specialized intracellular organelle dedicated to calcium homeostasis. The SR is divided into two functional parts: the longitudinal tubules and the **terminal cisternae**. 1. **Why Terminal Cisternae is correct:** The terminal cisternae are enlarged sacs of the SR located at the junction of the A and I bands. They serve as the primary storage site for calcium ions ($Ca^{2+}$). Within these sacs, calcium is bound to a protein called **calsequestrin**, which allows for high-capacity storage. When an action potential arrives, $Ca^{2+}$ is released from the terminal cisternae into the sarcoplasm via Ryanodine receptors (RyR1) to initiate contraction. 2. **Why other options are incorrect:** * **Transverse (T) tubule:** These are invaginations of the sarcolemma that conduct the action potential deep into the muscle fiber. They contain extracellular fluid, not stored calcium. * **Longitudinal canal:** This part of the SR connects the terminal cisternae and is primarily involved in the **reuptake** of calcium from the sarcoplasm via SERCA pumps, rather than long-term storage. * **Sarcolemma:** This is the plasma membrane of the muscle cell. While it contains calcium channels, it does not store the ions. **High-Yield NEET-PG Pearls:** * **The Triad:** In skeletal muscle, a triad consists of one T-tubule flanked by two terminal cisternae. (Note: Cardiac muscle has *diads* located at the Z-line). * **Calsequestrin:** The major calcium-binding protein in skeletal muscle (Calreticulin is the equivalent in non-muscle cells). * **Malignant Hyperthermia:** Caused by a mutation in the **Ryanodine receptor (RyR1)**, leading to excessive calcium release from the terminal cisternae.

Q: Muscle tone is maintained by?

Muscle spindle. **Explanation:** Muscle tone is defined as the continuous and passive partial contraction of the muscles, or the muscle's resistance to passive stretch during resting state. It is primarily maintained by the **Stretch Reflex** (Myotatic Reflex). **Why Muscle Spindle is Correct:** The muscle spindle acts as the sensory receptor for the stretch reflex. When a muscle is passively stretched, the muscle spindles (located in parallel with extrafusal fibers) are activated. They send afferent impulses via **Type Ia fibers** to the spinal cord, where they synapse directly with **Alpha Motor Neurons**. This results in a compensatory contraction of the same muscle, thereby maintaining tone. **Analysis of Incorrect Options:** * **Golgi Tendon Organ (GTO):** These are located in series with muscle fibers and respond to **muscle tension** (contraction), not stretch. They mediate the *Inverse Stretch Reflex*, which causes muscle relaxation to prevent injury from excessive force. * **Renshaw Cells:** These are inhibitory interneurons in the anterior horn of the spinal cord. They provide **recurrent inhibition** to alpha motor neurons to prevent over-activity and "sharpen" motor signals; they do not maintain tone. **High-Yield Clinical Pearls for NEET-PG:** * **Gamma Motor Neurons:** These innervate the intrafusal fibers of the muscle spindle. They regulate the sensitivity of the spindle, thereby "setting" the level of muscle tone. * **Supraspinal Control:** The Pontine Reticular Formation and Vestibular nuclei facilitate tone, while the Medullary Reticular Formation inhibits it. * **Clinical Correlation:** Lesions of the Upper Motor Neuron (UMN) lead to **Hypertonia** (Spasticity), while Lower Motor Neuron (LMN) lesions lead to **Hypotonia** (Flaccidity).

Q: What are the key molecules involved in the differences between skeletal and smooth muscle contraction and relaxation?

Troponin and Myosin. The fundamental difference between skeletal and smooth muscle contraction lies in the **regulatory proteins** and the **state of the myosin head**. ### 1. Why "Troponin and Myosin" is Correct * **Skeletal Muscle (Troponin-regulated):** Contraction is "actin-linked." Calcium binds to **Troponin C**, causing a conformational change in tropomyosin that uncovers the active sites on actin. * **Smooth Muscle (Myosin-regulated):** Contraction is "myosin-linked." Smooth muscle lacks troponin. Instead, Calcium binds to **Calmodulin**, which activates **Myosin Light Chain Kinase (MLCK)**. This enzyme phosphorylates the myosin head, enabling it to bind to actin. * **Relaxation:** In skeletal muscle, relaxation occurs via simple calcium sequestration. In smooth muscle, it requires **Myosin Light Chain Phosphatase** to dephosphorylate the myosin head. ### 2. Analysis of Incorrect Options * **Option A & D:** While MLCK is vital for smooth muscle, these options are incomplete. They focus only on the activation enzyme or ions, ignoring the structural protein differences (Troponin vs. Myosin state) that define the two systems. * **Option B:** Troponin is exclusive to skeletal and cardiac muscle; it does not explain the mechanism in smooth muscle. ### 3. High-Yield NEET-PG Pearls * **Calmodulin** in smooth muscle is functionally analogous to **Troponin C** in skeletal muscle. * **Latch-bridge mechanism:** Unique to smooth muscle; allows for prolonged tone with minimal ATP consumption. * **Caveolae:** Smooth muscle lacks a well-developed T-tubule system; instead, it uses these membrane invaginations. * **Multi-unit vs. Unitary:** Unitary (visceral) smooth muscle uses **gap junctions** for syncytial contraction (e.g., GI tract, uterus).

Q: In a neuron, where does graded electrogenesis occur?

Soma - dendritic zone. **Explanation:** **1. Why the Correct Answer is Right:** Graded electrogenesis refers to the production of **graded potentials** (local changes in membrane potential) that vary in amplitude depending on the strength of the stimulus. In a neuron, the **soma-dendritic zone** is the primary receptive area. It contains a high density of ligand-gated and modality-gated ion channels. When neurotransmitters bind to receptors here, they produce Excitatory Post-Synaptic Potentials (EPSPs) or Inhibitory Post-Synaptic Potentials (IPSPs). These potentials are non-propagated, decremental (fade over distance), and can be summed—the hallmarks of graded electrogenesis. **2. Why the Other Options are Incorrect:** * **Initial Segment:** This is the "Trigger Zone." It has the highest density of voltage-gated $Na^+$ channels and the lowest threshold for firing. Its role is to convert graded potentials into all-or-none **Action Potentials**, not to generate graded potentials. * **Axon:** The axon is specialized for the **conduction** of action potentials via saltatory or continuous conduction. It follows the "All-or-None" law, which is the opposite of graded electrogenesis. * **Nerve Ending:** This zone is specialized for **secretion** (neurotransmitter release) via voltage-gated $Ca^{2+}$ channels. While some local potentials can occur, it is not the primary site for electrogenesis. **3. High-Yield Facts for NEET-PG:** * **Summation:** Graded potentials can be **Temporal** (high frequency from one source) or **Spatial** (multiple sources). Action potentials cannot summate due to the refractory period. * **Threshold:** The initial segment (Axon Hillock) is the most excitable part of the neuron because it requires the least amount of depolarization to reach the threshold. * **Comparison:** Unlike action potentials, graded potentials have **no refractory period**.

Q: In a sarcomere, which band or zone is NOT characterized by the presence of actin filaments?

H band. **Explanation:** The sarcomere is the functional unit of skeletal muscle, extending from one Z-line to the next. Its appearance is defined by the arrangement of thick (myosin) and thin (actin) filaments. **Why the H band is correct:** The **H band** (from German *heller*, meaning brighter) is the central region of the A band. It contains **only thick (myosin) filaments**. During muscle contraction, actin filaments slide over myosin toward the center, narrowing the H band, but at rest, it is characterized specifically by the absence of actin. **Analysis of incorrect options:** * **I band:** This is the isotropic band which contains **only thin (actin) filaments**. It spans across two adjacent sarcomeres, bisected by the Z disc. * **M band:** Located in the dead center of the H zone, the M line (M band) contains proteins (like myomesin) that anchor the thick filaments. While it is within the H zone, the question asks for the "band or zone" defined by the absence of actin; the H zone is the primary anatomical region meeting this criteria. * **Z band (Z disc):** This is the boundary of the sarcomere where **actin filaments are anchored** via the protein alpha-actinin. **High-Yield NEET-PG Pearls:** 1. **"A" is for All:** The **A band** (Anisotropic) contains the entire length of the thick filament, including the area where actin and myosin overlap. Its length **remains constant** during contraction. 2. **Contraction Dynamics:** During contraction, the **H zone and I band shorten**, while the Z lines move closer together. 3. **Titans of the Sarcomere:** **Titin** is the largest protein in the body; it acts as a spring, connecting the Z disc to the M line, providing passive elasticity.

Q: Which of the following is NOT true about the tendon organ?

Contracting. The **Golgi Tendon Organ (GTO)** is a high-threshold mechanoreceptor located in the tendons of skeletal muscles. It functions as a tension sensor, monitoring the force of muscle contraction. ### Why "Contracting" is the Correct Answer The GTO is stimulated by **tension**, not by the act of contraction itself (though contraction creates tension). When tension becomes excessive, the GTO triggers the **Inverse Stretch Reflex** (Autogenic Inhibition). This reflex causes the muscle to **relax** rather than contract. Therefore, "Contracting" is the opposite of its physiological effect. ### Analysis of Incorrect Options * **A. Inhibitory:** The GTO sends impulses via **Ib afferent fibers** to the spinal cord, where they synapse with inhibitory interneurons. These interneurons inhibit the alpha motor neurons of the agonist muscle, leading to relaxation. * **B. Negative Feedback:** It operates on a negative feedback loop to regulate muscle tension. If tension increases, the GTO reduces motor output to prevent injury, maintaining tension within a physiological range. * **C. Protective:** Its primary role is to prevent tendon avulsion or muscle tearing during extreme exertion by forcing the muscle to "give way" (the **Clasp-knife response**). ### NEET-PG High-Yield Pearls * **Afferent Fiber:** Ib (Fast-conducting). * **Reflex:** Inverse Stretch Reflex (Disynaptic). * **GTO vs. Muscle Spindle:** The Muscle Spindle (Ia/II fibers) responds to **length/stretch** and causes contraction; the GTO responds to **tension** and causes relaxation. * **Clinical Sign:** The **Clasp-knife phenomenon** seen in upper motor neuron lesions is mediated by the GTO.

Question 1

A mutation in the gene coding for ryanodine receptors is associated with malignant hyperthermia. Which of the following statements best explains the increased heat production in malignant hyperthermia?

Accepted Answer

Increased muscle metabolism due to excess calcium ions

Answer

Thermic effect of food

Answer

Increased sympathetic discharge

Answer

Mitochondrial thermogenesis

Question 2

What is the effect of axon thickening on nerve impulse conduction speed?

Accepted Answer

Increased speed of conduction

Answer

Decreased speed of conduction

Answer

Increased absolute refractory period

Answer

Unmyelination

Question 3

Dynamic response is due to which type of muscle ending?

Accepted Answer

Primary ending

Answer

Secondary ending

Answer

Tertiary ending

Answer

All of the above

Question 4

What is the primary difference between skeletal and smooth muscle contraction and relaxation?

Accepted Answer

Troponin is present in skeletal muscle but absent in smooth muscle.

Answer

Myosin light chain kinase is involved in smooth muscle contraction.

Answer

Calcium ion regulation differs between skeletal and smooth muscle.

Answer

Actin filaments are structured differently in skeletal and smooth muscle.

Question 5

Where are calcium ions stored in skeletal muscles?

Accepted Answer

Terminal cistern

Answer

Transverse tubule

Answer

Longitudinal canal

Answer

Sarcolemma

Question 6

Muscle tone is maintained by?

Accepted Answer

Muscle spindle

Answer

Golgi tendon organ

Answer

Renshaw cells

Answer

None of the above

Question 7

What are the key molecules involved in the differences between skeletal and smooth muscle contraction and relaxation?

Accepted Answer

Troponin and Myosin

Answer

Troponin and Myosin light chain kinase

Answer

Troponin

Answer

Myosin light chain kinase and Calcium ions

Question 8

In a neuron, where does graded electrogenesis occur?

Accepted Answer

Soma - dendritic zone

Answer

Initial segment

Answer

Axon

Answer

Nerve ending

Question 9

In a sarcomere, which band or zone is NOT characterized by the presence of actin filaments?

Accepted Answer

H band

Answer

I band

Answer

M band

Answer

Z band

Question 10

Which of the following is NOT true about the tendon organ?

Accepted Answer

Contracting

Answer

Inhibitory

Answer

Negative feedback

Answer

Protective

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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