Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs

Question 441: Which of the following statements is NOT true regarding red muscle fibers?

A. Decreased glycolytic enzymes
B. Used for aerobic activity
C. Increased blood flow
D. Increased muscle fiber length (Correct Answer)

Explanation: ***Increased muscle fiber length*** - The length of muscle fibers is generally determined by the muscle's anatomical structure and function, not by whether they are red or white fibers. - While red muscle fibers (slow-twitch) are optimized for **endurance** and **sustained contractions**, this optimization does not involve an inherent increase in the length of individual muscle fibers. *Decreased glycolytic enzymes* - Red muscle fibers, also known as slow-oxidative fibers, primarily rely on **aerobic metabolism** for ATP production. - They have a lower content of glycolytic enzymes compared to white muscle fibers, which are specialized for **anaerobic glycolysis**. *Increased blood flow* - Red muscle fibers have a rich capillary supply, leading to **increased blood flow**, which is essential for delivering oxygen and nutrients for sustained aerobic activity. - This extensive vascularization contributes to their characteristic red appearance and their ability to resist fatigue. *Used for aerobic activity* - Red muscle fibers are well-suited for prolonged, low-intensity activities due to their high concentration of **mitochondria**, myoglobin, and oxidative enzymes. - They are primarily responsible for maintaining posture and performing **endurance activities** such as long-distance running.

Question 442: Which of the following proteins is primarily responsible for muscle relaxation?

A. SERCA (Sarcoplasmic Reticulum Ca2+ ATPase) (Correct Answer)
B. Both Actin and SERCA
C. Actin
D. None of the options

Explanation: ***SERCA (Sarcoplasmic Reticulum Ca2+ ATPase)*** - **SERCA** pumps calcium ions from the **cytosol** back into the **sarcoplasmic reticulum**, reducing cytosolic calcium levels and allowing muscle relaxation. - This active transport of **Ca2+** away from the contractile proteins (actin and myosin) is essential for stopping muscle contraction. *Both Actin and SERCA* - While **SERCA** is critical for relaxation, **actin** is a structural component of the thin filament involved in muscle contraction, not relaxation. - **Actin** interacts with myosin during the cross-bridge cycle to shorten the sarcomere, a process reversed by **Ca2+** removal. *Actin* - **Actin** is a primary component of the **thin filaments** and is directly involved in muscle **contraction** when bound by myosin. - It does not play a direct role in the active process of muscle relaxation; its interaction with myosin ceases when **calcium** is removed. *None of the options* - This option is incorrect because **SERCA** is a specific protein with a well-defined role in muscle relaxation by actively sequestering **calcium**. - The other options either include incorrect proteins or misattribute functionality for muscle relaxation.

Question 443: Which of the following techniques is used to study current flow across a single ion channel?

A. Galvanometry
B. Voltage clamp
C. Patch clamp (Correct Answer)
D. Iontophoresis

Explanation: ***Patch clamp*** - The **patch clamp** technique allows for the direct measurement of **ion current flow** through a single ion channel or a small group of channels. - It involves isolating a small patch of cell membrane with a micropipette to record the electrical activity. *Voltage clamp* - The **voltage clamp** technique is used to maintain a constant membrane potential while measuring the **total ionic current** across the entire cell membrane. - It is not typically used for studying current across a *single* ion channel, but rather for analyzing whole-cell currents. *Iontophoresis* - **Iontophoresis** is a method used to introduce ionized substances, such as drugs or neurotransmitters, into tissues using a small electric current. - It is a technique for drug delivery or localized stimulation, not for directly measuring ion channel current. *Galvanometry* - **Galvanometry** is a general term for the measurement of small electric currents using a galvanometer. - While ion channel activity involves electric currents, galvanometry is not a specific technique for isolating and studying single ion channels.

Question 444: Primary afferent fibers secrete which nociceptive substance at the dorsal horn?

A. Substance P (Correct Answer)
B. Acetylcholine
C. Norepinephrine
D. Epinephrine

Explanation: ***Substance P*** - **Substance P** is a neuropeptide released by **C fibers** and **A-delta fibers** (primary afferent nociceptors) in the dorsal horn of the spinal cord. - It acts as a **neurotransmitter** and **neuromodulator**, contributing to the transmission and amplification of pain signals. *Acetylcholine* - **Acetylcholine** is a primary neurotransmitter in the **neuromuscular junction** and the autonomic nervous system. - While it has some roles in the CNS, it is not the primary nociceptive substance secreted by afferent fibers in the dorsal horn. *Norepinephrine* - **Norepinephrine** (noradrenaline) is a neurotransmitter involved in the **fight-or-flight response** and mood regulation. - It can modulate pain, but it is not directly released by primary afferent fibers as a nociceptive substance in the dorsal horn. *Epinephrine* - **Epinephrine** (adrenaline) is a hormone and neurotransmitter primarily associated with the **sympathetic nervous system** and stress response. - It does not serve as a direct nociceptive transmitter released by primary afferent fibers in the spinal cord.

Question 445: Excitatory postsynaptic potentials are due to

A. K+ movement into the cell
B. Na+ movement out of the cell
C. Na+ movement into the cell (Correct Answer)
D. Ca++ movement into the cell

Explanation: ***Na+ movement into the cell*** - **Excitatory postsynaptic potentials (EPSPs)** are caused by the **depolarization** of the postsynaptic membrane. - This depolarization primarily occurs when **neurotransmitters** open **ligand-gated ion channels** that allow a net influx of **positively charged sodium (Na+) ions** into the cell. *K+ movement into the cell* - **K+ influx** would make the inside of the cell more positive, but the resting membrane potential is already close to the **equilibrium potential for K+**; its movement into the cell under normal circumstances does not cause the characteristic depolarization of an EPSP. - While **Na+/K+ ATPase** pumps K+ into the cell, this process is responsible for maintaining the resting potential, not generating rapid EPSP depolarization. *Na+ movement out of the cell* - **Na+ movement out of the cell** would lead to **hyperpolarization** or repolarization, making the inside of the cell more negative, which is characteristic of an inhibitory postsynaptic potential (IPSP) or the repolarization phase of an action potential. - The **Na+/K+ ATPase** pumps Na+ out of the cell, but this is a metabolic pump, not the ion channel mechanism responsible for EPSP generation. *Ca++ movement into the cell* - While **Ca++ influx** can also be excitatory and lead to depolarization, especially in certain types of neurons or during specific synaptic events (e.g., neurotransmitter release), it is not the primary mechanism responsible for most fast **EPSPs** in the central nervous system. - The influx of **Ca++** is more commonly associated with signaling pathways, muscle contraction, and sustained depolarization rather than the rapid, transient depolarization seen in typical EPSPs from ionotropic receptors.

Question 446: What is the primary site of action of tetanospasmin in the nervous system?

A. Presynaptic terminals of the spinal cord (Correct Answer)
B. Postsynaptic terminals of the spinal cord
C. Neuromuscular junction
D. Muscle fibers

Explanation: ***Presynaptic terminals of the spinal cord*** - **Tetanospasmin** is transported via **retrograde axonal transport** to the central nervous system, specifically targeting the **presynaptic terminals** of inhibitory interneurons in the spinal cord. - It interferes with the release of **inhibitory neurotransmitters** like **GABA** and **glycine**, leading to uncontrolled muscle spasms. *Postsynaptic terminals of the spinal cord* - This is incorrect because tetanospasmin acts by preventing the release of inhibitory neurotransmitters from the **presynaptic terminal**, rather than directly affecting the postsynaptic receptor. - While the absence of inhibition is perceived at the postsynaptic terminal, the direct mechanism of action is presynaptic. *Neuromuscular junction* - This is incorrect because **tetanospasmin** does not primarily act at the neuromuscular junction; that is the site of action for toxins like **botulinum toxin**. - Tetanospasmin is transported to the central nervous system to exert its effects. *Muscle fibers* - This is incorrect as **tetanospasmin** does not act directly on muscle fibers. - Its action is on the **nervous system**, leading to altered neuronal signaling that indirectly affects muscle contraction.

Question 447: The reflex originating from the Golgi tendon organ to relax the responding muscle is a

A. Disynaptic reflex (Correct Answer)
B. Polysynaptic reflex
C. Reflex with center in medulla oblongata
D. Monosynaptic reflex

Explanation: ***Disynaptic reflex*** - The reflex arc originating from the **Golgi tendon organ** involves an afferent neuron synapsing with an inhibitory interneuron, which then synapses with the alpha motor neuron. - This two-synapse pathway (Golgi tendon organ sensory neuron to interneuron, then interneuron to alpha motor neuron) makes it a **disynaptic reflex**. *Monosynaptic reflex* - This type of reflex involves only **one synapse** between the afferent sensory neuron and the efferent motor neuron, such as the **stretch reflex**. - The Golgi tendon reflex requires an **interneuron** to inhibit the motor neuron, thus making it more complex than monosynaptic. *Polysynaptic reflex* - This term describes reflexes involving **multiple interneurons** and more than two synapses. - While the Golgi tendon reflex involves an interneuron, the primary direct inhibition of the motor neuron is achieved through a **single inhibitory interneuron**, making disynaptic a more precise description. *Reflex with center in medulla oblongata* - Reflexes centered in the **medulla oblongata** typically regulate vital functions such as breathing, heart rate, and blood pressure. - The **Golgi tendon reflex** is a spinal reflex with its neural circuitry located within the spinal cord segments associated with the involved muscle.

Question 448: A person is able to sense fast, sharp pain and temperature normally. Which type of nerve fiber is primarily involved in carrying these fast pain and temperature sensations?

A. A-delta (Correct Answer)
B. C fibers
C. A-beta
D. A-alpha

Explanation: ***A-delta*** - **A-delta fibers** are **myelinated**, medium-diameter fibers responsible for transmitting **fast, sharp pain** and **cold temperature** sensations. - Their myelination allows for **rapid conduction** of nerve impulses, leading to the immediate perception of acute pain. *A-beta* - **A-beta fibers** are large-diameter, highly myelinated fibers primarily involved in transmitting **touch** and **pressure** sensations. - While they can transmit some non-painful signals from the skin, they are not the primary carriers of sharp pain or temperature. *C fibers* - **C fibers** are **unmyelinated**, small-diameter fibers that transmit **slow, dull, burning pain** and **warm temperature** sensations. - Their lack of myelination results in **slower conduction velocity**, which is why the duller pain is perceived after the sharp pain. *A-alpha* - **A-alpha fibers** are the **largest diameter**, most heavily myelinated fibers in the periphery. - They are primarily responsible for **proprioception** (sense of body position) and **motor control** to skeletal muscles, not pain or temperature.

Question 449: Which of the following statements about smooth muscles is true?

A. Presence of Caveolae (Correct Answer)
B. Ca2+ binds to troponin C
C. The anchorage for actin filaments is given by Z-lines
D. Voltage-gated L-type Ca2+ channels are present in T-tubules

Explanation: ***Presence of Caveolae*** - **Caveolae** are small invaginations of the plasma membrane in smooth muscle cells that function in **calcium handling** and cell signaling. - They increase the surface area and contain receptors and ion channels crucial for smooth muscle contraction. *Ca2+ binds to troponin C* - In **smooth muscle**, calcium ions (Ca2+) bind to **calmodulin**, not troponin C. - **Troponin C** is the calcium-binding protein found in **striated muscle** (skeletal and cardiac muscle). *The anchorage for actin filaments is given by Z-lines* - In **smooth muscle**, **dense bodies** serve as the anchor points for **actin filaments**, analogous to Z-lines in striated muscle. - **Z-lines** are characteristic structures of **sarcomeres** in skeletal and cardiac muscle. *Voltage-gated L-type Ca2+ channels are present in T-tubules* - While L-type Ca2+ channels are present in some smooth muscle cells, their distribution in **T-tubules** is primarily characteristic of **cardiac muscle**. - **Smooth muscle** generally lacks **T-tubules**, and Ca2+ entry occurs mainly through channels on the plasma membrane or from the sarcoplasmic reticulum.

Question 450: Which of the following types of nerve fibers are primarily responsible for transmitting slow, dull, and chronic pain sensations?

A. A-alpha fibers
B. A-beta fibers
C. A-delta fibers
D. C fibers (Correct Answer)

Explanation: ***C fibers*** - These are **unmyelinated**, small-diameter nerve fibers that conduct impulses slowly (0.5-2 m/s). - They are primarily responsible for transmitting **slow, dull, burning, or aching pain** (second pain or chronic pain), as well as temperature sensations and itch. - Their slow conduction velocity results in the delayed, poorly localized pain sensation that persists after initial injury. *A-alpha fibers* - These are the **largest and fastest-conducting** myelinated nerve fibers (70-120 m/s). - They are primarily involved in transmitting **proprioception** (sense of body position) and **motor information** to skeletal muscles. - They do **not transmit pain** signals. *A-beta fibers* - These are **large, myelinated** fibers with a fast conduction velocity (30-70 m/s). - They primarily transmit **touch and pressure sensations**, and can modulate pain perception through the gate control theory. - They are **not nociceptors** and do not directly transmit pain. *A-delta fibers* - These are **small, myelinated** nerve fibers that conduct impulses at 12-30 m/s. - They transmit **fast, sharp, well-localized pain** (first pain or acute pain) and cold sensations. - While they do transmit pain, they are responsible for the **initial sharp pain**, not the slow, dull, chronic pain that defines C fiber function.

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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