Nerve and Muscle Physiology — MCQs
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Which of the following types of nerve fibers are primarily responsible for transmitting slow, dull, and chronic pain sensations?
What is the primary reason for the unidirectional nature of neuronal synaptic conduction?
In excitable cells, repolarization is closely associated with one of the following events:
Post-tetanic potentiation is due to what?
Which of the following parameters decreases from its peak after the overshoot of an action potential?
Single-unit smooth muscles are seen in which of the following?
Golgi tendon organs are innervated by which type of nerve fibre?
Holstein-Lewis fracture is associated with injury to which nerve?
Which of the following statements is true regarding type 2b muscle fibers?
Hyperpolarization is caused by which ions?
Nerve and Muscle Physiology Indian Medical PG Practice Questions and MCQs
Question 451: 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.
Question 452: What is the primary reason for the unidirectional nature of neuronal synaptic conduction?
- A. Dendrites can be depolarized and repolarized.
- B. An area can be depolarized again after repolarization.
- C. Antidromic impulses are less effective than orthodromic impulses.
- D. Chemical mediators are released from the presynaptic terminal. (Correct Answer)
Explanation: ***Chemical mediators are released from the presynaptic terminal.*** - **Neurotransmitters** are stored in vesicles within the **presynaptic terminal** and are released into the synaptic cleft only from this side. - The **postsynaptic membrane** contains specific receptors for these neurotransmitters, ensuring that the signal transmission occurs exclusively in one direction. *Dendrites can be depolarized and repolarized.* - While dendrites do undergo changes in potential, their primary role is to **receive signals**, not to initiate chemical transmission across a synapse in a backward direction. - This property alone does not explain the **unidirectional chemical synapse**, as it pertains to electrical excitability, not the chemical release mechanism. *An area can be depolarized again after repolarization.* - This statement describes the **refractory period** and the ability of a neuron to fire subsequent action potentials, which is crucial for signal propagation along an axon, but not the unidirectional nature of a **synapse**. - It does not explain why a signal cannot cross the synapse from the postsynaptic to the presynaptic neuron. *Antidromic impulses are less effective than orthodromic impulses.* - An **antidromic impulse** is one traveling in the "wrong" direction along an axon, opposite to the normal physiological direction. - While they are indeed less effective or non-physiological, this refers to **axon conduction**, not the reason for unidirectional transmission at the **synaptic cleft**.
Question 453: In excitable cells, repolarization is closely associated with one of the following events:
- A. Na+ efflux
- B. Na+ influx
- C. K+ efflux (Correct Answer)
- D. K+ influx
Explanation: ***K+ efflux*** - Repolarization in excitable cells is primarily caused by the **outward movement of potassium ions (K+)** through voltage-gated potassium channels. - This **efflux of positive charge** makes the inside of the cell more negative, returning the membrane potential to its resting state. *Na+ efflux* - **Na+ efflux** is primarily mediated by the **Na+/K+ ATPase pump**, which is crucial for maintaining the resting membrane potential but does not directly cause repolarization during an action potential. - The pump expels 3 Na+ ions for every 2 K+ ions taken in, slowly contributing to the negative resting membrane potential. *Na+ influx* - **Na+ influx** is responsible for the **depolarization phase** of an action potential, where the membrane potential becomes more positive. - This occurs when voltage-gated sodium channels open rapidly, allowing sodium ions to rush into the cell. *K+ influx* - **K+ influx** occurs during the **resting membrane potential** and is maintained by the Na+/K+ ATPase pump, which brings K+ ions back into the cell. - This influx helps to establish the potassium concentration gradient, which is critical for K+ efflux during repolarization.
Question 454: Post-tetanic potentiation is due to what?
- A. Increased availability of Ca++ (Correct Answer)
- B. Hyperpolarization of muscle fibers
- C. Rapid K+ efflux
- D. Rapid Na+ influx
Explanation: ***Increased availability of Ca++*** - **Post-tetanic potentiation (PTP)** is a short-term enhancement of synaptic efficacy that occurs after a brief period of high-frequency stimulation (tetanus). - This phenomenon is primarily due to the **accumulation of residual intracellular calcium ions (Ca++)** in the presynaptic terminal, which leads to increased neurotransmitter release upon subsequent action potentials. *Hyperpolarization of muscle fibers* - **Hyperpolarization** makes the muscle fiber less excitable, thereby *reducing* its response to subsequent stimuli rather than enhancing it. - This effect would decrease muscle contractility, which is opposite to what is observed in PTP. *Rapid K+ efflux* - **Rapid K+ efflux** from a cell typically causes repolarization or hyperpolarization, which would decrease neuronal excitability and thus *reduce* neurotransmitter release. - This process is essential for repolarizing the neuron after an action potential but does not directly cause PTP. *Rapid Na+ influx* - **Rapid Na+ influx** is responsible for the depolarization phase of an action potential, triggering nerve impulse propagation. - While essential for neural activity, it doesn't directly explain the *potentiation* or enhanced neurotransmitter release following tetanic stimulation, which is primarily calcium-dependent.
Question 455: Which of the following parameters decreases from its peak after the overshoot of an action potential?
- A. Transference for potassium
- B. Membrane conductance for sodium (Correct Answer)
- C. Membrane conductance for potassium
- D. Transference for sodium
Explanation: ***Membrane conductance for sodium*** - During the **peak and overshoot phase** of an action potential, voltage-gated sodium channels are maximally open, resulting in **peak sodium conductance**. - **After the overshoot**, these sodium channels rapidly undergo **inactivation**, leading to a sharp decrease in **sodium conductance**. - This decrease in sodium conductance is essential for terminating sodium influx and initiating the **repolarization phase** of the action potential. *Membrane conductance for potassium* - **Potassium conductance** actually *increases* after the overshoot as **voltage-gated potassium channels** open in response to depolarization. - This increased **potassium conductance** facilitates potassium efflux, which drives membrane repolarization back toward the resting potential. *Transference for sodium* - **Transference** refers to the fraction of total current carried by a specific ion, a concept more relevant to solutions and ionic equilibria. - While sodium's contribution to membrane current decreases as channels inactivate, the primary physiological parameter is **membrane conductance**, which directly reflects channel activity. *Transference for potassium* - **Potassium transference** (or its contribution to current) would increase as potassium channels open after the overshoot. - The most physiologically precise parameter for describing rapid ion channel dynamics during action potentials is **membrane conductance**, not transference.
Question 456: Single-unit smooth muscles are seen in which of the following?
- A. Iris
- B. Ductus deferens
- C. Trachea
- D. Ureter (Correct Answer)
Explanation: ***Ureter*** - The ureter contains **single-unit smooth muscle** which exhibits spontaneous electrical activity and contraction, allowing for peristaltic movement of urine. - In single-unit smooth muscle, cells are connected by **gap junctions**, enabling them to contract as a coordinated unit. *Iris* - The iris contains **multi-unit smooth muscle**, which allows for fine, independent control of each muscle cell for precise pupil dilation and constriction. - Multi-unit smooth muscle cells are not connected by gap junctions and require individual neural stimulation. *Ductus deferens* - The ductus deferens primarily consists of **multi-unit smooth muscle**, which is necessary for the strong, rapid contractions required for sperm expulsion during ejaculation. - This type of muscle allows for graded contractions depending on the intensity of nervous stimulation. *Trachea* - The smooth muscle in the trachea (trachealis muscle) is primarily **multi-unit smooth muscle**, facilitating independent regulation of airway diameter. - Contraction of the trachealis muscle can reduce the tracheal lumen, aiding in coughing or regulating airflow.
Question 457: Golgi tendon organs are innervated by which type of nerve fibre?
- A. Ia
- B. Ib (Correct Answer)
- C. II
- D. III
Explanation: ***Ib*** - **Golgi tendon organs (GTOs)** are encapsulated sensory receptors located in the musculoskeletal junction that monitor **muscle tension**. - They are innervated by **Ib afferent nerve fibers**, which are large diameter, myelinated nerve fibers with a high conduction velocity that transmit information to the central nervous system. *Ia* - **Ia afferent nerve fibers** innervate **muscle spindles**, which detect changes in **muscle length** and the rate of change of muscle length. - While both Ib and Ia fibers are involved in proprioception, their specific sensory receptors and functions differ. *II* - **Type II afferent nerve fibers** also innervate **muscle spindles**, primarily sensing sustained changes in **muscle length** (static stretch). - They do not innervate Golgi tendon organs; their role is distinct in providing information about muscle position. *III* - **Type III afferent nerve fibers** are smaller, thinly myelinated fibers that respond mainly to **nociceptive (pain)** and **temperature stimuli** in muscles and joints. - They are not involved in sensing muscle tension or length and do not innervate Golgi tendon organs.
Question 458: Holstein-Lewis fracture is associated with injury to which nerve?
- A. Radial (Correct Answer)
- B. Median
- C. Ulnar
- D. Axillary
Explanation: ***Radial*** - Holstein's Lewis fracture is a specific fracture of the **distal humeral shaft** that characteristically entraps the **radial nerve**. - Injury to the radial nerve in this region typically results in **wrist drop** and sensory loss over the dorsal hand. *Median* - The median nerve is most commonly entrapped in the **carpal tunnel**, leading to carpal tunnel syndrome. - Injury to the median nerve affects the **flexors of the forearm** and sensation in the first 3.5 digits. *Ulnar* - The ulnar nerve is often injured at the **cubital tunnel** (elbow) or Guyon's canal (wrist). - Its injury leads to a characteristic **claw hand deformity** and sensory loss in the little finger and ulnar half of the ring finger. *Axillary* - The axillary nerve is most susceptible to injury during **shoulder dislocations** or fractures of the surgical neck of the humerus. - Damage to this nerve causes weakness in **shoulder abduction** (due to deltoid paralysis) and sensory loss over the lateral shoulder.
Question 459: Which of the following statements is true regarding type 2b muscle fibers?
- A. White, glycolytic, fast contracting (Correct Answer)
- B. White, glycolytic, slow contracting
- C. Red, oxidative, fast contracting
- D. Red, glycolytic, slow contracting
Explanation: ***White, glycolytic, fast contracting*** - **Type 2b muscle fibers** (also known as fast glycolytic fibers) are characterized by their **fast contraction speed** and high capacity for **anaerobic glycolysis**. - They appear **white** due to a lower myoglobin content and fewer mitochondria, relying on glycolytic metabolism for quick, powerful bursts of activity. - These are the classic fast-twitch white muscle fibers optimized for explosive power. *White, glycolytic, slow contracting* - While **Type 2b fibers** are indeed **white** and **glycolytic**, this option inaccurately describes them as **slow contracting**. - Their primary characteristic is their **fast contraction speed**, which allows for rapid, forceful movements. *Red, oxidative, fast contracting* - This describes **Type 2a fibers** (fast oxidative-glycolytic), not Type 2b fibers. - **Type 2b fibers** are predominantly **white** and **glycolytic**, not red and oxidative. - Type 2b fibers have lower myoglobin content and fewer mitochondria compared to Type 2a. *Red, glycolytic, slow contracting* - **Red, oxidative, slow contracting** fibers refer to **Type 1 (slow-twitch) fibers**, optimized for endurance. - This option incorrectly combines characteristics from different fiber types. - Type 2b fibers are neither red nor slow contracting.
Question 460: Hyperpolarization is caused by which ions?
- A. K+ (Correct Answer)
- B. Na+
- C. HCO3-
- D. Ca2+
Explanation: ***K+*** - **Efflux of K+ ions** out of the cell makes the inside of the cell more negative, leading to **hyperpolarization**. - This efflux is typically mediated by **voltage-gated potassium channels** opening, or by activation of **GABA-A** or **glycine receptors** that increase K+ conductance. *Na+* - **Influx of Na+ ions** into the cell makes the inside of the cell more positive, causing **depolarization**, not hyperpolarization. - This influx is responsible for the **rising phase of an action potential**. *Ca2+* - **Influx of Ca2+ ions** into the cell also contributes to **depolarization** and can trigger various intracellular processes. - Ca2+ influx is crucial for **neurotransmitter release** and muscle contraction, but not for hyperpolarization. *HCO3-* - Bicarbonate ions (**HCO3-**) play a significant role in **maintaining pH balance** in the body and are involved in various physiological processes. - While ion channels can conduct HCO3-, their movement is not typically the primary cause of cell membrane hyperpolarization.
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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