Nerve and Muscle Physiology Practice Questions

Q: Arrange the following nerve fibers sequentially in the descending order of nerve impulse transmission velocity: A. C fiber B. Aa fiber C. B fiber D. Ad fiber

B-D-C-A. **Explanation** The velocity of nerve impulse transmission is primarily determined by two factors: **fiber diameter** and the **presence of myelin**. According to the Erlanger-Gasser classification, thicker and more heavily myelinated fibers conduct impulses faster than thinner, unmyelinated ones. **The Correct Sequence (Descending Order):** 1. **Aα (Alpha) fiber:** The thickest (12–20 µm) and most heavily myelinated; conducts at 70–120 m/s. 2. **Aδ (Delta) fiber:** Thinner than Aα (2–5 µm) but still myelinated; conducts at 12–30 m/s. 3. **B fiber:** Small, preganglionic autonomic fibers ( Aδ > B > C**, which corresponds to sequence **B-D-C-A**. **Analysis of Options:** * **Option B (Correct):** Correctly follows the hierarchy of diameter and myelination. * **Options A, C, and D:** These are incorrect because they place C fibers (slowest) or B fibers (intermediate) ahead of A-group fibers in velocity. **High-Yield NEET-PG Pearls:** * **Sensitivity to Local Anesthetics:** The order of blockade is **B > C > Aδ > Aγ > Aβ > Aα**. Note that B fibers are blocked first despite being myelinated because of their small diameter and anatomy. * **Sensitivity to Pressure:** A fibers are most sensitive (e.g., "limb falling asleep"). * **Sensitivity to Hypoxia:** B fibers are most sensitive. * **Function of C fibers:** They carry "slow pain" (dull, aching), temperature, and postganglionic sympathetic signals. Aδ fibers carry "fast pain" (sharp, localized).

Q: Visceral noxious stimuli are mediated by which nerve fiber type?

C. **Explanation:** The correct answer is **D. C fibers**. **Underlying Medical Concept:** Sensory information from the viscera (internal organs) is primarily transmitted via two types of fibers: **A-delta (Aδ)** and **C fibers**. However, the vast majority of visceral afferents—especially those mediating dull, aching, and poorly localized **noxious stimuli** (pain)—are unmyelinated **C fibers**. These fibers have a small diameter and slow conduction velocity (0.5–2.0 m/s). In the viscera, these fibers respond to mechanical distortion, inflammation, and ischemia, often leading to the phenomenon of "referred pain" due to convergence in the spinal cord. **Analysis of Incorrect Options:** * **A. A-alpha (Aα):** These are the thickest, fastest myelinated fibers. They primarily carry proprioceptive information from muscles (muscle spindles) and tendons (Golgi tendon organs). * **B. A-beta (Aβ):** These are large, myelinated fibers that mediate "fine touch," pressure, and vibration. They are not involved in primary pain transmission. * **C. A-gamma (Aγ):** These are motor fibers (efferent) that innervate the intrafusal fibers of the muscle spindle to maintain spindle sensitivity; they do not carry sensory noxious stimuli. **NEET-PG High-Yield Pearls:** * **Fast Pain vs. Slow Pain:** Fast, sharp, localized pain (somatic) is carried by **A-delta** fibers. Slow, burning, chronic, or visceral pain is carried by **C fibers**. * **Fiber Sensitivity:** C fibers are the **most sensitive to local anesthetics**, while A-alpha fibers are the most sensitive to pressure/hypoxia. * **Visceral Pain Characteristics:** It is characterized by being poorly localized, often associated with autonomic symptoms (nausea, sweating), and typically referred to somatic structures (e.g., cardiac pain referred to the left inner arm).

Q: Which of the following best describes muscle B compared with muscle A?

Innervated by smaller nerve fibers. ***Innervated by smaller nerve fibers*** - According to **Henneman's size principle**, slow-twitch (Type I) muscle fibers are innervated by **smaller motor neurons** with smaller axon diameters. - Type I fibers require **lower threshold stimulation** and are recruited first during muscle activation, consistent with smaller nerve fiber innervation. *Adapted for rapid contraction* - This describes **fast-twitch (Type II) fibers**, not slow-twitch fibers; Type I fibers contract **slowly** and are built for endurance. - Fast-twitch fibers have rapid **calcium release and reuptake** mechanisms, enabling quick contraction-relaxation cycles. *Composed of larger muscle fibers* - **Type II (fast-twitch) fibers** are typically larger in diameter to generate more force quickly. - Type I fibers are generally **smaller** as they prioritize sustained activity over peak force production. *Fewer mitochondria* - Type I fibers have **abundant mitochondria** for sustained **aerobic metabolism** and fatigue resistance. - Type II fibers rely more on **anaerobic glycolysis** and have fewer mitochondria compared to oxidative Type I fibers.

Q: Which of the following is NOT a major calcium-binding protein?

Actinin. **Explanation:** The correct answer is **Actinin (Option C)**. **Why Actinin is the correct answer:** Actinin (specifically $\alpha$-actinin) is a **structural protein**, not a calcium-binding protein. Its primary role is to anchor actin filaments to the Z-discs in skeletal muscle and to dense bodies in smooth muscle. It belongs to the spectrin superfamily and functions as a cross-linking protein to maintain the architectural integrity of the sarcomere. **Analysis of Incorrect Options:** * **Troponin (Option A):** Specifically, **Troponin C** is a major calcium-binding protein in striated muscle. Upon binding $Ca^{2+}$, it undergoes a conformational change that moves tropomyosin away from the myosin-binding sites on actin, initiating contraction. * **Calmodulin (Option B):** This is the most ubiquitous calcium-binding messenger protein. It mediates various cellular processes (like activating Myosin Light Chain Kinase in smooth muscle) by forming a $Ca^{2+}$-calmodulin complex. * **Calcineurin (Option D):** Also known as Protein Phosphatase 2B, it is a calcium and calmodulin-dependent serine/threonine protein phosphatase. It plays a critical role in T-cell activation and cardiac hypertrophy. **High-Yield Clinical Pearls for NEET-PG:** * **Calsequestrin:** The major $Ca^{2+}$-binding protein located inside the **Sarcoplasmic Reticulum** (SR) that allows for high-capacity $Ca^{2+}$ storage. * **Calbindin:** A vitamin D-dependent calcium-binding protein found in the intestines and kidneys that facilitates $Ca^{2+}$ absorption/reabsorption. * **Clinical Correlation:** Calcineurin inhibitors (e.g., **Cyclosporine, Tacrolimus**) are potent immunosuppressants used to prevent organ transplant rejection.

Question 1

Arrange the following nerve fibers sequentially in the descending order of nerve impulse transmission velocity: A. C fiber B. Aa fiber C. B fiber D. Ad fiber

Accepted Answer

B-D-C-A

Answer

A-D-C-B

Answer

A-C-D-B

Answer

C-A-B-D

Question 2

In Myasthenia gravis, what is the fundamental defect at the neuromuscular junction?

Accepted Answer

Failure of Acetylcholine (ACh) to attach to the postsynaptic receptor

Answer

Deficiency of Acetylcholine (ACh)

Answer

Defect in the release of Acetylcholine (ACh)

Answer

Impeded presynaptic release of Acetylcholine (ACh)

Question 3

Where does the action potential in a spinal motor neuron generate from?

Accepted Answer

Initial segment

Answer

Cell body

Answer

Dendrites

Answer

Axon

Question 4

Visceral noxious stimuli are mediated by which nerve fiber type?

Accepted Answer

C

Answer

A alpha

Answer

A beta

Answer

A gamma

Question 5

Which of the following best describes muscle B compared with muscle A?

Accepted Answer

Innervated by smaller nerve fibers

Answer

Adapted for rapid contraction

Answer

Composed of larger muscle fibers

Answer

Fewer mitochondria

Question 6

Which of the following is NOT a major calcium-binding protein?

Accepted Answer

Actinin

Answer

Troponin

Answer

Calmodulin

Answer

Calcineurin

Question 7

Saltatory conduction in myelinated axons results from the fact that:

Accepted Answer

Voltage-gated sodium channels are concentrated at the nodes of Ranvier

Answer

Salt concentration is increased beneath the myelin segments

Answer

Non-gated ion channels are present beneath the segments of myelin

Answer

Membrane resistance is decreased beneath the segments of myelin

Question 8

Which of the following statements regarding intrafusal fibers is incorrect?

Accepted Answer

Primary sensory endings are excited by both nuclear chain and nuclear bag fibers.

Answer

Nuclear chain fibers are shorter and thinner than nuclear bag fibers.

Answer

Nuclear bag fibers are fewer in number than nuclear chain fibers.

Answer

Secondary sensory endings are excited by nuclear chain fibers only.

Question 9

Synaptic conduction is mostly orthodromic because?

Accepted Answer

Chemical mediators are located only in the presynaptic terminal.

Answer

Dendrites cannot be depolarized.

Answer

Once repolarized, an area cannot be depolarized.

Answer

The strength of the antidromic impulse is less.

Question 10

Post-tetanic facilitation is thought to be the result of which of the following?

Accepted Answer

A buildup of calcium in the presynaptic terminal

Answer

Opening of voltage-gated sodium channels

Answer

Opening of transmitter-gated potassium channels

Answer

Electrotonic conduction

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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