Nerve and Muscle Physiology Practice Questions

Q: Golgi tendon apparatus conveys sensory information to the CNS depending upon which of the following factors?

Tension in the muscle. **Explanation:** The **Golgi Tendon Organ (GTO)** is a specialized sensory receptor located at the junction of muscle fibers and tendons (musculotendinous junction). It is arranged **in series** with the extrafusal muscle fibers. **1. Why "Tension in the muscle" is correct:** The primary function of the GTO is to monitor **muscle tension**. When a muscle contracts or is stretched excessively, the collagen fibers in the tendon tighten, compressing the nerve endings of the **Ib afferent fibers**. This mechanical deformation triggers action potentials. Because it is in series, the GTO is particularly sensitive to the force of active muscle contraction, acting as a "force gauge" to prevent tendon avulsion or muscle tearing via the **inverse stretch reflex** (autogenic inhibition). **2. Why other options are incorrect:** * **Length of the muscle:** This is the primary function of the **Muscle Spindle**, which is arranged **in parallel** with muscle fibers and detects changes in length (stretch). * **Rapidity of contraction:** While GTOs can respond to the rate of tension change, the fundamental parameter they measure is tension itself. The "rapidity of length change" (velocity) is specifically sensed by **Nuclear Bag fibers** (Type Ia afferents) of the muscle spindle. * **Blood supply:** This is monitored by metabolic receptors (chemoreceptors) and does not involve the GTO. **High-Yield Clinical Pearls for NEET-PG:** * **Afferent Nerve:** Ib sensory fibers (Large, myelinated, fast-conducting). * **Reflex:** Inverse Stretch Reflex (leads to relaxation of the agonist muscle). * **Arrangement:** GTO = In Series (Tension); Muscle Spindle = In Parallel (Length). * **Function:** Protects the musculoskeletal system from damage due to excessive loading.

Q: What is the location of the lowest threshold potential in a motor nerve fiber?

Axon hillock. **Explanation:** The **Axon Hillock** (specifically the initial segment) is the site where the action potential is generated in a motor neuron. It possesses the **lowest threshold potential** because it contains the **highest density of voltage-gated sodium (Na+) channels** per unit area. Since the threshold for firing depends on the density of these channels, the axon hillock requires the least amount of depolarization (approximately -10 to -15 mV from resting membrane potential) to trigger an all-or-nothing response. **Analysis of Options:** * **Dendrites (A):** These are primarily involved in receiving inputs (EPSPs and IPSPs). They generally have a low density of voltage-gated Na+ channels, making them poor at generating action potentials; they conduct signals decrementally. * **Soma/Cell Body (B):** While the soma can conduct electrical impulses, its threshold is significantly higher than the axon hillock due to a lower concentration of Na+ channels. * **Axon (D):** Once the action potential is generated at the hillock, it is propagated along the axon. While the axon has enough channels to maintain the signal, the density is lower than at the initial segment/hillock. **High-Yield NEET-PG Pearls:** * **Trigger Zone:** The axon hillock is often referred to as the "integrative center" or "trigger zone" of the neuron. * **Threshold Values:** The typical threshold at the hillock is about **-55 mV**, whereas the rest of the soma may require depolarization to **-35 mV** to fire. * **Safety Factor:** The high density of Na+ channels at the hillock ensures a high "safety factor" for signal transmission from the cell body to the axon.

Q: What causes afterhyperpolarization?

Potassium efflux. **Explanation:** **1. Why Potassium Efflux is Correct:** Afterhyperpolarization (also known as the undershoot phase) occurs at the end of an action potential. During repolarization, voltage-gated **K⁺ channels** open to allow potassium to leave the cell (**efflux**). However, these channels are slow to close. Even after the membrane potential reaches the resting level (-70 mV), K⁺ continues to exit the cell, moving the potential closer to the equilibrium potential of Potassium (approximately -90 mV). This transient period where the interior of the cell becomes more negative than the resting state is called afterhyperpolarization. **2. Why Other Options are Incorrect:** * **Sodium Influx (B):** This causes **depolarization** (the rising phase of the action potential) as positive ions enter the cell, making the interior less negative. * **Sodium Efflux (A):** While the Na⁺-K⁺ ATPase pump moves sodium out, it is a slow active transport process and not the primary driver of the rapid voltage changes seen in afterhyperpolarization. * **Chloride Influx (C):** While Cl⁻ entry can cause hyperpolarization (e.g., via GABA receptors), it is not the mechanism responsible for the "undershoot" following a standard neuronal action potential. **3. NEET-PG High-Yield Pearls:** * **Equilibrium Potentials:** Remember the Nernst values: K⁺ ≈ -94 mV, Na⁺ ≈ +61 mV. The membrane potential always moves toward the equilibrium potential of the ion to which it is most permeable. * **Refractory Period:** Afterhyperpolarization contributes to the **Relative Refractory Period**, meaning a stronger-than-normal stimulus is required to trigger another action potential. * **Channel Kinetics:** The key takeaway is that Na⁺ channels are "fast" (quick to open/inactivate), while K⁺ channels are "slow" (delayed opening/closing).

Q: Which class of neurotransmitter would be most affected by a toxin that disrupted microtubules within neurons?

Peptide transmitters. **Explanation:** The correct answer is **Peptide transmitters (Option D)**. The core concept here is the **site of synthesis and the mechanism of transport**. 1. **Why Peptide Transmitters are correct:** Unlike small-molecule neurotransmitters, neuropeptides (e.g., Substance P, Enkephalins) are synthesized in the **cell body (soma)** because they require the Rough Endoplasmic Reticulum and Golgi apparatus for protein synthesis. Once synthesized and packaged into large dense-core vesicles, they must be transported to the distant axon terminal via **fast axonal transport**. This process is entirely dependent on **microtubules**, which act as "tracks" for motor proteins like **Kinesin** (anterograde transport). Therefore, disrupting microtubules halts the delivery of peptide transmitters to the synapse. 2. **Why other options are incorrect:** * **Amino acids (A)** (e.g., GABA, Glutamate) and **Catecholamines (B)** (e.g., Dopamine, Norepinephrine) are small-molecule transmitters. While their precursor enzymes are transported from the soma, the actual synthesis and recycling of the neurotransmitters occur locally within the **axon terminal**. They do not rely on continuous long-distance microtubule transport for their immediate availability. * **Membrane-soluble transmitters (C)** (e.g., Nitric Oxide) are synthesized on demand and diffuse directly across membranes; they are not stored in vesicles or transported via microtubules. **High-Yield NEET-PG Pearls:** * **Anterograde Transport:** Mediated by **Kinesin** (Soma → Terminal). * **Retrograde Transport:** Mediated by **Dynein** (Terminal → Soma). This is the route for Nerve Growth Factor (NGF) and certain viruses (Rabies, Herpes, Polio) and Tetanus toxin. * **Microtubule Inhibitors:** Drugs like **Colchicine** and **Vinca alkaloids** (Vincristine/Vinblastine) disrupt microtubules and can cause peripheral neuropathy by interfering with this axonal transport system.

Q: Which fibers carry pain sensation?

Ad fibers. Pain sensation is carried by two specific types of nerve fibers: **Aδ (A-delta) fibers** and **C fibers**. **Explanation of the Correct Answer:** * **Aδ fibers (Option D):** These are thin, myelinated fibers that conduct impulses at a rate of 6–30 m/s. They are responsible for **"fast pain"** (initial, sharp, pricking, and well-localized pain). They use glutamate as their primary neurotransmitter and terminate mainly in Laminae I and V of the dorsal horn. **Analysis of Incorrect Options:** * **Aα (Alpha) fibers (Option A):** These are the thickest and fastest myelinated fibers. They carry information related to **proprioception** (muscle spindles and Golgi tendon organs) and somatic motor function. * **Aβ (Beta) fibers (Option B):** These are large, myelinated fibers that carry sensations of **touch, pressure, and vibration**. According to the "Gate Control Theory," stimulation of these fibers can inhibit pain transmission in the spinal cord. * **Aγ (Gamma) fibers (Option C):** These fibers are motor efferents that innervate the **intrafusal fibers** of the muscle spindle, regulating muscle tone. **High-Yield Clinical Pearls for NEET-PG:** * **C Fibers:** These are **unmyelinated**, slow-conducting fibers (0.5–2 m/s) responsible for **"slow pain"** (dull, aching, burning, and poorly localized). They use Substance P as a neurotransmitter. * **Erlanger-Gasser Classification:** Remember that sensitivity to local anesthetics follows the order: **C > B > Aδ > Aγ > Aβ > Aα**. (Small, unmyelinated fibers are blocked first). * **First vs. Second Pain:** Aδ fibers mediate the immediate "first pain," while C fibers mediate the delayed "second pain."

Q: Proprioception is carried by which type of nerve fibers?

A alpha fibers. **Explanation:** Proprioception (the sense of body position and movement) requires the fastest possible transmission to the central nervous system to maintain balance and coordinate motor activity. This is achieved via **A-alpha (Aα) fibers**, which are the thickest and most heavily myelinated nerve fibers. **1. Why A-alpha is correct:** According to the Erlanger-Gasser classification, **A-alpha fibers** have the largest diameter (12–20 μm) and the fastest conduction velocity (70–120 m/s). They carry sensory information from **muscle spindles (Ia)** and **Golgi tendon organs (Ib)**, which are the primary receptors for proprioception. **2. Analysis of Incorrect Options:** * **A-beta (Aβ) fibers:** These are slightly smaller and slower than A-alpha. They primarily carry sensations of **fine touch, pressure, and vibration**. * **B fibers:** These are preganglionic autonomic fibers. They are small, myelinated, and have much slower conduction velocities than A-type fibers. * **C fibers:** These are the smallest, **unmyelinated** fibers. They carry "slow" pain (dull, aching), temperature, and crude touch. They have the slowest conduction velocity (0.5–2 m/s). **Clinical Pearls & High-Yield Facts:** * **Order of Susceptibility:** * **Hypoxia:** B > A > C (B fibers are most sensitive). * **Pressure:** A > B > C (A fibers are most sensitive; this is why your foot "falls asleep" when compressed). * **Local Anesthetics:** C > B > A (C fibers are most sensitive; pain is blocked before motor function). * **Fastest to Slowest:** Aα > Aβ > Aγ > Aδ > B > C. * **A-delta (Aδ) fibers** carry "fast" pain (sharp, localized) and cold temperature.

Q: Tetany may be a feature of the following conditions except?

Hyponatremia. **Explanation:** Tetany is a state of increased neuromuscular excitability characterized by carpopedal spasms, paresthesia, and laryngospasm. It is primarily driven by a decrease in the concentration of **ionized calcium ($Ca^{2+}$)** in the extracellular fluid. **Why Hyponatremia is the Correct Answer:** Hyponatremia (low serum sodium) does not cause tetany. In fact, severe hyponatremia typically leads to CNS symptoms like cerebral edema, seizures, and coma, but it does not increase neuromuscular irritability in the peripheral nerves. Therefore, it is the "except" in this list. **Analysis of Other Options:** * **Hyperventilation:** This leads to respiratory alkalosis. Alkalosis promotes the binding of ionized calcium to albumin, reducing the physiologically active $Ca^{2+}$ fraction, thereby triggering tetany. * **Hypokalemic Alkalosis:** Metabolic alkalosis (often seen in Conn’s syndrome or vomiting) similarly reduces ionized calcium levels [1]. While hypokalemia itself usually causes muscle weakness, the concomitant **alkalosis** is a classic trigger for tetany [1]. * **Hypomagnesemia:** Magnesium is a co-factor for adenylate cyclase; its deficiency leads to impaired PTH secretion and end-organ resistance to PTH, resulting in secondary hypocalcemia and tetany. **NEET-PG High-Yield Pearls:** 1. **Trousseau’s Sign:** Induction of carpal spasm by inflating a BP cuff above systolic pressure for 3 minutes (more sensitive than Chvostek’s). 2. **Chvostek’s Sign:** Tapping the facial nerve leads to twitching of facial muscles. 3. **The Ionized Calcium Rule:** Total calcium may be normal in alkalosis, but tetany occurs because the **ionized (free) fraction** decreases. 4. **Refractory Hypocalcemia:** If hypocalcemia does not respond to calcium gluconate, always check and correct **Magnesium** levels first.

Q: What is the chronaxie minimum in nerves?

Myelinated nerve. ### Explanation **Concept Overview:** Chronaxie is defined as the **minimum time** required to excite a tissue when using a stimulus intensity equal to twice the rheobase (the minimum current required for excitation). It is a measure of **excitability**: the shorter the chronaxie, the more excitable the tissue. **Why Myelinated Nerves are Correct:** Myelinated nerves (like Type A fibers) are designed for rapid signal transmission. Due to the presence of the myelin sheath and the Nodes of Ranvier, these fibers have a high density of voltage-gated sodium channels and low membrane capacitance. This makes them highly excitable, resulting in the **shortest chronaxie** among all nerve types. **Analysis of Incorrect Options:** * **Unmyelinated Nerves:** These fibers (like Type C fibers) lack myelin, leading to slower conduction velocities and lower excitability. Consequently, they have a **longer chronaxie** compared to myelinated fibers. * **Mixed Nerves:** A mixed nerve contains a variety of fibers (myelinated, unmyelinated, sensory, and motor). Its chronaxie would be an average or representative of its constituent parts, but it is not the "minimum" value. * **Sensory Nerves:** While many sensory nerves are myelinated, this category also includes unmyelinated fibers (e.g., slow pain). Therefore, "myelinated nerve" is a more precise physiological descriptor for minimum chronaxie. **High-Yield NEET-PG Pearls:** * **Excitability Relationship:** Chronaxie is inversely proportional to excitability ($Chronaxie \propto 1/Excitability$). * **Order of Chronaxie:** Myelinated Nerve < Unmyelinated Nerve < Skeletal Muscle < Cardiac Muscle < Smooth Muscle. * **Clinical Use:** Chronaxie is used in electrodiagnosis to assess nerve regeneration or muscle denervation (denervated muscles show a significantly increased chronaxie). * **Rheobase:** The minimum strength of current (intensity) required to evoke a response if applied for an infinite duration.

Question 1

Golgi tendon apparatus conveys sensory information to the CNS depending upon which of the following factors?

Accepted Answer

Tension in the muscle

Answer

Length of the muscle

Answer

Rapidity of contraction

Answer

Blood supply to the muscle

Question 2

What is the location of the lowest threshold potential in a motor nerve fiber?

Accepted Answer

Axon hillock

Answer

Dendrite

Answer

Soma (cell body)

Answer

Axon

Question 3

A 45-year-old lady suffers from botulism poisoning. This toxin blocks the release of acetylcholine. What is the primary effect of this toxin on synaptic transmission?

Accepted Answer

Inhibition of cholinergic synapse transmission

Answer

Inhibition of adrenergic synapse transmission

Answer

Facilitation of adrenergic synapse transmission

Answer

No effect on synaptic transmission

Question 4

What causes afterhyperpolarization?

Accepted Answer

Potassium efflux

Answer

Sodium efflux

Answer

Sodium influx

Answer

Chloride influx

Question 5

Which class of neurotransmitter would be most affected by a toxin that disrupted microtubules within neurons?

Accepted Answer

Peptide transmitters

Answer

Amino acid transmitters

Answer

Catecholamine transmitters

Answer

Membrane-soluble transmitters

Question 6

Which fibers carry pain sensation?

Accepted Answer

Ad fibers

Answer

Aa fibers

Answer

Ab fibers

Answer

Ag fibers

Question 7

Hyponatremia has no effect on the resting membrane potential (RMP) because:

Accepted Answer

The equilibrium potential for Na+ is +60 mV.

Answer

The cell membrane is impermeable to Na+.

Answer

Extracellular fluid Na+ concentration is high.

Answer

Intracellular K+ concentration is high.

Question 8

Proprioception is carried by which type of nerve fibers?

Accepted Answer

A alpha fibers

Answer

A beta fibers

Answer

B fibers

Answer

C fibers

Question 9

Tetany may be a feature of the following conditions except?

Accepted Answer

Hyponatremia

Answer

Hyperventilation

Answer

Hypokalemic alkalosis

Answer

Hypomagnesemia

Question 10

What is the chronaxie minimum in nerves?

Accepted Answer

Myelinated nerve

Answer

Mixed nerves

Answer

Unmyelinated nerve

Answer

Sensory nerves

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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