Nerve and Muscle Physiology Practice Questions

Q: Which of the following statements is TRUE regarding the Golgi tendon organ?

Senses muscle tension. **Explanation:** The **Golgi Tendon Organ (GTO)** is a high-threshold mechanoreceptor located in the tendons, arranged in **series** with extrafusal muscle fibers. **1. Why Option D is Correct:** The GTO is primarily sensitive to **muscle tension** (force). When a muscle contracts, it pulls on the tendon, compressing the nerve endings (Ib afferents) within the GTO. This triggers the **Inverse Stretch Reflex** (Autogenic Inhibition), which inhibits the agonist muscle to prevent tendon avulsion or muscle damage from excessive force. **2. Why Other Options are Incorrect:** * **Option A:** Sensing the **length** (and rate of change of length) of a muscle is the function of the **Muscle Spindle**, which is arranged in *parallel* with muscle fibers. * **Option B:** **Reciprocal innervation** is a feature of the *Stretch Reflex* (Muscle Spindle), where the agonist is excited and the antagonist is inhibited. The GTO reflex involves *autogenic inhibition* (inhibiting the same muscle). * **Option C:** GTOs are stimulated by muscle contraction or passive stretch, but they do not "result" from alpha motor neuron stimulation; rather, their activation leads to the **inhibition** of alpha motor neurons via inhibitory interneurons in the spinal cord. **High-Yield Facts for NEET-PG:** * **Afferent Nerve:** Type **Ib** sensory fibers (Fast conducting). * **Arrangement:** **Series** (GTO) vs. **Parallel** (Muscle Spindle). * **Function:** Acts as a "force transducer" and protective mechanism. * **Reflex Type:** Polysynaptic (involves an inhibitory interneuron). * **Clasp-Knife Phenomenon:** This clinical sign in upper motor neuron lesions is partly attributed to the exaggerated activity of the Golgi Tendon Organ reflex.

Q: Size of action potential is decreased as a result of?

Lower extracellular sodium. **Explanation:** The size (amplitude) of an action potential is primarily determined by the **electrochemical gradient of Sodium (Na⁺)**. During the depolarization phase, voltage-gated Na⁺ channels open, allowing Na⁺ to rush into the cell. This influx continues until the membrane potential approaches the **Equilibrium Potential for Sodium (E_Na)**, which is approximately +60 mV. **1. Why Option A is Correct:** When **extracellular sodium concentration is lowered**, the concentration gradient between the outside and inside of the cell decreases. According to the Nernst equation, this reduces the Equilibrium Potential for Sodium. Consequently, less Na⁺ enters the cell during depolarization, leading to a **lower peak (decreased amplitude)** of the action potential. **2. Why the other options are incorrect:** * **Option B & C (Extracellular Calcium):** Calcium levels primarily affect the **threshold** for firing an action potential, not its size. Low Ca²⁺ (Hypocalcemia) makes the cell more excitable (lowers threshold), while high Ca²⁺ (Hypercalcemia) stabilizes the membrane (raises threshold). * **Option D (Raised extracellular sodium):** Increasing extracellular Na⁺ would increase the concentration gradient, theoretically increasing the amplitude of the action potential (though physiological limits exist). **High-Yield Clinical Pearls for NEET-PG:** * **Amplitude vs. Velocity:** The *size* of the action potential depends on Na⁺ concentration, but the *conduction velocity* depends on myelination and axon diameter. * **All-or-None Law:** While an action potential follows the all-or-none law for a given set of conditions, changing the ionic environment (like hyponatremia) alters the "all" level. * **Hypokalemia:** Primarily affects the **Resting Membrane Potential (RMP)** by making it more negative (hyperpolarization), making it harder to initiate an action potential.

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."

Question 1

What is the ratio of actin : troponin : tropomyosin molecules in a thin filament?

Accepted Answer

7:1:1

Answer

1:1:1

Answer

1:3:5

Answer

1:7:7

Question 2

Which of the following statements is TRUE regarding the Golgi tendon organ?

Accepted Answer

Senses muscle tension

Answer

Senses the dynamic length of the muscle

Answer

Is involved in reciprocal innervation

Answer

Results from alpha motor neuron stimulation

Question 3

In a myelinated nerve fiber, if the refractory period is 1/2500 seconds, what is the maximum impulse rate per second?

Accepted Answer

2500 per sec

Answer

40 per sec

Answer

250 per sec

Answer

400 per sec

Question 4

Size of action potential is decreased as a result of?

Accepted Answer

Lower extracellular sodium

Answer

Raised extracellular calcium

Answer

Lower extracellular calcium

Answer

Raised extracellular sodium

Question 5

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 6

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 7

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 8

What causes afterhyperpolarization?

Accepted Answer

Potassium efflux

Answer

Sodium efflux

Answer

Sodium influx

Answer

Chloride influx

Question 9

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 10

Which fibers carry pain sensation?

Accepted Answer

Ad fibers

Answer

Aa fibers

Answer

Ab fibers

Answer

Ag fibers

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?