Nerve and Muscle Physiology Practice Questions

Q: Fast axonal transport is a component of orthograde axonal transport. What is the rate of fast axonal transport?

400 mm/d. **Explanation:** Axonal transport is the process by which organelles, proteins, and vesicles are moved along the axon. It is categorized based on direction (Orthograde/Anterograde vs. Retrograde) and speed (Fast vs. Slow). **1. Why Option C is Correct:** **Fast Axonal Transport** (specifically the orthograde component) occurs at a rate of approximately **400 mm/day**. This process is mediated by the motor protein **Kinesin**, which "walks" along microtubules using ATP. It is responsible for transporting membrane-bound organelles, mitochondria, and neurotransmitter vesicles from the cell body to the axon terminals. **2. Analysis of Incorrect Options:** * **Options A and B (100–200 mm/d):** These rates are too slow for fast transport. While some sources cite a range (200–400 mm/d), **400 mm/day** is the standard value cited in major physiology textbooks (like Guyton and Ganong) and is the high-yield figure for NEET-PG. * **Option D (600 mm/d):** This exceeds the physiological rate of standard fast axonal transport in human neurons. **3. Clinical Pearls & High-Yield Facts:** * **Slow Axonal Transport:** Occurs at a rate of **0.5 to 10 mm/day**. It transports structural elements like the axoplasmic matrix and cytoskeletal proteins (neurofilaments). * **Retrograde Transport:** Moves from the periphery to the cell body at a rate of **~200 mm/day** via the motor protein **Dynein**. * **Clinical Relevance:** Certain neurotropic viruses (e.g., **Rabies, Herpes Simplex**) and toxins (e.g., **Tetanus toxin**) exploit **Retrograde transport** to reach the Central Nervous System. * **Mnemonic:** **K**inesin moves to the **K**ick-off (Anterograde/Forward); **D**ynein moves to the **D**en (Retrograde/Backward).

Q: What is the twitch of a single motor unit called?

Fasciculation. **Explanation:** The correct answer is **Fasciculation**. **1. Why Fasciculation is correct:** A **motor unit** consists of a single alpha motor neuron and all the muscle fibers it innervates. A **fasciculation** is defined as the spontaneous, involuntary contraction of all muscle fibers within a single motor unit. Because an entire motor unit is involved, these twitches are often visible under the skin as brief ripples or flickers, though they are usually insufficient to move a joint. They typically result from pathological irritation of the lower motor neuron (LMN) or its axon. **2. Why the other options are incorrect:** * **Myoclonic jerk:** These are sudden, brief, shock-like involuntary movements caused by muscular contractions (positive myoclonus) or sudden loss of muscle tone (negative myoclonus). Unlike fasciculations, these involve large groups of muscles and result in visible limb movement. * **Tremor:** This is a rhythmic, oscillatory movement produced by alternating or synchronous contractions of antagonist muscles. It is not a single twitch but a repetitive movement. * **Fibrillation:** This is the spontaneous contraction of a **single muscle fiber**. Because only one fiber is contracting, fibrillations are **not visible** to the naked eye and can only be detected via Electromyography (EMG). They occur when a muscle fiber loses its nerve supply (denervation hypersensitivity). **3. High-Yield Clinical Pearls for NEET-PG:** * **Fasciculations vs. Fibrillations:** Fasciculations are visible; Fibrillations are invisible (EMG only). * **LMN Lesions:** Both fasciculations and fibrillations are classic signs of Lower Motor Neuron (LMN) lesions (e.g., Amyotrophic Lateral Sclerosis or Polio). * **Benign Fasciculations:** These can occur in healthy individuals due to fatigue, caffeine, or electrolyte imbalances (e.g., eyelid twitching).

Q: What does the 'all or none' phenomenon mean in physiology?

An action potential has a fixed, all-or-nothing value.. ### Explanation The **All-or-None Law** is a fundamental principle of neurophysiology. It states that if a stimulus is strong enough to reach the **threshold potential** (typically -55mV), an action potential will be generated at a constant, maximum amplitude. If the stimulus is subthreshold, no action potential occurs at all. **Why Option B is Correct:** Once the threshold is reached, voltage-gated sodium channels open in a regenerative cycle. The resulting action potential is independent of the stimulus strength; its amplitude and shape remain **fixed and constant** for that specific nerve or muscle fiber. Increasing the intensity of the stimulus will not increase the size of the action potential. **Analysis of Incorrect Options:** * **Option A & D:** These describe **graded potentials** (like receptor potentials or EPSPs), where the magnitude varies with the strength or duration of the stimulus. In contrast, action potentials are non-graded. * **Option C:** Subthreshold stimuli only cause local electronic changes (local response) but fail to trigger the rapid depolarization required for an action potential. **High-Yield NEET-PG Pearls:** * **Frequency Coding:** Since the amplitude of an action potential is fixed, the intensity of a stimulus is communicated to the CNS by the **frequency** of action potentials, not their size. * **Applicability:** The All-or-None law applies to **individual** nerve fibers and motor units. It does **not** apply to a whole nerve trunk (which contains many fibers with different thresholds) or a whole skeletal muscle, which show graded responses via recruitment. * **Refractory Period:** This ensures that action potentials remain discrete events, preventing them from merging or summing together.

Q: Rheobase indicates:

Magnitude of current. ### Explanation **Rheobase** is defined as the **minimum intensity (magnitude) of a constant electrical current** that, when applied for an indefinite period, is required to produce an action potential in an excitable tissue (nerve or muscle). #### Why Option A is Correct: The Strength-Duration curve illustrates the relationship between the intensity of a stimulus and the time required to excite a tissue. Rheobase represents the **threshold intensity** on the Y-axis. If the current magnitude is below the rheobase, the tissue will not fire, regardless of how long the stimulus is applied. #### Why Other Options are Incorrect: * **B. Rate of discharge:** This refers to the frequency of action potentials, which is determined by the intensity of a suprathreshold stimulus and the refractory period, not the rheobase. * **C. Velocity of nerve conduction:** This is a measure of speed (distance/time) determined by myelination and axon diameter, unrelated to the initial threshold intensity. * **D. Specificity of impulse transmission:** This refers to the "all-or-none" law or synaptic pathways, rather than the electrical parameters of stimulation. --- ### High-Yield Facts for NEET-PG: * **Chronaxie:** The minimum **time** required for a current of **double the rheobase** strength to excite the tissue. * **Excitability Inverse Relationship:** Chronaxie is inversely proportional to excitability. A shorter chronaxie means the tissue is more excitable. * **Chronaxie Values:** Nerve (0.1 ms) < Skeletal Muscle (0.25 ms) < Cardiac Muscle (2.0 ms) < Smooth Muscle (high). * **Utilization Time:** The minimum time required to excite a tissue using a current exactly at the rheobase strength. (Note: This is less clinically useful than chronaxie because it is difficult to measure accurately).

Q: Which of the following nerve fibers is unmyelinated?

C fiber. ### Explanation The classification of nerve fibers is a high-yield topic in NEET-PG, primarily based on the **Erlanger-Gasser classification**, which categorizes fibers according to their diameter, conduction velocity, and presence of myelin. **Why C fiber is the correct answer:** C fibers are the only nerve fibers in the human body that are **completely unmyelinated**. Because they lack a myelin sheath, they exhibit the smallest diameter and the slowest conduction velocity (0.5–2.0 m/s). They primarily transmit "slow" or "second" pain (dull, aching), temperature, and postganglionic autonomic signals. **Analysis of Incorrect Options:** * **A-alpha (Aα):** These are the thickest and most heavily myelinated fibers. They have the fastest conduction velocity and are responsible for proprioception and somatic motor function. * **A-beta (Aβ):** These are also myelinated fibers with a medium diameter. They primarily transmit sensations of touch and pressure. * **A-delta (Aδ):** While thinner than Aα and Aβ, these are still **myelinated**. They are responsible for "fast" or "first" pain (sharp, localized) and cold temperature. **High-Yield Clinical Pearls for NEET-PG:** * **Susceptibility to Blockade:** * **Local Anesthetics:** Block **Type C fibers first** (smallest diameter) and Type A-alpha last. * **Pressure:** Affects **Type A fibers first** (e.g., "limb falling asleep"). * **Hypoxia:** Affects **Type B fibers first**, followed by Type A and then C. * **Type B fibers:** These are preganglionic autonomic fibers; they are myelinated but have a smaller diameter than Type A fibers. * **Rule of Thumb:** Conduction velocity in myelinated fibers (m/s) is approximately **6 × diameter (μm)**.

Question 1

Fast axonal transport is a component of orthograde axonal transport. What is the rate of fast axonal transport?

Accepted Answer

400 mm/d

Answer

100 mm/d

Answer

200 mm/d

Answer

600 mm/d

Question 2

At the neuromuscular junction:

Accepted Answer

The motor nerve endings secrete acetylcholine.

Answer

The muscle membrane possesses muscarinic receptors.

Answer

The motor nerve endings secrete norepinephrine.

Answer

Curare leads to prolongation of neuromuscular transmission.

Question 3

What is the twitch of a single motor unit called?

Accepted Answer

Fasciculation

Answer

Myoclonic jerk

Answer

Tremor

Answer

Fibrillation

Question 4

What does the 'all or none' phenomenon mean in physiology?

Accepted Answer

An action potential has a fixed, all-or-nothing value.

Answer

The magnitude of a response is related to the strength of the stimulus.

Answer

Action potentials can be developed by subthreshold stimuli.

Answer

The magnitude of a response is related to the duration of the stimulus.

Question 5

Rheobase indicates:

Accepted Answer

Magnitude of current

Answer

Rate of discharge

Answer

Velocity of nerve conduction

Answer

Specificity of impulse transmission

Question 6

The 'all or none' law is obeyed by which of the following potentials?

Accepted Answer

Action potential

Answer

Postsynaptic potential

Answer

Non-propagated potential

Answer

Spike potential

Question 7

All of the following statements regarding activity at the neuromuscular junction are true EXCEPT:

Accepted Answer

Ach stimulates muscarinic cholinergic receptors on the muscle endplate.

Answer

Action potentials on the alpha motor neuron release acetylcholine (Ach) from the axon terminal.

Answer

Cholinergic receptors on the muscle endplate are cation channels.

Answer

Alpha motor neurons excite, and Renshaw cells inhibit, the muscle endplate.

Question 8

A 10-year-old boy presents with increasing muscle weakness and elevated creatine phosphokinase (CPK) levels. What is the most likely cellular defect?

Accepted Answer

Muscle fibers

Answer

Neurons

Answer

Basement membrane

Answer

All body cells

Question 9

Which electrical event is characteristic of inhibitory synaptic interactions?

Accepted Answer

A neurotransmitter agent that selectively opens ligand-gated chloride channels is the basis for an inhibitory postsynaptic potential.

Answer

Because the Nernst potential for chloride is about -70 mV, chloride ions tend to move out of the cell along its electrochemical gradient.

Answer

A neurotransmitter that selectively opens potassium channels will allow potassium to move into the cell.

Answer

An increase in the extracellular sodium concentration usually leads directly to an inhibitory postsynaptic potential.

Question 10

Which of the following nerve fibers is unmyelinated?

Accepted Answer

C fiber

Answer

A-alpha

Answer

A-beta

Answer

A-delta

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?