Nerve and Muscle Physiology Practice Questions

Q: What is the relationship between nerve thickness and conduction velocity of myelinated nerves?

Linear. **Explanation:** The relationship between the diameter of a myelinated nerve fiber and its conduction velocity is **linear**. According to the **Hursh transformation**, the conduction velocity (V) in meters per second is approximately 6 times the external diameter (D) in micrometers ($V \approx 6 \times D$). In myelinated nerves, conduction occurs via **saltatory conduction**. As the fiber diameter increases, internal axial resistance decreases significantly, and the distance between the Nodes of Ranvier (internodal length) increases. This allows the action potential to "jump" further and faster, resulting in a direct proportionality between thickness and speed. **Analysis of Options:** * **B. Parabolic & C. Hyperbolic:** These are incorrect because the increase in velocity does not follow a squared or inverse curve. While unmyelinated fibers show a relationship where velocity is proportional to the *square root* of the diameter (making it non-linear), myelinated fibers strictly follow a linear progression. * **D. No relation:** This is incorrect as fiber diameter is the primary anatomical determinant of conduction speed. **High-Yield Facts for NEET-PG:** * **Erlanger-Gasser Classification:** Type A fibers (thickest, myelinated) have the highest velocity, while Type C fibers (thinnest, unmyelinated) have the slowest. * **Myelination Effect:** Myelin increases conduction velocity by increasing membrane resistance and decreasing membrane capacitance. * **Clinical Correlation:** In demyelinating diseases like **Guillain-Barré Syndrome (GBS)** or **Multiple Sclerosis**, the loss of myelin disrupts this linear relationship, leading to significantly slowed conduction or conduction block.

Question 1

During a neurophysiology practical, a student applies two successive electrical stimuli to an isolated frog sciatic nerve preparation. The first stimulus is suprathreshold. The second stimulus, applied 4 milliseconds after the first (corresponding to the relative refractory period in this preparation), is stronger than the original suprathreshold stimulus, and is delivered at point C shown on the waveform. The recorded intracellular waveform is displayed on the oscilloscope (Image 1). Which of the following best explains the membrane state at the time the second stimulus was delivered?

Accepted Answer

B. Voltage-gated sodium channels are partially recovered from inactivation, allowing a suprathreshold stimulus to generate a reduced-amplitude action potential

Answer

A. Voltage-gated sodium channels are completely inactivated and cannot be reopened regardless of stimulus strength

Answer

C. Voltage-gated potassium channels are fully closed and the membrane potential has returned exactly to resting level

Answer

D. The membrane is in a state of hyperpolarisation due to electrogenic sodium-potassium ATPase activity, preventing any action potential generation

Question 2

Type I muscle fibers are rich in myosin heavy chain. What is their characteristic property?

Accepted Answer

Slow contracting, resistant to fatigue

Answer

Fast contracting, susceptible to fatigue

Answer

Slow contracting, susceptible to fatigue

Answer

Fast contracting, resistant to fatigue

Question 3

The number of muscle fibers innervated by a single motor axon is smallest in which of the following?

Accepted Answer

Orbicularis oculi

Answer

Gastrocnemius

Answer

Single unit smooth muscle

Answer

Soleus

Question 4

What is the relationship between nerve thickness and conduction velocity of myelinated nerves?

Accepted Answer

Linear

Answer

Parabolic

Answer

Hyperbolic

Answer

No relation

Question 5

In tetany, what causes the increased membrane excitability?

Accepted Answer

Depolarization of the nerve and muscle membranes

Answer

Decreased release of inhibitory neurotransmitter from nerve terminals

Answer

Spontaneous release of calcium from the sarcoplasmic reticulum

Answer

Activation of sodium channels at more negative membrane potentials

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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