& x20;In Myasthenia Gravis, why does repetitive nerve stimulation show a decremental response? - INI-CET EXAM -2025

INI-CET EXAM -2025

Pharmacology

Medium

Appeared in: INI-CET EXAM -2025

Explanation

Myasthenia Gravis (MG) is an autoimmune disease where antibodies destroy acetylcholine receptors (AChRs) on the postsynaptic membrane of the neuromuscular junction.
This destruction leads to a significant reduction in the number of functional AChRs, which diminishes the 'safety factor' for neuromuscular transmission.
During repetitive nerve stimulation, a normal physiological decrease in acetylcholine release (presynaptic rundown) occurs.
The combination of fewer available receptors and less acetylcholine with each impulse causes a progressive failure of muscle fibers to activate, resulting in a decremental (weakening) response.

Option A: This describes the primary pathophysiology of Lambert-Eaton Myasthenic Syndrome (LEMS), not Myasthenia Gravis. In LEMS, autoantibodies target presynaptic calcium channels, impairing acetylcholine release.
Option B: The activity of acetylcholinesterase is not increased in Myasthenia Gravis. In fact, medications used to treat MG are acetylcholinesterase inhibitors, which work by increasing the amount of acetylcholine in the synaptic cleft to improve the chance of receptor binding.
Option D: This is the specific pathophysiology of Lambert-Eaton Myasthenic Syndrome (LEMS). Autoantibodies block presynaptic voltage-gated calcium channels, which prevents the normal release of acetylcholine.

Visual 1: Diagram - A comparison of a normal neuromuscular junction and a myasthenic neuromuscular junction. The myasthenic junction should show a simplified postsynaptic fold and a greatly reduced number of acetylcholine receptors.
Visual 2: Graph - An example of a decremental response on an electromyogram (EMG) during repetitive nerve stimulation, showing the amplitude of the compound muscle action potential (CMAP) decreasing with each successive stimulus.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Pathophysiology of Myasthenia Gravis and its electrophysiological findings as background academic context rather than a clinical decision trigger.
Repetitive nerve stimulation is a key diagnostic test to confirm Myasthenia Gravis and differentiate it from other conditions that cause weakness.
A decremental response of greater than 10% is considered positive for a postsynaptic neuromuscular junction disorder like MG.
Understanding this mechanism is crucial for nurses to appreciate why patients with MG experience fatigability that worsens with activity and improves with rest.

How to Approach the Question

First, identify the core disease in the question: Myasthenia Gravis (MG).
Recall the fundamental pathophysiology of MG. The key defect is autoimmune destruction of postsynaptic acetylcholine receptors (AChRs).
Analyze the question's specific focus: the 'decremental response' to 'repetitive nerve stimulation'.
Connect the pathophysiology to the clinical test. How would a reduced number of receptors affect the muscle's response to repeated signals?
Evaluate each option: Option A and D describe Lambert-Eaton syndrome (a presynaptic problem). Option B is incorrect as the enzyme's activity isn't the primary issue. Option C directly states the core pathology of MG (reduced postsynaptic receptors), which explains the phenomenon.
Therefore, the reduced number of receptors is the reason why the muscle's response weakens (decrements) as the naturally declining acetylcholine release with each stimulus has fewer and fewer targets to hit.

Concept Tested & Keywords

Concept Tested: Pathophysiology of Myasthenia Gravis and its electrophysiological findings.
Stem keywords: Myasthenia Gravis, repetitive nerve stimulation, decremental response
Lead-in keywords: why
Negative lead-in flag: false

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