Repetitive nerve stimulation in myasthenia gravis
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Repetitive nerve stimulation in myasthenia gravis
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[ID] => 554608
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[post_date] => 2024-12-23 18:14:00
[post_date_gmt] => 2024-12-23 23:14:00
[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
Repetitive nerve stimulation (RNS) electromyography is a test commonly used to diagnose myasthenia gravis (MG), a disease caused by autoantibodies that target and competitively inhibit the acetylcholine receptor (AChR) on the motor end plate of the neuromuscular junction. The test measures the amplitude of action potentials while a muscle is repetitively stimulated at a frequency of 2 to 5 Hz. A reproducible decrease in the amplitude of the signal by ≥ 10% by the fourth or fifth stimuli is considered diagnostic for MG. RNS results for normal and MG individuals are shown in Figure 1.
Figure 1 RNS tracings from a normal subject (A), an individual with MG (B and C), the individual with MG immediately following 30 seconds of exercise (D), and the individual with MG 4 minutes after 30 seconds of exercise (E)
Single-fiber electromyography (SFEMG) is a more sensitive diagnostic test for MG. SFEMG is performed with a specialized needle electrode that allows simultaneous recording of the action potentials of two muscle fibers innervated by the same motor axon. The variability in the interpotential interval (IPI) – the time between the action potentials of the two muscle fibers – is called "jitter". Increased jitter occurs when neuromuscular transmission is impaired because the physiological excess in the amount of neurotransmitter required to produce an action potential is increased, leading to variable timing of nerve impulse transmission between the first and second muscle. SFEMG results for an MG patient are shown in Figure 2.
Figure 2 Single-fiber electromyography (SFEMG) set-up that is monitoring the action potentials of the muscle fibers innervated by the same motor neuron (Figure 2A); SFEMG results show the end-plate potential (EPP2) of the second muscle fiber (Figure 2B, left pane) and the corresponding interpotential interval (IPI) between the first and second muscle fiber (Figure 2B, right pane)
Information and images adapted from Myasthenia Gravis: Diagnosis. Meriggioli Matthew et al. Semin Neurol 2004; 24(1): 31-39
[post_title] => Repetitive nerve stimulation in myasthenia gravis
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[question] => Which best describes the neuron shown in Figure 2A?
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[answer] => 2
[description] => Reason for Correct Answer:
The nervous system is divided into the central nervous system (brain and spinal cord) and peripheral nervous system (nerves and neurons in the body). The peripheral nervous system is further divided into the somatic and autonomic nervous systems.
The somatic nervous system is the voluntary component of the nervous system that includes motor neurons (like that shown in Figure 2A) that innervate skeletal muscles, whereas the autonomic nervous system includes sympathetic and parasympathetic divisions that control automatic/involuntary processes in the body.
Afferent neurons carry information from the body back to the central nervous system, whereas efferent neurons carry information from the brain to the body. Motor neurons are therefore efferent neurons.
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[each_answer] => A. Afferent neuron in the somatic nervous system
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[each_answer] => B. Efferent neuron in the somatic nervous system
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[each_answer] => C. Afferent neuron in the autonomic nervous system
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[each_answer] => D. Efferent neuron in the autonomic nervous system
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Given the pathology of MG, what difference in action potential generation would be expected between an MG patient and a person without MG?
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[answer] => 1
[description] => Reason for Correct Answer:
Remember that an action potential is an “all or nothing” response that occurs once the neuronal membrane is depolarized to its threshold potential. Binding of neurotransmitters to their receptors opens ion channels that contribute to this depolarization.
The passage states that MG patients suffer from antibodies that act as competitive inhibitors of acetylcholine receptors (AChR) at the neuromuscular junction. The antibodies targeting the receptors also likely cause receptor degradation by the immune system.
The competitive inhibition can be overcome by increasing the substrate concentration. And, if fewer receptors are present, this likely also means that a larger percentage of receptors will need to be activated to stimulate an action potential.
This all means that higher than normal levels of neurotransmitter (ACh) in the synapse will be needed in order to bind to and stimulate enough receptors to trigger an action potential in the postsynaptic muscle cells.
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[each_answer] => A. MG patients require more neurotransmitters in the synaptic cleft to produce an action potential.
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[each_answer] => B. MG patients have a higher than normal threshold potential required to trigger an action potential.
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[each_answer] => C. MG patients have lower than normal levels of neurotransmitter in the synaptic cleft.
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[each_answer] => D. MG patients have a postsynaptic membrane that is more polarized at rest.
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => What does Figure 1 suggest about the relationship between MG symptoms and exercise?
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[answer] => 2
[description] => Reason for Correct Answer:
Figure 1 shows how individuals with MG (graphs B and C) have reduced amplitudes of axon potentials on repeat nerve stimulation (RNS), compared to normal individuals.
Figures 1D and 1E show the results of RNS on a patient directly after exercise (graph D) or 4 minutes after exercise (graph E).
According to Figure 1D, the patient had a more normal RNS result directly after exercise; however, according to Figure 1E, this result became abnormal again by 4 minutes after exercise.
This suggests that the effects and therefore symptoms of MG may improve immediately after exercise but only temporarily.
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[each_answer] => A. Longer exercise durations exacerbate symptoms more than shorter durations.
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[each_answer] => B. The symptoms of MG are temporarily relieved following exercise.
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[each_answer] => C. The symptoms of MG tend to improve with time following exercise.
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[each_answer] => D. Exercise is likely harmful for individuals who have MG.
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[question] => Which statement about SFEMG results is supported by Figure 2B?
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[answer] => 1
[description] => Reason for Correct Answer:
The dotted lines that lead from bars 3 and 6 to the corresponding waveforms make it clear that the EPP2s are ordered from left to right and the corresponding axon potentials are ordered from top to bottom. So, you can assess how the EPP2 (end plate potential of the second muscle) relates to the IPI (interpotential interval) on each wave form.
You can see from the data that the IPI varies a lot between the waveforms, so you can eliminate Choice D. Also, notice that measurements with the highest EPP (like those marked 1 and 5 here) correspond to the shortest IPI, so you can eliminate Choice C as well.
The dotted lines on the graph point from the EPPs below the threshold value to their corresponding waveforms (all marked red here). Notice that there is no second axon potential shown on the waveforms – there is just a flat line after the first axon potential is generated. This means that when the EPP2 is below threshold, the second muscle does not fire; in other words, neuromuscular transmission fails in this case.
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[each_answer] => A. Neuromuscular transmission may fail if the EPP2 amplitude fails to reach a threshold value.
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[each_answer] => B. An EPP2 well above the threshold value may result in an absence of muscle firing.
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[each_answer] => C. longer IPI corresponds to a greater EPP2 in the second muscle that fires.
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[each_answer] => D. The IPI is fairly consistent between recordings in patients with MG.
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[quiz_unique_key] => 1403770772
[question] => Based on these findings, how does neostigmine compare to pyridostigmine in regards to onset of action and duration of action?
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[answer] => 3
[description] => Reason for Correct Answer:
In the case of these treatments for MG, a medication “working” in most patients would be seen as most patients testing negative for MG on RNS analysis.
The graph shows that, for neostigmine, most patients start responding to the drug by 30 minutes, but positive responses to the medication start to decline by 90 minutes.
The graph also shows that, for pyridostigmine, most patients start responding to the drug by 60 minutes, and positive responses do not decline during the 150-minute study.
This suggests that, compared to neostigmine, pyridostigmine takes longer to kick in (longer onset) but its effects last longer (longer duration). Vice versa, neostigmine has a shorter onset and a shorter duration of action.
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[each_answer] => A. Neostigmine has a shorter onset of action and a longer duration of action.
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[each_answer] => B. Neostigmine has a longer onset of action and a shorter duration of action.
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[each_answer] => C. Neostigmine has a shorter onset of action and a shorter duration of action.
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[each_answer] => D. Neostigmine has a longer onset of action and a longer duration of action.
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[quiz_unique_key] => 1325138223
[question] => According to these findings, what percentage of subjects with myasthenia gravis have a false negative result with RNS electromyography testing?
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[answer] => 2
[description] => Reason for Correct Answer:
A false negative occurs when a subject with a disease (ie. myasthenia gravis) tests negative for the disease (ie. negative RNS test).
All of the test subjects have myasthenia gravis, but the placebo group can be used as a control to determine what percentage of subjects with myasthenia gravis will have a false negative, without any pharmacological intervention.
According to the graph, about 20–30% of this group has a false negative result on RNS.
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[each_answer] => A. 0-10%
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[each_answer] => B. 20-30%
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[each_answer] => C. 40-50%
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[each_answer] => D. 70-80%
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Figure 1
Figure 2
