Cells of the Nervous System
Which structure is responsible for receiving input in a neuron?
a) Axon
b) Dendrite
c) Myelin sheath
d) Synapse
Correct Answer: b) Dendrite
Explanation: Dendrites are the parts of the neuron that receive incoming signals, while axons send signals out.
A 60-year-old man with a known history of small cell lung carcinoma presents with weakness predominantly affecting the proximal muscles. Neurological examination reveals hyporeflexia, and his symptoms improve with sustained muscle activity. Despite treatment with pyridostigmine, his weakness persists. Electromyography reveals an incremental response with repetitive nerve stimulation. What is the primary pathophysiological process underlying this condition?
A) Autoimmune destruction of the postsynaptic acetylcholine receptors
B) Inhibition of acetylcholinesterase activity at the neuromuscular junction
C) Impaired release of acetylcholine due to antibodies against presynaptic voltage-gated calcium channels
D) Toxin-mediated blockade of acetylcholine release at the neuromuscular junction
Answer: C) Impaired release of acetylcholine due to antibodies against presynaptic voltage-gated calcium channels
Explanation: The patient’s symptoms, history of small cell lung cancer, and electrophysiological findings of an incremental response on repetitive nerve stimulation are consistent with Lambert-Eaton Myasthenic Syndrome (LEMS). In LEMS, autoantibodies target presynaptic voltage-gated calcium channels, impairing acetylcholine release. This differentiates it from myasthenia gravis, which involves postsynaptic acetylcholine receptor antibodies.
A 35-year-old woman presents with fluctuating muscle weakness, which worsens with activity and improves with rest. She has a history of ptosis and diplopia that fluctuate throughout the day. Electromyography shows a decremental response on repetitive nerve stimulation. What is the next best step in diagnosis?
A) Serum anti-acetylcholine receptor antibody test
B) Tensilon test (edrophonium test)
C) Muscle biopsy
D) Lumbar puncture
Answer: A) Serum anti-acetylcholine receptor antibody test
Explanation: The clinical presentation and decremental response on repetitive nerve stimulation are suggestive of myasthenia gravis (MG), a condition characterized by autoantibodies against acetylcholine receptors at the neuromuscular junction. The serum anti-acetylcholine receptor antibody test is the next best step to confirm the diagnosis. The Tensilon (edrophonium) test is less commonly used due to the availability of more specific and safer tests. Muscle biopsy and lumbar puncture are not typically used for diagnosing MG.
A 38-year-old man presents with complaints of fatigue and muscle weakness that worsens throughout the day. On examination, he has ptosis and diplopia. He is started on pyridostigmine, which leads to symptomatic improvement. What is the mechanism of action of this medication in treating his condition?
A. Inhibition of acetylcholinesterase, increasing acetylcholine availability at the neuromuscular junction
B. Blockade of presynaptic calcium channels, enhancing neurotransmitter release
C. Direct stimulation of nicotinic receptors at the neuromuscular junction
D. Reduction of autoantibody production against acetylcholine receptors
E. Inhibition of acetylcholine release from presynaptic vesicles
A. Inhibition of acetylcholinesterase, increasing acetylcholine availability at the neuromuscular junction
Explanation: Pyridostigmine is a cholinesterase inhibitor that increases the availability of acetylcholine at the neuromuscular junction by inhibiting the enzyme acetylcholinesterase. This action enhances neuromuscular transmission and muscle contraction, improving the symptoms of myasthenia gravis. Other options do not represent the mechanism of action for pyridostigmine in this context.
A newborn is delivered at term to a mother with myasthenia gravis. The neonate presents with poor sucking and generalized hypotonia. Which of the following is the most likely diagnosis?
A) Neonatal myasthenia gravis
B) Infant botulism
C) Spinal muscular atrophy
D) Congenital hypothyroidism
E) Down syndrome
A) Neonatal myasthenia gravis
Explanation: Neonatal myasthenia gravis occurs due to the transplacental transfer of maternal antibodies against acetylcholine receptors, leading to symptoms like hypotonia and poor sucking in the newborn. These symptoms are typically transient, resolving as the maternal antibodies are cleared from the infant’s circulation. Other conditions, such as infant botulism or spinal muscular atrophy, present with similar features but have different etiologies and would not be directly related to maternal myasthenia gravis.
What stain is used to visualize dendrites and cell bodies?
a) Hematoxylin
b) Nissl stain
c) Eosin
d) Gram stain
Correct Answer: b) Nissl stain
Explanation: Nissl staining is used to stain the rough endoplasmic reticulum (RER) in neurons, which is present in dendrites and cell bodies but not in axons.
A 48-year-old woman presents with bilateral facial weakness, difficulty swallowing, and generalized weakness. She reports eating at a local restaurant two days ago where she had a homemade salad. Neurological examination shows ptosis, ophthalmoplegia, and a descending pattern of paralysis with preserved mental status. Which diagnostic test would most likely confirm the diagnosis?
A) Repetitive nerve stimulation test showing incremental response
B) Detection of botulinum toxin in serum, stool, or gastric contents
C) Detection of acetylcholine receptor antibodies in serum
D) MRI of the brain and spinal cord
Answer: B) Detection of botulinum toxin in serum, stool, or gastric contents
Explanation: The patient’s presentation of acute, descending paralysis, cranial nerve involvement (ptosis and ophthalmoplegia), and recent consumption of homemade food suggests botulism, a neuromuscular junction disorder caused by botulinum toxin. Confirmation is through detection of the toxin in biological samples. This is crucial for differentiating botulism from other causes of neuromuscular paralysis, such as Guillain-Barré syndrome or myasthenia gravis.
A 28-year-old woman is suspected of having myasthenia gravis due to fluctuating muscle weakness. Repetitive nerve stimulation shows a decremental response. She tests negative for anti-AChR and anti-MuSK antibodies. Which of the following is the next best step in management?
A. Start treatment with corticosteroids
B. Perform a chest CT to look for thymoma
C. Repeat antibody testing in 6–12 months
D. Order a muscle biopsy
E. Refer for thymectomy
C. Repeat antibody testing in 6–12 months
Explanation: In patients suspected of having myasthenia gravis who are negative for anti-AChR and anti-MuSK antibodies, it is appropriate to repeat antibody testing in 6–12 months as some patients may seroconvert over time. A chest CT to evaluate for thymoma is also indicated but does not confirm MG on its own. Starting corticosteroids or proceeding with thymectomy would typically be considered after diagnosis is confirmed.
1. Seronegative Myasthenia Gravis:
2. Sensitivity of the Test:
A 50-year-old woman with generalized myasthenia gravis is being treated with pyridostigmine but continues to experience significant muscle weakness. What is the most appropriate next step in management?
A. Increase the dose of pyridostigmine
B. Start glucocorticoids and/or azathioprine
C. Perform plasmapheresis
D. Refer for thymectomy
E. Prescribe neostigmine
B. Start glucocorticoids and/or azathioprine
Explanation: For patients with myasthenia gravis who have inadequate symptom control with pyridostigmine, the next step is to initiate immunosuppressive therapy, such as glucocorticoids or azathioprine. Increasing pyridostigmine beyond the optimal dose does not usually improve symptoms and can cause side effects. Plasmapheresis is typically reserved for acute exacerbations or myasthenic crisis. Thymectomy is considered in select cases but is not the immediate next step for treatment escalation.
A 32-year-old woman with well-controlled myasthenia gravis is planning her delivery. Which of the following factors most likely increases her risk of requiring a cesarean section?
A) Intrauterine growth restriction
B) Ineffective uterine contractions
C) Macrosomia
D) Preeclampsia
E) Premature rupture of membranes
B) Ineffective uterine contractions
Explanation: Myasthenia gravis affects skeletal muscles, which can lead to ineffective uterine contractions during labor, increasing the likelihood of a cesarean section. Muscle weakness can lead to prolonged labor and risk of maternal exhaustion, necessitating surgical intervention. Other factors listed, such as macrosomia or preeclampsia, are not specific to myasthenia gravis and do not have the same direct impact on muscle strength or function.
Which glial cells form the blood-brain barrier?
a) Schwann cells
b) Ependymal cells
c) Astrocytes
d) Oligodendrocytes
Correct Answer: c) Astrocytes
Explanation: Astrocytes are involved in forming the blood-brain barrier, which protects the brain from harmful substances in the bloodstream.
A 28-year-old woman with myasthenia gravis presents with acute onset of dyspnea and inability to lift her head off the bed. She recently increased her dose of pyridostigmine due to worsening symptoms. Physical examination shows severe generalized muscle weakness and diaphoresis. Vital signs are significant for bradycardia and hypotension. Her pupils are constricted and non-reactive to light. What is the most appropriate next step in management?
A) Increase dose of pyridostigmine
B) Administer high-dose corticosteroids
C) Initiate plasmapheresis or IVIG
D) Discontinue pyridostigmine and administer atropine
Answer: D) Discontinue pyridostigmine and administer atropine
Explanation: The patient's symptoms of severe muscle weakness, bradycardia, hypotension, miosis, and diaphoresis suggest a cholinergic crisis, which is due to excessive acetylcholine from overuse of acetylcholinesterase inhibitors like pyridostigmine. This can mimic myasthenic crisis but is managed differently by stopping the acetylcholinesterase inhibitor and administering atropine, an anticholinergic agent. Myasthenic crisis would be treated with IVIG or plasmapheresis, not atropine.
A 55-year-old man presents with proximal muscle weakness and autonomic symptoms, including dry mouth and constipation. Physical examination reveals reduced reflexes and muscle strength that improves with repetitive activity. What is the most likely finding on repetitive nerve stimulation?
A. Decremental response
B. Incremental response
C. No change
D. Continuous muscle activity
E. Myotonia
B. Incremental response
Explanation: The patient's symptoms are indicative of Lambert-Eaton myasthenic syndrome (LEMS), which is characterized by proximal muscle weakness, autonomic symptoms, and reduced reflexes that improve with repetitive activity. The typical finding on repetitive nerve stimulation in LEMS is an incremental response, which differentiates it from myasthenia gravis (where a decremental response is seen).
A 42-year-old patient with myasthenia gravis is scheduled for a thymectomy. Which of the following statements regarding the potential benefits of thymectomy is true?
A. It only benefits patients with thymoma.
B. It is not recommended for patients with MuSK-antibody positive myasthenia gravis.
C. It guarantees complete remission of symptoms.
D. It is contraindicated in patients under 18 years old.
E. It should be avoided if immunosuppressive therapy is already effective.
B.
Thymectomy is a surgical option in the treatment of myasthenia gravis (MG) that can lead to reduced symptoms, fewer exacerbations, and a decreased need for immunosuppressive medications. The potential benefits and indications of thymectomy vary depending on the type of MG and the presence of thymoma:
Patients with Thymoma: Thymectomy is recommended for most patients with thymoma, regardless of antibody status, as it is part of the treatment for the tumor itself. Thymectomy can improve MG symptoms even in the absence of thymoma in some cases.
AChR-Antibody Positive MG: Thymectomy can also benefit patients with AChR-antibody positive generalized MG who do not have a thymoma. These patients often experience a reduction in symptoms and medication requirements post-thymectomy.
MuSK-Antibody Positive MG: Thymectomy is generally not recommended for patients with MuSK-antibody positive MG because this subtype of MG is not associated with thymic pathology, and these patients typically do not benefit from thymectomy.
Other Misconceptions:
A 28-year-old pregnant woman with myasthenia gravis is diagnosed with preeclampsia at 34 weeks of gestation. Which of the following medications is contraindicated for seizure prophylaxis in this patient?
A) Magnesium sulfate
B) Labetalol
C) Nifedipine
D) Hydralazine
E) Methyldopa
A) Magnesium sulfate
Explanation: Magnesium sulfate is contraindicated in patients with myasthenia gravis because it can worsen muscle weakness by inhibiting neuromuscular transmission. It competes with calcium at the neuromuscular junction, further exacerbating the symptoms of myasthenia gravis. Alternative medications such as labetalol or hydralazine, which do not impair neuromuscular transmission, are preferred for blood pressure control in preeclampsia.
Which cell type has a "fried egg" appearance histologically?
a) Astrocytes
b) Microglia
c) Oligodendrocytes
d) Schwann cells
Correct Answer: c) Oligodendrocytes
Explanation: Oligodendrocytes have a characteristic "fried egg" appearance on histological slides.
A 52-year-old man with a history of smoking and recently diagnosed small cell lung cancer presents with progressive muscle weakness, dry mouth, and bilateral ptosis. He reports symptoms of erectile dysfunction and difficulty initiating urination. On examination, he has absent deep tendon reflexes, and muscle strength improves significantly after repeated contractions. His symptoms have not improved with pyridostigmine. Which of the following additional clinical findings would most strongly support the diagnosis?
A) Autonomic dysfunction such as orthostatic hypotension
B) Presence of thymoma on chest imaging
C) Cranial nerve involvement with facial palsy and dysphagia
D) Sensory deficits in the lower extremities
Answer: A) Autonomic dysfunction such as orthostatic hypotension
Explanation: This patient likely has Lambert-Eaton Myasthenic Syndrome (LEMS), which is often paraneoplastic and associated with small cell lung cancer. LEMS involves impaired acetylcholine release at the neuromuscular junction due to autoantibodies against presynaptic voltage-gated calcium channels. A hallmark of LEMS is autonomic dysfunction (e.g., orthostatic hypotension, dry mouth). Myasthenia gravis typically does not present with autonomic symptoms, making autonomic dysfunction highly supportive of LEMS.
A 45-year-old woman with suspected myasthenia gravis undergoes a repetitive nerve stimulation study which shows a decremental response. Which additional test should be performed to assess for an associated condition commonly seen in myasthenia gravis?
A. MRI of the brain
B. Chest CT scan
C. Muscle biopsy
D. Genetic testing
E. Lumbar puncture
B. Chest CT scan
Explanation: A chest CT scan is recommended in all patients with confirmed myasthenia gravis to assess for the presence of a thymoma or thymic hyperplasia, which are commonly associated conditions. These thymic abnormalities are seen in a significant number of patients with MG and may influence treatment decisions. Other tests listed, such as MRI of the brain, muscle biopsy, genetic testing, and lumbar puncture, are not specifically indicated for MG or for assessing associated conditions like thymoma.
A 35-year-old man with myasthenia gravis is being managed on pyridostigmine and azathioprine. He presents to the clinic with new-onset dyspnea and worsening muscle weakness. Which of the following medications is most likely to exacerbate his symptoms?
A. Ibuprofen
B. Ciprofloxacin
C. Omeprazole
D. Acetaminophen
E. Lisinopril
B. Ciprofloxacin
Certain medications are known to exacerbate myasthenia gravis symptoms by affecting neuromuscular transmission. Ciprofloxacin, a fluoroquinolone antibiotic, is one of the medications that can worsen MG symptoms or even trigger a myasthenic crisis due to its neuromuscular blocking effects:
Fluoroquinolones (e.g., Ciprofloxacin): These antibiotics can impair neuromuscular transmission, making them particularly dangerous for patients with MG. They are known to block the neuromuscular junction, exacerbating muscle weakness and potentially leading to respiratory failure in severe cases.
Other Exacerbating Medications:
A 35-year-old woman with myasthenia gravis is concerned about her ability to breastfeed her newborn due to her condition and medications. Which of the following aspects should be discussed as a potential challenge during breastfeeding?
A) Risk of transmitting myasthenia gravis through breast milk
B) Possible exacerbation of maternal symptoms due to physical exertion
C) Decreased milk production due to anticholinesterase medications
D) Increased risk of neonatal hypoglycemia
E) Increased risk of neonatal jaundice due to maternal antibodies
B) Possible exacerbation of maternal symptoms due to physical exertion
Explanation: Breastfeeding can be physically demanding and may exacerbate the fatigue and muscle weakness experienced by women with myasthenia gravis. The condition itself does not transmit through breast milk, and while some medications may affect breastfeeding, the primary concern in this scenario is the potential physical exertion worsening maternal symptoms. Other listed options do not accurately reflect common challenges specific to breastfeeding in myasthenia gravis.
Which glial cells are derived from mesoderm?
a) Oligodendrocytes
b) Astrocytes
c) Microglia
d) Schwann cells
Correct Answer: c) Microglia
Explanation: Microglia, the phagocytic cells of the CNS, are derived from mesoderm, unlike other CNS glial cells.
A 6-month-old boy is brought to the emergency department by his mother because of poor feeding, especially when trying to breastfeed. The mother also reports that the boy's grip is weak and that he is constipated. He has met all developmental milestones and his immunizations are up-to-date. His diet consists primarily of breast milk, applesauce, and oatmeal sweetened with honey. Physical examination shows diffuse weakness and decreased breath sounds bilaterally. Deep tendon reflexes are decreased throughout. Which of the following labeled processes in the cholinergic synapse is most likely directly affected in this patient?
D shows exocytosis of acetylcholine-filled vesicles into the synaptic cleft. Botulinum toxin inhibits this process by cleaving the SNAP-25 SNARE protein that mediates the fusion of vesicles with the presynaptic membrane in response to a rise in intracellular Ca2+ concentration, which is caused by neuron depolarization. Without acetylcholine release into the synaptic cleft, muscle cells will not contract, leading to flaccid paralysis. Because botulinum toxin can inhibit cholinergic innervation of both nicotinic and muscarinic neurons, autonomic innervation of tear, sweat, and salivary glands can also be affected.
A 50-year-old woman is evaluated for ptosis and diplopia that worsen with prolonged activity. Physical examination reveals a positive Cogan lid twitch sign and an ice-pack test that improves her ptosis by 2 mm. What is the mechanism behind the ice-pack test in diagnosing her condition?
A. It increases acetylcholinesterase activity at the neuromuscular junction.
B. It decreases the degradation of acetylcholine in the synaptic cleft.
C. It improves neuromuscular transmission by reducing acetylcholine receptor degradation.
D. It inhibits the production of autoantibodies against acetylcholine receptors.
E. It stabilizes the neuromuscular junction by lowering local temperature.
Answer: E. It stabilizes the neuromuscular junction by lowering local temperature.
Explanation: The ice-pack test helps in diagnosing myasthenia gravis by lowering the local temperature around the neuromuscular junction, which reduces the activity of acetylcholinesterase. This leads to increased acetylcholine availability at the synaptic cleft, thereby temporarily improving muscle strength and reducing ptosis. This test is highly specific for MG and is used when ocular symptoms are present.
A 29-year-old woman with myasthenia gravis is started on prednisone for inadequate control of symptoms with pyridostigmine. Two weeks later, she develops increased weakness and difficulty swallowing. What is the most likely cause of her new symptoms?
A. Adverse reaction to prednisone
B. Worsening of myasthenia gravis due to prednisone-induced myasthenic crisis
C. Development of a new autoimmune condition
D. Incorrect pyridostigmine dosing
E. Infection triggering a myasthenic crisis
B. Worsening of myasthenia gravis due to prednisone-induced myasthenic crisis
Explanation: Glucocorticoids like prednisone can trigger a myasthenic crisis, especially within the first two weeks of initiating therapy. This exacerbation is due to a transient increase in weakness associated with steroid treatment. Monitoring and gradual dose adjustments are recommended to minimize this risk. Infections or other triggers should also be evaluated, but the timeline and recent initiation of prednisone suggest a steroid-induced myasthenic crisis.
A 31-year-old woman with myasthenia gravis is at 39 weeks of gestation. She has been treated with pyridostigmine and prednisone throughout her pregnancy, and her condition has remained stable. The patient goes into spontaneous labor and delivers a full-term male infant. Shortly after birth, the neonate exhibits generalized hypotonia, weak cry, and difficulty feeding. Which of the following interventions is most appropriate for the management of this neonate, and what is the primary concern for the infant’s immediate health?
A) Start intravenous calcium gluconate for suspected neonatal hypocalcemia
B) Begin phototherapy for jaundice due to maternal autoimmune disease
C) Administer intravenous immunoglobulin (IVIG) or perform plasmapheresis for severe neonatal myasthenia gravis
D) Provide respiratory support and consider acetylcholinesterase inhibitors for transient neonatal myasthenia gravis
E) Initiate immediate blood transfusion for suspected anemia of prematurity
D) Provide respiratory support and consider acetylcholinesterase inhibitors for transient neonatal myasthenia gravis
Explanation: The neonate's symptoms of generalized hypotonia, weak cry, and feeding difficulties are indicative of transient neonatal myasthenia gravis, caused by the transplacental transfer of maternal acetylcholine receptor antibodies. Management primarily involves supportive care, including respiratory support if necessary, and the use of acetylcholinesterase inhibitors to improve muscle strength temporarily. IVIG or plasmapheresis (Option C) is generally reserved for severe cases and is not the first-line treatment for transient symptoms in neonates. Other options, such as calcium gluconate (Option A), phototherapy (Option B), and blood transfusion (Option E), do not address the underlying condition of neonatal myasthenia gravis and are unrelated to the symptoms observed.
The primary concern for the infant’s immediate health is respiratory support, as neonatal myasthenia can compromise respiratory function, making it essential to monitor and manage breathing effectively.