A 34-year-old man comes to the emergency department complaining of weakness, dizziness, and paresthesias involving his face and extremities. He has never had symptoms like this before. Two hours earlier, he ate fish at a new Japanese restaurant. Neurological examination shows reduced muscle strength and decreased deep-tendon reflexes. Which of the following parts of the action potential curve is most likely impaired in this patient?

Question

A 34-year-old man comes to the emergency department complaining of weakness, dizziness, and paresthesias involving his face and extremities.  He has never had symptoms like this before.  Two hours earlier, he ate fish at a new Japanese restaurant.  Neurological examination shows reduced muscle strength and decreased deep-tendon reflexes.  Which of the following parts of the action potential curve is most likely impaired in this patient?

Quizplus · Accepted Answer

11ec160b_3b11_1852_9249_53d5169ff48a_MD0001_00 This patient's constellation of symptoms (ie, paresthesias, weakness, dizziness, nausea) and history of fish consumption are strongly suggestive of food poisoning by pufferfish, a Japanese delicacy. Pufferfish can be lethal if not properly prepared. It is necessary to remove the organs containing high quantities of tetrodotoxin, a toxin produced by microorganisms associated with the fish. Tetrodotoxin binds to voltage-gated sodium channels in nerve and cardiac tissue, preventing sodium influx and depolarization. Symptoms of tetrodotoxin poisoning occur quickly after ingestion and include dizziness, weakness, loss of reflexes, paresthesias of the face and extremities, nausea, vomiting, and diarrhea. Higher exposures can cause severe hypotension and general paralysis; death can occur from respiratory failure and hypotension. Treatment includes supportive care and intestinal decontamination with gut lavage and charcoal. (Choice A) The resting potential is equal to approximately -70 mV. It is maintained by high membrane K⁺ permeability and low Na⁺permeability. K⁺ efflux occurs via non-gated K⁺ channels (leak channels). (Choice C) Overshoot represents the maximal value of the action potential during which the membrane potential is positive. At this point, open sodium channels become inactivated and close, preventing further increase in the membrane potential. (Choice D) Repolarization results from closure of Na⁺ channels and simultaneous opening of K⁺ channels. This results in a sharp decrease in Na⁺ conductance and a significant increase in K⁺ conductance. Potassium efflux is responsible for returning the membrane potential to the resting potential. (Choice E) Hyperpolarization occurs for a short moment following repolarization due to the slow closure of voltage-gated potassium channels. The membrane potential becomes more negative than the normal resting potential and approaches the K⁺ equilibrium potential of -85 mV. When the voltage-gated K⁺ channels close, the membrane potential returns to the resting value. Educational objective: Pufferfish poisoning is caused by tetrodotoxin, a neurotoxin produced by microorganisms associated with the fish. Tetrodotoxin binds to voltage-gated sodium channels in nerve and cardiac tissue, preventing sodium influx and depolarization. __________ References: Tetrodotoxin: a brief history (http://www.ncbi.nlm.nih.gov/pubmed/18941294)

A 34-Year-Old Man Comes to the Emergency Department Complaining of Weakness