Bad Fish: a Case on Nervous Tissue

One Evening during a recent trip to Indonesia, Dr. Marshall Westwood sat down to a meal of pufferfish and rice. Within an hour of returning to his hotel room, Dr. Westwood felt numbness in his lips and tongue, which quickly spread to his face and neck. Before he could call the front desk, he began to fell pains in his stomach and throat, which produced feelings of nausea and eventually severe vomiting.

Fearing that he had eaten some “bad fish” for dinner, Dr. Westwood called a local hospital to describe his condition. The numbness is his lips and face made it almost impossible for him to communicate, but the hospital staff managed to at least understand the address he gave them and they sent an ambulance. As Dr. Westwood was rushed to the hospital, he breathing became increasingly labored.

The patient presented in the ED with diaphoresis, motor dysfunction, paresthesias, nausea, and an ascending paralysis that started in his legs and spread to the upper body, arms, face, and head. The patient was cyanotic and hypoventilating. Within 30 minutes of presenting in the ED, Dr. Westwood developed bradycardia with a BP of 90/50 mmHg. Atropine was administered in response to the bradycardia. Intravenous hydration, gastric lavage, and activated charcoal followed a presumptive diagnosis of tetrodotoxin poisoning that was based on the clinical presentation in the ED. Five hours after treatment, the following vital signs were noted:

¥ BP 125/79 mmHg

¥ HR 78 bpm

¥ Oxygen saturation: 97% on room air

After discussing his case with his physician, he learned that he had probably been the victim of puffer fish poisoning. The active toxin in the tissues of this fish is a chemical called tetrodotoxin (TTX). Tetrodotoxin is in a class of chemicals known as neurotoxins because it exerts its effects on neurons. The specific action of tetrodotoxin is that it blocks voltage-gated sodium ion channels.

Part A

Which of the following best describes a voltage-gated sodium ion...