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 feel 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 in 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, his 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 pufferfish 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.
Short Answer Questions
- Define the following phrases and terms associated with the signs and symptoms of Dr. Westwood’s TTX poisoning:
Diaphoresis - sweating, especially to an unusual degree as a symptom of disease or a side effect of a drug.
motor dysfunction - motor, pertaining to a muscle, nerve, or center that produces or affects movement. motorneuron, one of the various efferent nerve cells that transmit nerve impulses from the brain or from the spinal cord to muscular or glandular tissue.
Paresthesias - Paresthesia is an abnormal sensation such as tingling, tickling, pricking, numbness or burning of a person's skin with no apparent physical cause.
cyanotic - Cyanosis refers to bluish discoloration of skin, nail beds and mucous membranes.
hypoventilating In medicine, hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (hypo meaning "below") to perform needed gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia) and respiratory acidosis.
bradycardia – abnormally slow heart beat
Gastric lavage, also commonly called stomach pumping or gastric irrigation, is the process of cleaning out the contents of the stomach. It has been used for over 200 years as a means of eliminating poisons from the stomach.
oxygen saturation - Oxygen saturation is a term referring to the fraction of oxygen-saturated hemoglobin relative to total hemoglobin (unsaturated + saturated) in the blood. The human body requires and regulates a very precise and specific balance of oxygen in the blood. Normal blood oxygen levels in humans are considered 95-100 percent.
- As mentioned in the case description, tetrodotoxin is a molecule that blocks voltage-gated sodium ion channels. What is a voltage-gated sodium ion channel and what is its function?
A voltage gated sodium channel is protein located in the cell membrane that allows sodium to pass into the cell when it reaches a certain voltage.
- When nerve cells are at rest, there is an unequal amount of positive and negative charges on either side of a nerve cell membrane. This charge difference creates an electrical potential. Describe how the resting membrane potential (resting potential)is generated.
The resting membrane potential is generated by the Na+K+ ATPase pump which pumps three sodium out and two potassium in for each ATP that is used.
- What is happening to the electrical potential of a neuron when it generates an action potential?What is the function of the action potential in neurons?
An action potential arises when there is a domino like influx of sodium due to the wave-like influx of sodium ions along the axon of a nerve cell.
- What would happen to a neuron if it was exposed to tetrodotoxin? Be specific regarding its effect on the ability of a neuron to communicate.
Tetrodotoxin is a sodium channel blocker. It inhibits the firing of action potentials in neurons by binding to the voltage-gated sodium channels in nerve cellmembranes and blocking the passage of sodium ions (responsible for the rising phase of an action potential) into the neuron. This prevents the nervous system from carrying messages and thus muscles from flexing in response to nervous stimulation
- Now that you have addressed some of the basic biology of this case, explain why Dr. Westwood experienced numbness after eating the pufferfish meal.
Tetrodotoxin is a sodium channel blocker which interferes with action potentials from being fired along an axon.