Case Study One – Bad Fish: A Case on Nervous Tissue One evening during a recent trip to Indonesia, Dr. Marshall Westwood sat down to a meal of puffer fish 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 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. Define the following phrases and terms associated with the signs and symptoms of Dr. Westwood’s TTX poisoning: diaphoresis motor dysfunction paresthesias cyanotic hypoventilating bradycardia gastric lavage oxygen saturation 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? 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. 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? Describe the role of sodium ions and sodium channels in generating an action potential. 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. Now that you have addressed some of the basic biology of this case, explain why Dr. Westwood experienced numbness after eating the puffer fish meal. Paralysis is a term used to describe the loss of muscle function. If tetrodotoxin’s effect is on neurons, why did Dr. Westwood experience paralysis? Case Study 2 – “My Leg is on Fire”: A Case Study on Spinal and Peripheral Nerve Anatomy Sarah Mitchell is a 68-year-old female who is normally healthy. However, about five days ago she began to feel very fatigued and started to experience a burning and tingling sensation on her right thigh. You ask to see the area and upon visual inspection you notice 3–4 small, red, swollen areas with vesicles on the posterior aspect of her right thigh. She describes the pain to you, saying “it feels like the back of my leg is on fire and it hurts so bad.” She denies being exposed to any excessive heat sources, any changes in her diet, and any changes in the type of body soap, lotion, or laundry detergent she is using. All other physical findings are within normal limits, but her oral temperature is 100.6˚F. She complains about being under a lot of stress for the past three months because she has been helping take care of her husband, who is in the end stages of Alzheimer’s disease. She has no known drug allergies, is a non-smoker, and attends a water aerobics class twice a week. You suspect she may be suffering from a particular viral infection, so you ask if she had chicken pox as a child. Sarah confirms that she had chicken pox and measles during childhood. Her answer confirms your suspicions that she is likely suffering from shingles (herpes zoster) due to varicella-zoster virus infection. Define the following terms, used in the case and also in associated questions. paresthesia skin vesicle dorsal ventral dormant The list below classifies sensory receptors by the stimulus that activates them. Using your textbook or other resource, name the receptor for each stimulus type. activated by touch, pressure, vibration, and stretch. activated by changes in temperature activated by light or changes in light wavelengths activated by changes in chemical concentrations in a solution activated by tissue injury or situations that may lead to tissue injury Based on what you know about their stimuli, which type of sensory receptor is activated and causing Sarah’s symptoms of pain and paresthesia? The virus infecting Sarah lies dormant in the dorsal root ganglion. What part of a neuron is located in the dorsal root ganglion? Does the dorsal root and its ganglion carry sensory input, motor output, or both? Based on the pattern of skin vesicles and pain, which of Sarah’s peripheral nerves is infected? To which peripheral nerve plexus does this nerve belong? If Sarah had skin vesicles on the anterolateral region of her neck, which peripheral nerve is infected by the virus? To which peripheral nerve plexus does this nerve belong? If Sarah had a viral infection that affected neuron function in the ventral root of the same spinal nerve, how would the signs and symptoms be different than those she has now? If we traced the sensory pathways that carry Sarah’s thigh pain up to her brain, which cerebral lobe AND which part of the cortex would be active when she feels this sensation?
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