Case 1 Derek Zoolander is a 32-year-old fashion model for a world famous modelin

Question

Case 1 Derek Zoolander is a 32-year-old fashion model for a world famous modeling agency. Over the last 8 months, he experienced "strange" symptoms. He had severe eyestrain when he modeled for longer than 15 minutes. He became tired when he did common tasks, such as eating, and he had extreme fatigue on the job. Despite his strong work ethic, Derek had to excuse himself from several photo shoots because he simply could not pose. Derek is not a complainer, but he began to worry about these vague symptoms. His physician suspected myasthenia gravis. While awaiting the results of a serum antibody test, the physician initiated a trial of an acetylcholinesterase inhibitor. Derek immediately felt better while taking the drug; his strength returned and he could work normally. Meanwhile, the results of the antibody test were positive, confirming the diagnosis of myasthenia gravis.

1. What are the steps of neuromuscular transmission? 2. What step of neuromuscular transmission is blocked by myasthenia gravis (review pages 86-87 in Guyton and Hall)? 3. Explain why muscle weakness occurs in myasthenia gravis. 4. How does curare compare to myasthenia gravis in action?

Explanation / Answer

1. A connection between a neuron and another neuron/mucle fibre is called as a synapse. A neuromuscular transmission takes place in between a neuron and a muscle fibre. The synapse formed between neurons and the muscle fibre is chemical in nature. Upon an excitatory action potential the voltage gated channels of the pre synaptic neurons open up allowing Calcium ions to enter it. Calcium ions bind to sensor proteins called synaptotagmin on synaptic vesicles and release neurotransmitter called acetylcholine (ACh). These ACh molecules in turn then bind to the nicotinic ACh receptors which causes cascade of reactions causing the mucle to contract appropriately.

2. Myasthenia gravis is an autoimmune disease where a body's immune system produces antibodies against self molecules. In this case the antibodies bind to postsynaptic ACh receptors on the muscles which inhibit the excitatory signals received from the presynaptic neuron.

3. Weakness occurs in this disease because the necessary muscle movement fails to take place which can aid to proper functioning of the muscles.

4. Curare is an alkaloid poison. The action of curare is similar to the condition in myasthenia gravis. Here also curare competitively and reversibly blocks the nicotinic ACh receptors found on postsynaptic mucle fibres which fails to depolarize the muscle.

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