Depаrtmentаl Neurоmusculаr Junctiоn Quiz Jarrоd was an active man who was extremely interested in physical fitness. Running, going to the gym, and being outdoors were a major part of Jarrod’s life. When Jarrod became unable to complete a run or his weight exercises, he became worried. He was experiencing muscle weakness and fatigue more and more frequently in his daily life. Where Jarrod had once been able to go to the gym three days a week for weight training and lift one hundred sixty pounds, he was now lucky to lift seventy-five pounds for just one full repetition before his muscles were no longer able to continue. His doctor told him he had an autoimmune disorder called Myasthenia gravis where the body’s immune system attacks the skeletal muscle cells, making them respond slower to the nervous system and present weaker contractions. Jarrod didn’t understand what any of that meant so the doctor proceeded to explain to him the importance of the nerves and muscles working together. The neuromuscular junction is an important communication point in the human body. This junction illustrates how the nervous system and the muscular system use electric and chemical signals to communicate. This communication allows for our conscious control of skeletal muscles, so it's vital to our daily functions. For this junction to work we must first produce an action potential in a motor neuron. This action potential will propagate down an axon and cause voltage-gated calcium channels to open. The calcium ion will enter the presynaptic terminal and initiate the release of the neurotransmitter acetylcholine. The acetylcholine will be released by exocytosis and diffuse across the synapse and bind to ligand-gated sodium channels on the postsynaptic membrane (skeletal muscle). This will cause the ligand-gated sodium channels to open and allow sodium to enter the cell membrane of the skeletal muscle. Once threshold is passed, an action potential is generated along the cell membrane of the skeletal muscle. Muscle contraction will occur soon after. This figure is associated with the related text information and to answers to questions 1 – 10 following. For the muscle to relax, acetylcholine must be removed from the sodium channels by the enzyme acetylcholinesterase. This will break down the acetylcholine into acetic acid and choline. The choline will be transported back into the terminal end of the axon and the acetic acid diffuses away from the synapse. The doctor proceeded to explain that in individuals with Myasthenia gravis, the person’s immune system attacks and destroys the ligand-gated sodium channels on the postsynaptic membrane of the skeletal muscle cell. With fewer ligand-gated sodium channels, it takes longer for the threshold to be met to generate an action potential and weakens the contraction strength of the muscle. Jarrod was shocked to learn that it was his own body that had caused the disease and to also learn that his condition was incurable. His doctor informed him that he could use corticosteroids and immunosuppressant medicines to prevent any further attacks, but the current damage was permanent. Jarrod would be unable to continue living as physically active as he had previously been, but in time he would learn the extent of his physical capabilities and slowly define a new normal for his life. Action potentials must propagate down what structure of a neuron to reach the appropriate ion channels?
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