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Name: Mona
Status: student
Grade: 9-12
Location: Outside U.S.
Country: USA
Date: Winter 2012-2913


Question:
What is the effect of decreasing Ca+2 on the muscles? In tetany decrease of Ca+2 ions causes increased irritability and contraction of muscles. But Ca+2 bind to troponin thus straightening tropomyosin filament in turn exposing the myosin binding site. This helps in muscle contraction. So if Ca+2 ions help in muscle contraction how does their decrease cause increased contraction of muscles in tetany?



Replies:
It is true that calcium ions are used inside muscle cells to drive contractions. However, you are also correct that low calcium levels cause tetany, or involuntary muscle contractions. This seeming contraction is resolved by understanding that the low calcium levels that cause tetany are talking about systemic, rather than cellular levels of calcium. When calcium in the body is lower than normal, it can have specific effects on the membranes of nerve cells that carry signals to the muscles. These cells will then start to spontaneously produce action potentials (signals for the muscles to contract), even without a specific command from the brain. When the action potential reaches the muscle cell, it will involuntarily contract even if the local calcium levels are a bit lower than normal.

Dr. Shimon Unterman



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