No glycongen is found in other organs as well in very low concentration, like in kidneys, white blood cells, glial cells and the brain. But yes liver and muscles are the main source.
Only the liver can store a glycogen storage that is accessible. Other cells that need glucose fast can't access the ones in muscle cells. And the liver is the first organ after glucose enters into your bloodstream from the entrails. This way, its purpose beside many things is to store the glucose, and if you would ever use it, make glucose and pump it to your bloodstream. Now it wouldn't be so smart to store glycogen in every cell or every organ would it? Why? Because when the glucose you ate, would reach those organs it would already disappear, because the cells on the way sucked it up! Also when you would need glucose because your running for example, the time every organ would get the signal to make glucose from glycogen, beside their usual function would would die.
Now muscles are different because they are the ones that need energy fast. For example when you are running a 100 m, in 10 seconds, you need massive energy to your muscles, but when you start running and the muscles use up your blood's glucose, the brain sends signal to the liver, and it hidrolizes glycogen and glucose enter into the bloodstream, and it makes it to your muscles, you are already done with running.
So it's best to store glycogen in muscles so they won't run out of energy until the glucose from the liver arrives.
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