How and why does specific parts of the body's muscules jump like a frog is in there?

i tried to put it in the best of wordz possibleHow and why does specific parts of the body's muscules jump like a frog is in there?
they are called "muscle spasms" and they occur when your muscles are growing or beating. perfectly normal and natural. I get them all the time.
Ur nervous system usually gets a shock or thier might be a history of parkinsons disease in the family!
I hope im right