Check out this video of a turtle's heart beating outside the body:
Most muscles contract via electrical impulses from the brain. Then obviously they would cease to function if they were disconnected from the brain.
However, the heart follows a pattern different than most muscles in the body. The beating of the heart itself is not regulated by the brain, but actually within the heart itself. The only function of the brain is tell the heart how fast it needs to beat. Nerve cells within the heart continue firing for an extended period of time, promoting the process of beating.
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Believe it or not, human hearts continue to beat after removal from the body for a short period. A turtle's heart can last much longer after removal, easily up to several hours.
Interesting fact: Doctors define death as the point where the heart stops beating. If you were a turtle you could have your head cut off, be drained of blood, completely dissected, and eaten, but still be considered alive since the heart can continue to beat for many hours after removal.