Not to be picky, but the heart isn't a CNS shot. People can and have continued to fight for up to three minutes with their heart destroyed. The brain can continue to function for that long without blood actively being pumped to it.
The reason a Central Nervous System shot can stop a fight is because the brain can no longer communicate with the rest of the body (muscles). The heart shot has more to do with the individuals willingness to fight rather than a disconnect of the brain to the body.