When the heart stops beating and somebody dies, the body’s systems shut down and its natural processes cease.

Or do they?

A new study suggests that at least one type of cell remains alive after death — and that for hours, they experience increased genetic activity and even massive growth.

The research, published in the journal Scientific Reports, looked at how brain tissue acts in the hours after death. To simulate what happens after death, researchers examined tissue samples taken from patients during routine brain surgeries.

They found a surprising amount of activity in genes unique to glial cells. Although they’re part of the nervous system, these cells don’t transmit or receive electrical signals. Instead, they help support other brain cells, binding neurons together and helping them function.

After simulated death, the ­glial cells actually increased their genetic activity, ballooning in size and growing long arms. The activity of these “zombie genes” peaked at 12 hours after “death.” Other genes involved in brain activities like memory and thinking degraded quickly, while others remained stable for about 24 hours without much change.

That didn’t surprise researchers. Glial cells work as a cleanup crew after events such as illnesses and brain injuries, so the scientists expected their genes could stay active.

But the researchers said the amount of growth they observed in the glial cells has implications for brain science in general. Since post-mortem brain tissue is integral to brain research, it’s widely used. But past research hasn’t taken those post-mortem changes into account.

Additional scrutiny is needed to properly interpret the results of postmortem brain studies, the researchers write.

“Most studies assume that ­everything in the brain stops when the heart stops beating, but this is not so,” said Jeffrey Loeb, a ­University of Illinois at Chicago professor who co-wrote the paper, in a news release. “The good news from our findings is that we now know which genes and cell types are stable, which degrade, and which increase over time so that results from postmortem brain studies can be better understood.”