HONG KONG (Reuters) - Mature brains need a continuous supply of new nerve cells to sustain functions like smelling and memory, an experiment with mice in Japan has shown.

While the adult brain can make new nerve cells, experts have never been sure of their roles until now. These findings may explain why some stroke survivors never recover certain faculties because their brains no longer generate new cells.

In an article published in Nature Neuroscience, the researchers said they found a way to insert a fluorescent protein into adult mice, which helped identify new brain cells.

Over the course of a year, they found that nearly all nerve cells in the olfactory bulb -- responsible for smelling -- had been replaced with new ones. New nerve cells were also seen in the hippocampus, which is linked to memory.

“These mice were normal and we could tell which were new nerve cells ... and they should be functional,” Ryoichiro Kageyama, director and professor at the Institute for Virus Research in Kyoto University told Reuters by telephone.

“We believe those neurons are very important for olfactory system (sense of smell).”

In another group of adult mice, the researchers blocked their ability to grow new brain nerve cells.

“Normal mice quickly learn which hole to get to where it’s dark and has bedding. For the mutants, they learn where to hide, but after a week, they totally forget, they completely lose their memory,” said Kageyama.

However, this group of mice continued to be able to smell four months into the experiment. The researchers are waiting to see if that faculty might be affected further out.

“We are waiting a while more to see if there could be loss of the sense of smell,” Kageyama said.

He added that the findings had implications for people suffering brain damage.

“In some damaged brains, like after a stroke, there is no neurogenesis (generation of new brain cells). We are interested in knowing where nerve cells come from, and whether we can stimulate neurogenesis,” he said.