Researchers in the US have succeeded in plotting the paths of some of the largest nerve cells in an attempt to understand the causes of dementia.

By studying laboratory mice, scientists at The Johns Hopkins University have mapped the mammalian brain and the path of cholinergic neurons, the first cells to degenerate in people with Alzheimer’s disease.

“For us, this was like scaling Mount Everest,” said Dr Jeremy Nathans. “This work reveals the amazing challenges that cholinergic neurons face every day. Each of these cells is like a city connected to its suburbs by a single, one-lane road, with all of the emergency services located only in the city.

“You can imagine how hard it would be in a crisis if all of the emergency vehicles had to get to the suburbs along that one road. We think something like this might be happening when cholinergic neurons trying to repair the damage done by Alzheimer’s disease.”

Each cholinergic neuron, Dr Nathans said, has roughly 1,000 branch points. If lined up end to end, one neuron’s branches would add up to about 15 times the length of the mouse brain. But all of the branches are connected by a single, extremely thin “pipeline” to one hub — the cell body — that provides for the needs of the branches.

The challenge of moving material through this single pipeline could make it very difficult for cholinergic neurons to combat the challenges that come with a disorder like Alzheimer’s disease, he said. Now, by mapping the branches and pipelines, scientists will likely get a better fix on what happens when the neurons fail to meet the challenges.

A summary of the research was published online last month.