Brain cells that produce the protective coating around nerve fibers can become dysfunctional with age and may actively contribute to cognitive decline, according to new research.

A study found that people with more severe cognitive decline had changes to nerve fibers and myelin—the protective coating around nerve fibers that helps messages travel efficiently through the brain. The study is published in the journal Nature Medicine.

Experiments in mice showed that disrupting oligodendrocytes—the cells that produce myelin—also altered the coating and impaired cognitive performance.