Effects of a 4Â month enriched environment on the hippocampus and the myelinated fibers in the hippocampus of middle-aged rats

► An enriched environment improved the performance of middle‐aged female rats in the Morris water maze. ► An enriched environment induced the increase of the total length of the myelinated fibers in the hippocampus. ► An enriched environment induced the increase of the total volume of the myelinated fibers in the hippocampus.