Scientists said they have restored sensation to laboratory rats by repairing nerves severed and crushed at the spine, a breakthrough that may someday allow doctors to help people with disabling spinal injuries. The technique involves implanting a “bridge,” made of fetal cells, to guide regrowth of the nerves connecting the rat’s hind leg to the spinal cord, said Dr. Jerry Silver of Case Western Reserve University in Cleveland. “We believe we really have regeneration into the spinal cord” for the first time, he said. Previous attempts to get nerves to grow back into the spinal cord failed because scar tissue got in the way. Silver is to present his findings Monday at a New Orleans symposium of the Society for Neuroscience.