Glial cells, not neurons.

scientificamerican.com/…/human-brain-cells-make-m…

Human myelin sheaths are better, faster, and, in some mutants, can repair damaged sheaths better than rodent ones. Since myelin is what makes the action potential travel fast and efficiently along a neutron, this means smarter mice.

www.ncbi.nlm.nih.gov/pmc/articles/PMC6700730/#ABS…