The modifications of the placental nervous system
included an enlarged cerebrum with many shared regions.
The ability to move the hand with fine, skilled movements evolved before
primates and is present in a number of mammalian lineages, such as mice.
In most placentals, the lateral cortico-spinal tract is no longer located
in the dorsal column (as in monotremes, marsupials, ungulates and rodents).
In placental mammals, there is an increase in the number of small and
medium sized cells in and around trigeminal nucleus and a reduction in
forward extension of efferent facial nucleus. Placental mammals possess
multiple nuclei of the anterior, medial, and intralaminar groups and a
lateral posterior-pulvinar complex, unlike monotremes.
In placental mammals, the cerebrum increased in size. Its expansion over
the midbrain ends the "midbrain exposure" typical of other groups
of vertebrates. Most placental mammals further increase the surface area
of the cerebrum by developing gyri and sulci. This wrinkling of brain
tissue to increase surface area is not unique to the cerebrum: a number
of brain areas have increased their size through folding such as substantia
gelatinosa, nucleus laminaris, dentate nucleus , inferior and superior
olivary nuclei, lateral geniculate nuclei and cerebellar cortex. Folding
of the cerebrum exists in some marsupials. While the majority of placental
mammals possess gyri and sulci on the surface of the cerebrum, smooth
cerebrums are known in some rodents, insectivores, and prosimians. The
presence of the lipid lysophosphatidic acid (LPA) to the developing brain
of a mouse converts its smooth cerebral cortex to a gyrated one, resembling
the brain surface of higher mammals.
A corpus callosum developed in placental mammals to connect the two cerebral
hemispheres (although some marsupials have a dorsal corpus callosum).
In placental mammals, the hippocampus is positioned more laterally. Eutherian
mammals share M2 primary and secondary motor fields. Placental mammals
possess the following sulci: intercalates, rostralis, occipito-temporalis,
suprasylvian, coronal, ansate (which contributes to the central sulcus
in primates), and an interparietal (Simian) sulcus. Placental mammals
lack oil droplets in retinal cones.