Ontogeny of interhemispheric evoked potentials in the rat: Significance of myelination of the corpus callosum

Myelination of the corpus callosum of the rat was studied together with the ontogeny of the interhemispheric and transcallosal evoked responses from birth to adulthood. Electron microscopic examination of the corpus callosum revealed: that this structure was entirely unmyelinated in the neonatal animal; that the frequency of myelinated callosal axons increased from 0.1% at 36 days postconception to 54% in the adult and that myelination may at first be localized to a small area of axon and that longitudinal extension of the sheath occurs later. Electrocortical maturation of both evoked responses was characterized by wave-form, amplitude, and latency changes consistent with the structural development of the neocortex and corpus callosum.