The corpus callosum is a very thick bundle of nerve fibers containing both myelinated and unmyelinated axons.
In the 18th century, the corpus callosum was considered the site of the soul (Maurice Ptito), and in the early 20th it was assigned the mere role of preventing the cerebral hemispheres from collapsing onto each other. It was only in the 1950's that the corpus callosum, in the pioneering work of Myers and Sperry, was attributed the function of transferal of information between the two hemispheres. This was followed by the development, in the early 1960's, of a surgical intervention aimed at reducing the interhemispheric transmission of abnormal electrical discharges in epileptic patients. This involved the sectioning of the corpus callosum and other commissural structures of severe epileptic patients in which drug treatment was ineffective. The study of these patients by Sperry, Gazzaniga, and collaborators have greatly contributed to our knowledge of the functions of this midline structure. Even today the corpus callosum is still the focus interest for many neuroscientists who study interhemispheric communication.
Each hemisphere contains neurons which project callosal axons not only to homologous (homotopic) areas in the contralateral hemisphere but also to heterologous (heterotopic) areas. There are approximately 200,000,000 callosal axons in humans! The corpus callosum is undoubtedly the most important commissure to connect the two hemispheres, not only by virtue of its size, but also due to the wealth of its neural connections. It is through these projections that information is shared between the two halves of the brain. Very little is known about the neural signals that pass between the hemispheres, but recent studies have used modern tract tracing techniques to determine precisely the sites of origin and termination of neurons which project across the corpus callosum. Using a retrograde tracer (horseradish peroxidase), Lomber et al. (1994) were able to link the functional divisions of the cerebral cortex to fiber trajectory through the corpus callosum. The motor cortex sends fibers through the rostrum and genu of the corpus callosum. The adjacent somatosensory cortex projects fibers through the anterior half of the corpus callosum whereas axons arising from auditory regions pass through the posterior two-thirds of the corpus callosum and the dorsal splenium. Axons from the limbic cortex also help to form these regions of the corpus callosum. Finally, axons from visual cortices which occupy the greatest single fraction of the cortical mantle pass through the largest portion of the corpus callosum; the fibers are present throughout the splenium and extend well into the body and the anterior portion. Ramifications of the axonal breakdown of the corpus callosum are discussed in out section on
Above is a view of a living brain as seen with magnetic resonance imaging, or MRI, corpus callosum well exposed.