The corpus callosum is a flat bundle of commissural fibers beneath the cerebral cortex in the brains of placental mammals. It spans part of the longitudinal fissure, connects the left and right cerebral hemispheres, and enables communication between the hemispheres. It is the largest white matter structure in the human brain, consisting of 200–250 million axonal projections.
The corpus callosum is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials, as well as other vertebrates such as birds, reptiles, amphibians and fish.
Other groups do have other brain structures that allow for communication between the two hemispheres, such as the anterior commissure, which serves as the primary mode of interhemispheric communication in marsupials, and which carries all the commissural fibers arising from the neocortex (also known as the neopallium, whereas in placental mammals, the anterior commissure carries only some of these fibers.) In primates, the speed of nerve transmission depends on its degree of myelination, or lipid coating.
Corpus Callosum Structure
The posterior end of the corpus callosum, near to the cerebellum, is called the splenium. This is the thickest part, and overlaps the tela chorioidea of the third ventricle and the mid-brain, and ends in a thick, convex, free border. Splenium translates as bandage in Greek.
A sagittal section of the brain shows that the posterior end of the corpus callosum is acutely bent forward, the upper and lower parts being applied to each other.
The anterior end near to the frontal lobes is called the genu (“knee”). The genu of the corpus callosum is bent downward and backward in front of the septum pellucidum; diminishing rapidly in thickness.
It is prolonged backward under the name of the rostrum, and is connected below with the lamina terminalis, which stretches from the interventricular foramina to the recess at the base of the optic stalk. The rostrum is so named for its resemblance to a bird’s beak.
The anterior cerebral arteries are in contact with the under surface of the rostrum; they then arch over the front of the genu, and are carried backward above the body of the corpus callosum.
The body or truncus of the corpus callosum is between the splenium and the genu. A narrowed part between the body and the splenium is known as the isthmus of the corpus callosum.
On either side of the corpus callosum, the fibers radiate in the white matter and pass to the various parts of the cerebral cortex; those curving forward from the genu into the frontal lobe constitute the forceps anterior, and those curving backward into the occipital lobe, the forceps posterior.
Between these two parts is the main body of the fibers which constitute the tapetum and extend laterally on either side into the temporal lobe, and cover in the central part of the lateral ventricle.
Thinner axons in the genu connect the prefrontal cortex between the two halves of the brain; these fibres arise from a fork-like bundle of fibers from the tapetum, the forceps anterior.
Thicker axons in the mid body, or trunk of the corpus callosum, interconnect areas of the motor cortex, with proportionately more of the corpus callosum dedicated to supplementary motor regions including Broca’s area.
The posterior body of the corpus, known as the splenium, communicates somatosensory information between the two halves of the parietal lobe and the visual cortex at the occipital lobe, these are the fibres of the forceps posterior.
In a study of five- to eighteen-year-olds there was found to be a positive correlation between age and callosal thickness.
The front portion of the human corpus callosum has been reported to be significantly larger in musicians than non-musicians, and to be 0.75 cm2 or 11% larger in left-handed and ambidextrous people than right-handed people. This difference is evident in the anterior and posterior regions of the corpus callosum, but not in the splenium.
Other magnetic resonance morphometric study showed corpus callosum size correlates positively with verbal memory capacity and semantic coding test performance. Children with dyslexia tend to have smaller and less-developed corpus callosums than their nondyslexic counterparts.
Musical training has shown to increase plasticity of the corpus callosum during a sensitive period of time in development. The implications are an increased coordination of hands, differences in white matter structure, and amplification of plasticity in motor and auditory scaffolding which would serve to aid in future musical training.
The study found children who had begun musical training before the age of six (minimum 15 months of training) had an increased volume of their corpus callosum and adults who had begun musical training before the age of 11 also had increased bimanual coordination.
The corpus callosum and its relation to sex has been a subject of debate in the scientific and lay communities for over a century.
Initial research in the early 20th century claimed the corpus to be different in size between men and women. That research was in turn questioned, and ultimately gave way to more advanced imaging techniques that appeared to refute earlier correlations.
However, advanced analytical techniques of computational neuroanatomy developed in the 1990s showed that sex differences were clear but confined to certain parts of the corpus callosum, and that they correlated with cognitive performance in certain tests. One recent study using magnetic resonance imaging (MRI) found that the midsagittal corpus callosum cross-sectional area is, after controlling brain size, on average, proportionately larger in females.
The symptoms of refractory epilepsy can be reduced by cutting the corpus callosum in an operation known as a corpus callosotomy. This is usually reserved for cases in which complex or grand mal seizures are produced by an epileptogenic focus on one side of the brain, causing an interhemispheric electrical storm.
The work up for this procedure involves an electroencephalogram, MRI, PET scan, and evaluation by a specialized neurologist, neurosurgeon, psychiatrist, and neuroradiologist before surgery can be considered.
Top Image: Henry Vandyke Carter – Henry Gray (1918): Gray’s Anatomy, Plate 733