Corpus Callosum
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Corpus Callosum
Corpus callosum
Gray733.png
Corpus callosum from above, front part at the top of the image.
Gray720.png
Cross-section of brain, with a person facing forward to the left. The corpus callosum can be seen in the center, in light gray
Details
Pronunciation
Part ofHuman brain
PartsGenu, trunk, splenium
Identifiers
MeSHD003337
NeuroNames191
NeuroLex IDbirnlex_1087
TAA14.1.09.241
FMA86464
Anatomical terms of neuroanatomy

The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick, nerve tract consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mammals.[1] It spans part of the longitudinal fissure, connects the left and right cerebral hemispheres, enabling communication between them. It is the largest white matter structure in the human brain, about ten centimetres in length and consisting of 200–300 million axonal projections.[2][3]

A number of separate nerve tracts, classed as subregions, of the corpus callosum connect different parts of the hemispheres. The main ones are known as the genu, the rostrum, the trunk or body, and the splenium.[4]

Structure

Callosal sulcus is shown in red

The corpus callosum forms the floor of the longitudinal fissure that separates the two cerebral hemispheres. It also forms part of the roof of the lateral ventricles.[5]

The corpus callosum has four main parts; individual nerve tracts that connect different parts of the hemispheres. These are the rostrum, the genu, the trunk or body, and the splenium.[4] A narrowed part between the trunk and the splenium is known as the isthmus.

The front part of the corpus callosum, towards the frontal lobes is called the genu ("knee"). The genu curves downward and backward in front of the septum pellucidum, diminishing greatly in thickness. The lower much thinner part is 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 named for its resemblance to a bird's beak.

The end part of the corpus callosum, towards the cerebellum, is called the splenium. This is the thickest part, and overlaps the tela choroidea of the third ventricle and the midbrain, and ends in a thick, convex, free border. Splenium translates as bandage in Greek.

The trunk of the corpus callosum lies between the splenium and the genu.

The callosal sulcus separates the corpus callosum from the cingulate gyrus.

Relations

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 lobes constitute the forceps minor (also forceps anterior) and those curving backward from the splenium into the occipital lobes, the forceps major (also forceps posterior).[4] 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. The tapetum and anterior commissure share the function of connecting left and right temporal lobes.

The anterior cerebral arteries are in contact with the under surface of the rostrum, they arch over the front of the genu and are carried along the trunk, supplying the front four-fifths of the corpus callosum.[6]

Nerve fibres

Fiber tracts from six segments of the corpus callosum.gif

The size, amount of myelination, and density of the fibers in the subregions relate to the functions of the brain regions they connect.[7] Myelination is the process of coating neurons with myelin, which helps the transfer of information between neurons. The process is believed to occur until an individual's thirties with peak growth in the first decade of one's life.[8] Thinner, lightly myelinated fibres are slower conducting and they connect the association and prefrontal areas. Thicker and fast-conducting fibers connect the visual and motor areas.[9]

The tractogram pictured, shows the nerve tracts from six segments of the corpus callosum, providing linking of the cortical regions between the cerebral hemispheres. Those of the genu are shown in coral, of the premotor - green, of the sensory-motor - purple, of the parietal - pink, of the temporal - yellow, and of the splenium - blue.[10]

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 minor. Thicker axons in the 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 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 major.[11][12]

In a study of five- to eighteen-year-olds there was found to be a positive correlation between age and callosal thickness.[3]

Variation between sexes

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.[13] One recent study using magnetic resonance imaging (MRI) found that the midsagittal corpus callosum cross-sectional area is, after controlling for brain size, on average, proportionately larger in females.[14]

Using diffusion tensor sequences on MRI machines, the rate at which molecules diffuse in and out of a specific area of tissue, anisotropy can be measured and used as an indirect measurement of anatomical connection strength. These sequences have found consistent sex differences in human corpus callosal shape and microstructure.[which?][15][16][17]

Analysis by shape and size has also been used to study specific three-dimensional mathematical relationships with MRIs, and have found consistent and statistically significant differences across genders.[18][19] Specific algorithms have found significant differences between the two sexes in over 70% of cases in one review.[20]

Other correlations

The front portion of the human corpus callosum has been reported to be significantly larger in musicians than nonmusicians,[21] and to be 0.75 cm2[22] or 11% larger in left-handed and ambidextrous people than right-handed people.[22][23] This difference is evident in the anterior and posterior regions of the corpus callosum, but not in the splenium.[22] Other magnetic resonance morphometric study showed corpus callosum size correlates positively with verbal memory capacity and semantic coding test performance.[24] Children with dyslexia tend to have smaller and less-developed corpus callosums than their nondyslexic counterparts.[25][26]

Musical training has shown to increase plasticity of the corpus callosum during a sensitive period of time in development, known as synaptogenesis. Synaptogenesis is the surge in the formation of new synapses in the brain. When this process is over, synaptic pruning begins. This pruning process strengthens synaptic connections that are habitually used and eliminates the ones that are not.[27] 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.[28]

Clinical significance

Epilepsy

Electroencephalography is used to find the source of electrical activity causing a seizure as part of the surgical evaluation for a corpus callosotomy.

The symptoms of refractory (difficult to treat) epilepsy can be reduced by cutting through the corpus callosum in an operation known as a corpus callosotomy.[29] 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 diagnostic 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.[30]

Failure to develop

The formation of the corpus callosum begins with the first midline crossing of pioneer axons around week 12 in the prenatal development of the human,[31] or day 15 in the embryogenesis of the mouse.[32]Agenesis of the corpus callosum (ACC) is a rare congenital disorder that is one of the most common brain malformations observed in human beings,[33] in which the corpus callosum is partially or completely absent. ACC is usually diagnosed within the first two years of life, and may manifest as a severe syndrome in infancy or childhood, as a milder condition in young adults, or as an asymptomatic incidental finding. Initial symptoms of ACC usually include seizures, which may be followed by feeding problems and delays in holding the head erect, sitting, standing, and walking. Other possible symptoms may include impairments in mental and physical development, hand-eye coordination, and visual and auditory memory. Hydrocephaly may also occur. In mild cases, symptoms such as seizures, repetitive speech, or headaches may not appear for years. Some syndromes that are often associated with ACC are Aicardi syndrome, Andermann syndrome, Shapiro syndrome, and acrocallosal syndrome.

ACC is usually not fatal. Treatment usually involves management of symptoms, such as hydrocephaly and seizures, if they occur. Although many children with the disorder lead normal lives and have average intelligence, careful neuropsychological testing reveals subtle differences in higher cortical function compared to individuals of the same age and education without ACC. Children with ACC accompanied by developmental delay and/or seizure disorders should be screened for metabolic disorders.[34]

In addition to agenesis of the corpus callosum, similar conditions are hypogenesis (partial formation), dysgenesis (malformation), and hypoplasia (underdevelopment, including too thin).

Recent studies have also linked possible correlations between corpus callosum malformation and autism spectrum disorders.[35]

Kim Peek, a savant and the inspiration behind the movie Rain Man, was found with agenesis of the corpus callosum, as part of FG syndrome.

Other disease

Anterior corpus callosum lesions may result in akinetic mutism or anomic aphasia. See also:

History

The first study of the corpus with relation to gender was by R. B. Bean, a Philadelphia anatomist, who suggested in 1906 that "exceptional size of the corpus callosum may mean exceptional intellectual activity" and that there were measurable differences between men and women. Perhaps reflecting the political climate of the times, he went on to claim differences in the size of the callosum across different races. His research was ultimately refuted by Franklin Mall, the director of his own laboratory.[36]

Of more mainstream impact was a 1982 Science article by Holloway and Utamsing that suggested sex difference in human brain morphology, which related to differences in cognitive ability.[37]Time published an article in 1992 that suggested that, because the corpus is "often wider in the brains of women than in those of men, it may allow for greater cross-talk between the hemispheres—possibly the basis for women's intuition."[38]

More recent publications in the psychology literature have raised doubt as to whether the anatomic size of the corpus is actually different. A meta-analysis of 49 studies since 1980 found that, contrary to de Lacoste-Utamsing and Holloway, no sex difference could be found in the size of the corpus callosum, whether or not account was taken of larger male brain size.[36] A study in 2006 using thin slice MRI showed no difference in thickness of the corpus when accounting for the size of the subject.[39]

Other animals

The corpus callosum is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials,[40] as well as other vertebrates such as birds, reptiles, amphibians and fish.[41] (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,[42][43] 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.[44]) In primates, the speed of nerve transmission depends on its degree of myelination, or lipid coating. This is reflected by the diameter of the nerve axon. In most primates, axonal diameter increases in proportion to brain size to compensate for the increased distance to travel for neural impulse transmission. This allows the brain to coordinate sensory and motor impulses. However, the scaling of overall brain size and increased myelination have not occurred between chimpanzees and humans. This has resulted in the human corpus callosum's requiring double the time for interhemispheric communication as a macaque's.[11] The fibrous bundle at which the corpus callosum appears, can and does increase to such an extent in humans that it encroaches upon and wedges apart the hippocampal structures.[45]

Additional images

References

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