Corpus callosum

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Brain: Corpus callosum
Gray733.png
Corpus callosum from above. (Anterior portion is at the top of the image.)
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Median sagittal section of brain (person faces to the left). Corpus callosum visible at center, in light gray
Gray's p.828
NeuroNames hier-173
MeSH Corpus+Callosum
NeuroLex ID birnlex_1087
Corpus callosum with Anatomography

The corpus callosum (from Latin: "tough body"), also known as the colossal commissure, is a wide, flat bundle of neural fibers beneath the cortex in the eutherian brain at the longitudinal fissure. It connects the left and right cerebral hemispheres and facilitates interhemispheric communication. It is the largest white matter structure in the brain, consisting of 200–250 million contralateral axonal projections.

Structure[edit]

The posterior (back) portion of the corpus callosum is called the splenium; the anterior (front) is called the genu (or "knee"); between the two is the truncus, or "body", of the corpus callosum. The part between the body and the splenium is often markedly narrowed and thus referred to as the "isthmus". The rostrum is the part of the corpus callosum that projects posteriorly and inferiorly from the anteriormost genu, as can be seen on the sagittal image of the brain displayed on the right. The rostrum is so named for its resemblance to a bird's beak.

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.[1][2]

Variation[edit]

Agenesis of the corpus callosum (ACC) is a rare congenital disorder that is one of the most common brain malformations observed in the humans,[3] 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.

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.[4]

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

Recent studies have also linked possible correlations between corpus callosum malformation and autism spectrum disorders.[5]

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

Sexual dimorphism[edit]

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.[6] One recent study using magnetic resonance imaging (MRI) found that the midsagittal corpus callosum cross-sectional area is, on average, proportionately larger in females.[7]

Function[edit]

Physiologic imaging[edit]

The ability to evaluate the form and function of the human mind has undergone almost exponential growth and a paradigm shift in recent years. Magnetic resonance imaging, for example, is now being used to analyze physiology in addition to anatomy. Using diffusion tensor sequences on MRI machines, the rate at which molecules diffuse in and out of a specific area of tissue, directionality or anisotropy, and rates of metabolism can be measured. These sequences have found consistent sex differences in human corpus callosal morphology and microstructure.[which?][8][9][10]

Morphometric analysis has also been used to study specific three-dimensional mathematical relationships with MRIs, and have found consistent and statistically significant differences across genders.[11][12] Specific algorithms have found significant gender differences in over 70% of cases in one review.[13]

Gender identity disorder[edit]

Research has been done on the shape of the corpus callosum in those with gender identity disorder. Researchers were able to demonstrate that the shape dimorphism of the corpus callosum at birth in biological males who self-identified as female was actually reversed, and that the same held true for biological females who self-identified as male. The publishers of this article argued that the shape of the corpus callosum correlates better with the 'mental sex' of individuals rather than their 'physical sex'.[13] The relationship between the corpus callosum and gender remains an active subject of debate in the scientific community.

Other correlations[edit]

The front portion of the corpus callosum has been reported to be significantly larger in musicians than nonmusicians,[14] and to be 0.75 cm2[15] or 11% larger in left-handed and ambidextrous people than right-handed people.[15][16] This difference is evident in the anterior and posterior regions of the corpus callosum, but not in the splenium.[15] Other magnetic resonance morphometric study showed corpus callosum size correlates positively with verbal memory capacity and semantic coding test performance.[17] Children with dyslexia tend to have smaller and less-developed corpus callosums than their nondyslexic counterparts.[18][19]

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.[20]

Clinical significance[edit]

Epilepsy[edit]

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 epilepsy can be reduced by cutting the corpus callosum in an operation known as a corpus callosotomy.[21] 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.[22]

Other disease[edit]

Anterior corpus callosum lesions may result in akinetic mutism or tactile anomia. Posterior corpus callosum (splenium) lesions may result in alexia without agraphia.

See also:

Brain split procedure[edit]

Main article: Corpus callosotomy

The cerebral cortex is divided into two hemispheres, connected by the corpus callosum. A procedure to help patients alleviate the severity of seizures is called split-brain procedure. As a result, a seizure that starts in one hemisphere is isolated in that hemisphere, since a connection to the other side no longer exists. However, this procedure is dangerous and risky.

History[edit]

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.[23]

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.[24] 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."[25]

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.[23] A study in 2006 using thin slice MRI showed no difference in thickness of the corpus when accounting for the size of the subject.[26]

In other animals[edit]

The corpus callosum is found only in placental mammals (the eutherians), while it is absent in monotremes and marsupials,[27] as well as other vertebrates such as birds, reptiles, amphibians and fish.[28] (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,[29][30] 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.[31]) 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.[1]

Sagittal post mortem section through the midline brain. The corpus callosum is the curved band of lighter tissue at the center of the brain above the hypothalamus. Its lighter texture is due to higher myelin content, resulting in faster neuronal impulse transmission.

The fibrous bundle as 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.[32]

Additional images[edit]

References[edit]

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