Corpus callosum

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Corpus callosum
Corpus callosum from above, front part at the top of the image.
Sagittal section of brain, front part to the left. The corpus callosum can be seen in the center, in light gray
Details
Pronunciation/ˈkɔːrpəs kəˈlsəm/
Part ofHuman brain
PartsGenu, rostrum, trunk, splenium
Identifiers
MeSHD003337
NeuroNames191
NeuroLex IDbirnlex_1087
TA98A14.1.09.241
TA25604
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, connecting the left and right cerebral hemispheres, enabling communication between them. It is the largest white matter structure in the human brain, about 10 in (250 mm) 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

MRI
of corpus callosum and its named parts
Corpus callosum

The corpus callosum forms the floor 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. Fibres from the trunk and the splenium known together as the tapetum form the roof of each lateral ventricle.[6]

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 is a

cingulate gyrus
.

Relations

On either side of the corpus callosum, the fibers radiate 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 undersurface 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.[7]

Neuronal fibers

The size, amount of myelination, and density of the fibers in the subregions relate to the functions of the brain regions they connect.[8] 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.[9] Thinner, lightly myelinated fibers are slower conducting and they connect the association and prefrontal areas. Thicker and fast-conducting fibers connect the visual and motor areas.[10]

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.[11]

Thinner

somatosensory information between the two halves of the parietal lobe and the visual cortex at the occipital lobe, these are the fibers of the forceps major.[12][13]

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

Variation between sexes

The corpus callosum and its

MRI study found that the midsagittal corpus callosum cross-sectional area is, after controlling for brain size, on average, proportionately larger in females.[15]

Using

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 between sexes.[19][20] Specific algorithms have found significant differences between the two sexes in over 70% of cases in one review.[21]

A 2005 study on the sizes and structures of the corpus callosum in transgender people found it to be structurally more in line with their declared gender than their assigned sex. [21]

Correlates of size with handedness

One study reported that the front portion of the human corpus callosum was 0.75 cm2 or 11% larger in

ambidextrous people than right-handed people.[22][23] This difference was evident in the anterior and posterior regions of the corpus callosum, but not in the splenium.[22] However, a 2022 meta-analysis failed to confirm any substantial differences in the corpus callosum related to left vs. right- vs. mix-handedness.[24] Others have instead suggested that the degree of handedness negatively correlates with the size of the corpus callosum, meaning that individuals who are capable of using both hands with dexterity would have the largest corpus callosum and vice versa for either left or right hand.[25]

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)

electroencephalogram, MRI, PET scan, and evaluation by a neurologist, neurosurgeon, psychiatrist, and neuroradiologist before a partial lobotomy surgery can be considered.[27]

Failure to develop

The formation of the corpus callosum begins with the first midline crossing of pioneer axons around week 12 in the

Hydrocephaly may also occur. In mild cases, symptoms such as seizures, repetitive speech, or headaches may not appear for years. Some syndromes often associated with ACC include 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.[31]

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

Other studies have also linked possible correlations between corpus callosum malformation and autism spectrum disorders.[32][33]

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.[34]

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.[35] 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."[36]

Later 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 any account was taken of larger male brain size.[34] A study in 2006 using thin slice MRI showed no difference in thickness of the corpus when accounting for the size of the subject.[37]

Other animals

The corpus callosum is found only in

chimpanzees and humans. This has resulted in the human corpus callosum's requiring double the time for interhemispheric communication as a macaque's.[12] 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.[43]

Additional images

  • Corpus callosum
    Corpus callosum
  • Coronal T2 (grey scale inverted) MRI of the brain at the level of the caudate nuclei emphasizing corpus callosum
    Coronal T2 (grey scale inverted) MRI of the brain at the level of the caudate nuclei emphasizing corpus callosum
  • Tractography of Corpus callosum
  • Corpus callosum with Anatomography
    Corpus callosum with Anatomography
  • 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.
    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.

References

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External links