| Corpus callosum | |
|---|---|
 Corpus callosum from above, front part at the top of the image | |
 Sagittal section of abrain, front part to the left. The corpus callosum can be seen in the center, in light gray | |
| Details | |
| Pronunciation | /ˈkɔːrpəskəˈloʊsəm/ |
| Part of | Human brain |
| Location | Between the two brain hemispheres |
| Parts | Genu, rostrum, trunk, splenium |
| Function | Facilitating communication between thetwo brain hemispheres, allowing them to share information and coordinate functions like movement, sensory processing, and cognitive tasks |
| Identifiers | |
| MeSH | D003337 |
| NeuroNames | 191 |
| NeuroLex ID | birnlex_1087 |
| TA98 | A14.1.09.241 |
| TA2 | 5604 |
| FMA | 86464 |
| Anatomical terms of neuroanatomy | |
Thecorpus callosum (Latin for "tough body"), alsocallosal commissure, is a wide, thicknerve tract, consisting of a flat bundle ofcommissural fibers, beneath thecerebral cortex in thebrain. The corpus callosum is only found inplacental mammals.[1] It spans part of thelongitudinal fissure, connecting the left and rightcerebral hemispheres, enabling communication between them. It is the largestwhite matter structure in thehuman brain, about 10 cm (3.9 in) in length and consisting of 200–300 millionaxonal 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]
The corpus callosum forms the floor of thelongitudinal fissure that separates the twocerebral hemispheres. Part of the corpus callosum forms the roof of thelateral ventricles.[5]
The corpus callosum has four main parts – individual nerve tracts that connect different parts of the hemispheres. These are therostrum, thegenu, thetrunk orbody, and thesplenium.[4] Fibres from the trunk and the splenium, known together as thetapetum ("carpet"), form the roof of each lateral ventricle.[6]
The front part of the corpus callosum, towards thefrontal lobes, is called the genu ("knee"). The genu curves downward and backward in front of theseptum pellucidum, diminishing greatly in thickness. The lower, much thinner part is the rostrum and is connected below with thelamina terminalis, which stretches from theinterventricular foramina to the recess at the base of theoptic stalk. The rostrum is named for its resemblance to abird's beak.
The end part of the corpus callosum, towards thecerebellum, is called the splenium. This is the thickest part, and overlaps thetela choroidea of thethird ventricle and themidbrain, and ends in a thick, convex, free border. Splenium translates as "bandage" inGreek.
The trunk of the corpus callosum lies between the splenium and the genu.
Thecallosal sulcus is asulcus that separates the corpus callosum from thecingulate gyrus.
On either side of the corpus callosum, the fibers radiate in thewhite matter and pass to the various parts of thecerebral cortex; those curving forward from the genu into thefrontal lobes constitute the forceps minor (also forceps anterior) and those curving backward from the splenium into theoccipital 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 thetemporal lobe, and cover in thecentral part of the lateral ventricle. The tapetum andanterior commissure share the function of connecting left and right temporal lobes.
Theanterior 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]
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]
Thetractogram 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]
Thinneraxons in the genu connect theprefrontal cortex between the two halves of the brain; these fibers 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 themotor cortex, with proportionately more of the corpus callosum dedicated to supplementary motor regions includingBroca's area. The splenium communicatessomatosensory information between the two halves of theparietal lobe and thevisual cortex at theoccipital 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]
The corpus callosum and itsrelation 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 ofcomputational 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.[14] AnMRI study found that the midsagittal corpus callosum cross-sectional area is, after controlling for brain size, on average, proportionately larger in females.[15]
Usingdiffusion 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: specifically, increased signal intensity and decreased fractional anisotropy in the female corpus callosum, as compared with that of the male.[16][17][18]
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 intransgender people found it to be structurally more in line with their declared gender than their assigned sex.[21]
One study reported that the front portion of the human corpus callosum was 0.75 cm2 or 11% larger inleft-handed andambidextrous 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]
The formation of the corpus callosum begins with the first midline crossing of pioneer axons around week 12 in theprenatal development of the human,[26] or day 15 in theembryogenesis of the mouse.[27]
The symptoms of refractory (difficult to treat)epilepsy can be reduced by cutting through the corpus callosum in an operation known as acorpus callosotomy lobotomy paralysis.[28] This is usually reserved for cases in whichcomplex orgrand malseizures are produced by anepileptogenic focus on one side of the brain, causing an interhemispheric electrical storm. Thediagnostic work up for this procedure involves anelectroencephalogram,MRI,PET scan, and evaluation by a neurologist, neurosurgeon, psychiatrist, and neuroradiologist before a partial lobotomy surgery can be considered.[29]
Agenesis of the corpus callosum (ACC) is a rarecongenital disorder that is one of the most common brain malformations observed in human beings,[30] 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 includeseizures, 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 often associated with ACC includeAicardi syndrome,Andermann syndrome,Shapiro syndrome, andacrocallosal 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 andautism spectrum disorders.[32][33]
Kim Peek, asavant and the inspiration behind the movieRain Man, was found with agenesis of the corpus callosum, as part ofFG syndrome.
Anterior corpus callosumlesions may result inakinetic mutism oranomic aphasia. See also:
The first study of the corpus with relation to gender was byR. 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 byFranklin Mall, the director of his own laboratory.[35]
Of more mainstream impact was a 1982Science article byHolloway and Utamsing that suggested sex difference in human brainmorphology, which related to differences in cognitive ability.[36]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."[37]
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.[35] A study in 2006 using thin slice MRI showed no difference in thickness of the corpus when accounting for the size of the subject.[38]
The corpus callosum is found only inplacental mammals, while it is absent inmonotremes andmarsupials,[39] as well as other vertebrates such as birds, reptiles, amphibians and fish.[40] Other groups do have other brain structures that allow for communication between the two hemispheres, such as theanterior commissure, which serves as the primary mode of interhemispheric communication in marsupials,[41][42] and which carries all thecommissural fibers arising from theneocortex (also known as the neopallium), whereas in placentals, the anterior commissure carries only some of these fibers.[43]
Inprimates, the speed of nerve transmission depends on the degree ofmyelination, 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 increasedmyelination have not occurred betweenchimpanzees andhumans. This has resulted in the human corpus callosum's requiring double the time for interhemispheric communication as amacaque'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.[44]
