Allocortex , or atypical cortex (also known as heterogenetic cortex ), is one of the two main types of cerebral cortex . Another, opposed to allocortex, type of cortex is the isocortex , or “typical cortex,” also known as the neocortex, or “new cortex”. The atypical nature of allocortex is that, unlike the isocortex, in adulthood it has only three or four layers of cortical cells, and in some areas even only two layers. This brings him closer to the cloak of the brain of the lower chordates , in particular reptiles . At the same time, the isocortex in adulthood has a structure typical of the cortex of the mammalian mammalian cerebral hemispheres, usually consisting of six (only rarely and in a small number of regions, from a smaller number) layers of the cortex. In addition, it is also very important in this definition that the allocortex regions at no stage of the embryonic development of the brain pass through the stage of having six layers in them, followed by a decrease in the number of layers during their further development (their disappearance, intergrowth, or mixing). It is for this reason that the allocortex is called a heterogenetic cortex, in contrast to a heterotypic but homogenetic cortex, such as, for example, the Brodman 4 region, which during the period of embryonic development passes through the stage of six layers of the cortex, but subsequently loses the layer of granular cells, and the layers in it become five [1] . Allocortex in mammals, especially higher primates and humans, occupies only a small fraction of the total volume of the cortex: more than 90% is isocortex (“typical bark”) or neocortex (“new bark”).
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A heterotypic homogenetic cortex, which in an adult state has a number of layers less than six, but passes through the stage of having six layers in an embryo, is referred not to an allocortex, but to an isocortex (neocortex), despite its apparent external “atypicality” in an adult state, according to phylogenetic and ontogenetic grounds. Allocortex is subdivided, on the basis of the number of layers available in it, patterns of embryonic development and estimated phylogenetic age, into three subtypes: the evolutionarily oldest bark (or archicortex, archipallium), the evolutionarily younger old bark (or paleocortex, paleopallium), which, however , still phylogenetically much older than the new cortex, and the so-called transitional cortex (periallocortex), located at the junction of the ancient or old cortex with the new cortex and representing a smooth histological transition the transition from one type of bark to another with a gradual increase in the number of layers and the complexity of the organization [2] .
Areas of the cerebral cortex, usually classified as an allocortex, include the areas of the olfactory brain , the hippocampus formation (including the hippocampus proper) and the limbic system .
Anatomical structure
Allocortex consists of only three or four layers of cortical neurons, and in some areas even just two layers, as opposed to the isocortex (neocortex), which consists of six layers of cortical cells (only in some areas it consists of fewer layers, for example, five, but these areas still pass through the stage of the presence of six layers in the process of embryonic development). In the areas of allocortex, three subtypes of areas are distinguished: areas of the ancient cortex (archicortex), areas of the old cortex (paleocortex), and areas of the transitional cortex (periallocortex, also called mesocortex) [3] .
The old cortex, or old cloak (paleocortex, paleopallium) is a subtype of evolutionarily very ancient, very thin, rather primitive in histological structure, cortical tissue, which consists of four layers of cortical neurons [4] [5] . Two layers of granular cells, namely layers II and IV present in the isocortex (neocortex), are absent in the paleocortex. The main areas of the old bark are the olfactory bulb , the olfactory tubercle and the piriform cortex .
The ancient cortex, or ancient cloak (archicortex, archipallium) is a subtype of an even more evolutionarily ancient, even more primitive in histological structure, cortical tissue, which consists of three, and in some places even only two, layers of cortical neurons [6] . The main areas of the ancient bark are the hippocampus formation , in particular the hippocampus itself, and the limbic system , in particular the dentate gyrus .
The transitional cortex (periallocortex) is the area of a smooth histological transition at the junction between the new cortex (neocortex or neopallium) and either the old bark (paleocortex, paleopallium) or the ancient bark (archicortex, archipallium). Therefore, the transitional cortex, in turn, can be subdivided into the transitional old cortex , or peripaleocortex (peripaleopallium), and the transitional ancient cortex , or periarchicortex (periarchipallium) [7] . The transitional old cortex primarily refers to the anterior insular cortex [8] . The transitional ancient cortex includes the entorhinal cortex , the presubicular cortex , the retrosplenal cortex , the suprasolar region , the popliteal region [9] .
The “computational power” of a particular section of the cortex (its ability to quickly process incoming information flows) and its functional flexibility (the ability to solve non-standard information problems in rapidly changing conditions) directly depend on the number of layers of the cortex and the complexity of its histological structure. More primitive cortical structures, consisting of a smaller number of layers of cells, are evolutionarily highly conservative both in their histological structure and location in the brain, and resemble reptiles in the structure of the brain cloak . In addition, the sense of smell is the main and most ancient sensory modality in phylogenetically more ancient mammals. Therefore, it is assumed that the number of layers of the cortex is inversely proportional to the phylogenetic age of a particular site of the cortex (the more layers of the cortex, the given site is probably phylogenetically younger, and vice versa - the fewer layers of the cortex in it, the more likely it is phylogenetically older). On this basis, it is believed that the most phylogenetically ancient and most primitive are the areas of the so-called "ancient bark", or archicortex [5] .
See also
- Paralymbic bark
Notes
- ↑ Crosby EC; Humphrey T; Lauer EW (1962) Correlative Anatomy of the Nervous System. New York: MacMillan
- ↑ Allocortex . BrainInfo University of Washington. Date of appeal October 13, 2017.
- ↑ Filimonoff, IN A rational subdivision of the cerebral cortex (neopr.) // Arch Neurol Psychiatry. - 1947.- T. 58 . - S. 296-311 .
- ↑ Paleocortex . BrainInfo University of Washington. Date of appeal October 13, 2017.
- ↑ 1 2 Neuroscience. - 5th. - Sinauer Associates Inc, 2011 .-- P. 666. - ISBN 9780878936465 .
- ↑ Archicortex . BrainInfo University of Washington. Date of appeal October 13, 2017.
- ↑ Periallocortex . BrainInfo University of Washington. Date of appeal October 13, 2017.
- ↑ Peripaleocortex . BrainInfo University of Washington. Date of appeal October 13, 2017.
- ↑ Periarchicortex . BrainInfo University of Washington. Date of appeal October 13, 2017.
- “Allocortex.” Stedman's Medical Dictionary, 27th ed. (2000).
- Haines, Duane E. Fundamental Neuroscience, 2nd. (2002).