The allocortex (also known as heterogenetic cortex) is one of the two types of cerebral cortex, the other being the neocortex. It is characterized by having just three or four cell layers, in contrast with the six layers of the neocortex, and takes up a much smaller area than the neocortex. There are three subtypes of allocortex: the paleocortex, the archicortex, and the periallocortex – a transitional zone between the neocortex and the allocortex.
Allocortex is termed heterogenetic cortex, because during development it never has the six-layered architecture of homogenetic neocortex. It differs from heterotypic cortex, a type of cerebral cortex, which during prenatal development, passes through a six-layered stage to have fewer layers, such as in Brodmann area 4 that lacks granule cells.
The allocortex has just three or four layers of neuronal cell bodies in contrast to the six layers of the neocortex. There are three subtypes of allocortex, the paleocortex, archicortex and periallocortex.
Paleocortex is a type of thin, primitive cortical tissue that consists of three to five cortical laminae (layers of neuronal cell bodies). In comparison, the neocortex has six layers and the archicortex has three or four layers.
Archicortex is a type of cortical tissue that consists of three laminae (layers of neuronal cell bodies). It has fewer layers than both neocortex, which has six, and paleocortex, which has either four or five. Because the number of laminae that compose a type of cortical tissue seems to be directly proportional to both the information-processing capabilities of that tissue and its phylogenetic age, paleocortex is thought to be an intermediate between neocortex and archicortex in both aspects, and archicortex is thought to be the oldest and most basic type of cortical tissue.
Paleocortex is present in the parahippocampal gyrus, olfactory bulb, accessory olfactory bulb, olfactory tubercle, piriform cortex, periamygdalar area, anterior olfactory nucleus, anterior perforated substance, and prepyriform area.
Archicortex is most prevalent in the olfactory cortex and the hippocampus, which are responsible for processing smells and forming memories, respectively. Because olfaction is considered to be the phylogenetically oldest sensory modality, and the limbic system, of which the hippocampus is a part, is one of the oldest systems in the brain, it is likely that archicortex was one of the first types of tissue to develop in primitive nervous systems.
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