Periallocortex is one of three subtypes of
allocortex
The allocortex or heterogenetic cortex, and neocortex are the two types of cerebral cortex in the brain. The allocortex is the much smaller area of cortex taking up just 10 %, the neocortex takes up the remaining 90 %. It is characterized by havi ...
, the other two subtypes being
paleocortex
In anatomy of animals, the paleocortex, or paleopallium, is a region within the telencephalon in the vertebrate brain. This type of cortical tissue consists of three cortical laminae (layers of neuronal cell bodies). In comparison, the neocortex ...
and
archicortex
The archicortex, or archipallium, is the phylogenetically oldest region of the brain's cerebral cortex. It is often considered contiguous with the olfactory cortex, but its extent varies among species. In older species, such as fish, the archipal ...
. The periallocortex is formed at transition areas where any of the other two subtypes of allocortex borders with the
neocortex
The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, sp ...
(which is also called isocortex).
Thus, the periallocortex is also subdivided to two subtypes. One subtype is called
peripaleocortex Peripaleocortex is one of two subtypes of periallocortex, the other being periarchicortex. Peripaleocortex is formed at borders between isocortex (neocortex) and paleocortex (a subtype of allocortex). It shows slow histological transition from the ...
, which is formed at borders between paleocortex and neocortex. Areas considered to belong to peripaleocortex are for example anterior
insular cortex
The insular cortex (also insula and insular lobe) is a portion of the cerebral cortex folded deep within the lateral sulcus (the fissure separating the temporal lobe from the parietal and frontal lobes) within each hemisphere of the mammalian bra ...
.
Another subtype of periallocortex is called
periarchicortex Periarchicortex is one of two subtypes of periallocortex, the other being peripaleocortex. It is formed at borders between archicortex (a subtype of allocortex) and isocortex and shows slow histological transition from the four-layered structure ty ...
. It is formed at borders between archicortex and neocortex. Areas considered to belong to periarchicortex include
entorhinal cortex
The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in the ...
,
perirhinal cortex
The perirhinal cortex is a cortical region in the medial temporal lobe that is made up of Brodmann areas 35 and 36. It receives highly processed sensory information from all sensory regions, and is generally accepted to be an important region f ...
,
presubiculum
Area 27 of Brodmann-1909 is a cytoarchitecturally defined cortical area that is a rostral part of the parahippocampal gyrus. It is commonly regarded as a synonym of presubiculum.
The dorsal part of the presubiculum is more commonly known as the ...
,
parasubiculum
In the rodent, the parasubiculum is a retrohippocampal isocortical structure, and a major component of the subicular complex. It receives numerous subcortical and cortical inputs, and sends major projections to the superficial layers of the entor ...
,
retrosplenial cortex
The retrosplenial cortex (RSC) is a cortical area in the brain comprising Brodmann areas 29 and 30. It is secondary association cortex, making connections with numerous other brain regions. The region's name refers to its anatomical location imm ...
,
subcallosal area and
subgenual area.
No one allocortex or even periallocortex area borders, contacts or transitions immediately to the so-called true isocortex. Instead, they border and slowly transition to another transitional area from the neocortex side, called
proisocortex Proisocortex or pro-isocortex is one of two subtypes of cortical areas in the areas belonging to the neocortex. The other subtype is termed the true isocortex. Proisocortical areas are transitional areas placed between areas of true isocortex and ...
, which then slowly transitions to the true isocortex. Thus, at borders between isocortex and allocortex there are two transitional areas, one from allocortex side and histologically more resembling allocortex, so it is called periallocortex, and another from isocortex side, histologically more resembling true isocortex, so it is called proisocortex.
Those two transitional areas (one from isocortex or neocortex side, called proisocortex, and another from allocortex side, called periallocortex) together form what is called
mesocortex Mesocortex (also called juxtallocortex) is the transitional areas of the cerebral cortex, formed at borders between true isocortex and true allocortex (either paleocortex or archicortex). Parts of mesocortex that lie closer to the true isocortex an ...
.
References
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Cerebral cortex