Neocortex

Source: Wikipedia, the free encyclopedia.
For other uses, see Neocortex (disambiguation).
Neocortex
A representative column of neocortex. Cell body layers are labeled on the left, and fiber layers are labeled on the right.
Identifiers
MeSHD019579
NeuroNames757
NeuroLex IDbirnlex_2547
TA98A14.1.09.304
A14.1.09.307
TA25532
FMA62429
Anatomical terms of neuroanatomy

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,[1] spatial reasoning and language.[2] The neocortex is further subdivided into the true isocortex and the proisocortex.[3]

In the human brain, the cerebral cortex consists of the larger neocortex and the smaller allocortex, respectively taking up 90% and 10%.[4] The neocortex is made up of six layers, labelled from the outermost inwards, I to VI.

Etymology

The term is from cortex,

Greek
, "new". Neopallium is a similar hybrid, from Latin pallium, "cloak". Isocortex and allocortex are hybrids with Greek isos, "same", and allos, "other".

Anatomy

The neocortex is the most developed in its organisation and number of layers, of the cerebral tissues.[5] The neocortex consists of the grey matter, or neuronal cell bodies and unmyelinated fibers, surrounding the deeper white matter (myelinated axons) in the cerebrum. This is a very thin layer though, about 2–4 mm thick.[6] There are two types of cortex in the neocortex, the proisocortex and the true isocortex. The pro-isocortex is a transitional area between the true isocortex and the periallocortex (part of the allocortex). It is found in the cingulate cortex (part of the limbic system), in Brodmann's areas 24, 25, 30 and 32, the insula and the parahippocampal gyrus.

Of all the mammals studied to date (including humans), a species of oceanic dolphin known as the long-finned pilot whale has been found to have the most neocortical neurons.[7]

Geometry

The neocortex is smooth in

dolphins and primates and other larger mammals it has deep grooves (sulci) and ridges (gyri). These folds allow the surface area of the neocortex to be greatly increased. All human brains have the same overall pattern of main gyri and sulci, although they differ in detail from one person to another.[8] The mechanism by which the gyri form during embryogenesis is not entirely clear, and there are several competing hypotheses that explain gyrification, such as axonal tension,[9] cortical buckling[10] or differences in cellular proliferation rates in different areas of the cortex.[11]

Layers

Neurons form distinct layers in mouse visual cortex.  Layer II/III (green), Layer IV (purple), Layer V (red), Layer VI (yellow). 3D reconstructions from the MICrONS cubic millimeter.

The neocortex contains both excitatory (~80%) and inhibitory (~20%)

axons to other areas of neocortex, while those in the deeper layers V and VI often project out of the cortex, e.g. to the thalamus, brainstem, and spinal cord. Neurons in layer IV receive the majority of the synaptic connections from outside the cortex (mostly from thalamus), and themselves make short-range, local connections to other cortical layers.[12]
Thus, layer IV is the main recipient of incoming sensory information and distributes it to the other layers for further processing.

Cortical columns

The column is the function unit of computation in the cortex. Neurons are color-coded by their layer: Layer II/III (green), Layer IV (purple), Layer V (red), Layer VI (yellow).

The neocortex is often described as being arranged in vertical structures called

cortical columns, patches of neocortex with a diameter of roughly 0.5 mm (and a depth of 2 mm, i.e., spanning all six layers). These columns are often thought of as the basic repeating functional units of the neocortex, but their many definitions, in terms of anatomy, size, or function, are generally not consistent with each other, leading to a lack of consensus regarding their structure or function or even whether it makes sense to try to understand the neocortex in terms of columns.[15]

Function

The neocortex is derived embryonically from the dorsal

primary auditory cortex. Further subdivisions or areas of neocortex are responsible for more specific cognitive processes. In humans, the frontal lobe contains areas devoted to abilities that are enhanced in or unique to our species, such as complex language processing localized to the ventrolateral prefrontal cortex (Broca's area).[12] In humans and other primates, social and emotional processing is localized to the orbitofrontal cortex
.

The neocortex has also been shown to play an influential role in sleep, memory and learning processes.

depolarizing phase and are firing briefly at a high rate, a period of excitation occurs during a slow oscillation, called the up state.[16]

Clinical significance

Lesions that develop in

semantic memories). These symptoms can also be replicated by transcranial magnetic stimulation of this area. If damage is sustained to this area, patients do not develop anterograde amnesia and are able to recall episodic information.[18]

Evolution

See also: Pallium (neuroanatomy) § Evolution

The neocortex is the newest part of the cerebral cortex to evolve (hence the prefix neo meaning new); the other part of the cerebral cortex is the allocortex. The cellular organization of the allocortex is different from the six-layered neocortex. In humans, 90% of the cerebral cortex and 76% of the entire brain is neocortex.[12]

For a species to develop a larger neocortex, the brain must evolve in size so that it is large enough to support the region. Body size, basal

metabolic rate and life history are factors affecting brain evolution and the coevolution of neocortex size and group size.[19] The neocortex increased in size in response to pressures for greater cooperation and competition in early ancestors. With the size increase, there was greater voluntary inhibitory control of social behaviors resulting in increased social harmony.[20]

The six-layer cortex appears to be a distinguishing feature of mammals; it has been found in the brains of all mammals, but not in any other animals.[2] There is some debate,[21][22] however, as to the cross-species nomenclature for neocortex. In avians, for instance, there are clear examples of cognitive processes that are thought to be neocortical in nature, despite the lack of the distinctive six-layer neocortical structure.[23] Evidence suggest the avian pallium to be broadly equivalent to the mammalian neocortex.[24][25][26] In a similar manner, reptiles, such as turtles, have primary sensory cortices. A consistent, alternative name has yet to be agreed upon.

Neocortex ratio

The neocortex ratio of a species is the ratio of the size of the neocortex to the rest of the brain. A high neocortex ratio is thought to correlate with a number of social variables such as group size and the complexity of social mating behaviors.[27] Humans have a large neocortex as a percentage of total brain matter when compared with other mammals. For example, there is only a 30:1 ratio of neocortical gray matter to the size of the medulla oblongata in the brainstem of chimpanzees, while the ratio is 60:1 in humans.[28]

See also

References

  1. PMID 26359774
    . The neocortex is the part of the brain responsible for execution of higher-order brain functions, including cognition, sensory perception, and sophisticated motor control.
  2. ^
    PMID 21729779
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  3. ^ "BrainInfo". braininfo.rprc.washington.edu.
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  6. ISBN 978-0071390118
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  7. PMID 25505387
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  8. PMID 25120426
    . For example, in the human brain, the auditory cortex presents an expansion of cortical surface, with additional gyri and with a much larger inter-individual variability...
  9. S2CID 4355025
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  10. PMID 1135626
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  11. PMID 23542881
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  16. ^
    ISBN 978-0-205-239481
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  17. ISBN 1-57331-263-0
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  19. PMID 17301028
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  27. doi:10.1006/jhev.1995.1021
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  28. S2CID 5921065
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External links
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