Cerebral cortex
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226083154
2008-07-16T19:22:51Z
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[[WP:UNDO|Undid]] revision 226048419 by [[Special:Contributions/71.250.133.66|71.250.133.66]] ([[User talk:71.250.133.66|talk]])
{{otheruses|Cortex}}
[[Image:Cerebral Cortex location.jpg|thumb|200px|Location of the cerebral cortex]]
[[Image:Cerebral Cortex 10.5mm.jpg|thumb|200px|Slice of the cerebral cortex, ca. 10.5mm wide]]
[[Image:NeuronGolgi.png|thumb|250px|Golgi-stained neurons in the somatosensory cortex of the macaque monkey.]]
The '''cerebral cortex''' is a structure within the [[brain]] that plays a key role in [[memory]], [[attention]], perceptual [[awareness]], [[thought]], [[language]], and [[consciousness]]. In dead, preserved brains, the outermost layer of the [[telencephalon|cerebrum]] has a [[grey]] color, hence the name ''grey matter''. Grey matter is formed by [[neurons]] and their [[myelin|unmyelinated]] fibers, whereas the white matter below the grey matter of the cortex is formed predominantly by myelinated [[axon]]s interconnecting different regions of the [[central nervous system]]. The human cerebral cortex is 2-4 mm (0.08-0.16 inches) thick.
The surface of the cerebral cortex is folded in large mammals, wherein more than two-thirds of the cortical surface is buried in the grooves, called "[[sulcus (neuroanatomy)|sulci]]." The [[phylogeny|phylogenetically]] most recent part of the cerebral cortex, the [[neocortex]], is differentiated into [[Neocortex#Anatomy|six horizontal layers]]; the more ancient part of the cerebral cortex, the [[hippocampus]] (also called archicortex), has at most three cellular layers, and is divided into [[Hippocampal subfields|subfields]]. Relative variations in thickness or cell type (among other parameters) allow us to distinguish between different neocortical architectonic fields. The geometry of these fields seems to be related to the anatomy of the cortical folds, and, for example, layers in the upper part of the cortical grooves (called [[gyri]]) are more clearly differentiated than in its deeper parts (called sulci).
== Development ==
The cerebral cortex develops from the most anterior part of the [[neural plate]], a specialized part of the embryonic [[germ layer|ectoderm]]. The neural plate folds and closes to form the [[neural tube]]. From the cavity inside the neural tube develops the [[ventricular system]], and, from the epithelial cells of its walls, the [[neurons]] and [[glia]] of the nervous system. The most anterior (frontal) part of the neural tube, the [[telencephalon]], gives rise to the cerebral hemispheres and cortex.
Cortical neurons are generated within the ventricular zone, next to the ventricles. At first, this zone contains "progenitor" cells, which divide to produce glial and neuronal cells <ref>{{Cite journal
| author = [[Stephen C. Noctor]], Alexander C. Flint, Tanily A. Weissman, Ryan S. Dammerman & [[Arnold R. Kriegstein]]
| year = [[2001]]
| title = Neurons derived from radial glial cells establish radial units in neocortex
| journal = [[Nature (journal)|Nature]]
| volume = 409
| issue = 6821
| pages = 714–720
| pmid = 11217860
| doi = 10.1038/35055553
| url = http://www.nature.com/nature/journal/v409/n6821/abs/409714a0.html
}}</ref>. The glial fibers produced in the first divisions of the progenitor cells are radially oriented, spanning the thickness of the cortex from the ventricular zone to the outer, [[Pia_mater|pia]]l surface, and provide scaffolding for the migration of neurons outwards from the ventricular zone. The first divisions of the progenitor cells are symmetric, which duplicates the total number of progenitor cells at each [[Mitosis|mitotic cycle]]. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates along the radial glial fibers, leaving the ventricular zone, and one progenitor cell, which continues to divide until the end of development, when it differentiates into a [[astrocyte|glial cell]] or an [[ependyma|ependymal cell]]. The migrating daughter cells become the [[pyramidal neurons]] of the cerebral cortex.<ref>{{Cite journal
| author = P. Rakic
| year = [[1988]]
| title = Specification of cerebral cortical areas
| journal = [[Science (journal)|Science]]
| volume = 241
| issue = 4862
| pages = 170–176
| doi = 10.1126/science.3291116
| url = http://www.sciencemag.org/cgi/content/abstract/241/4862/170
| pmid = 3291116
}}</ref>
The layered structure of the mature cerebral cortex is formed during development. The first [[pyramidal neurons]] generated migrate out of the ventricular zone and form the [[preplate]]. Next, a cohort of neurons migrating into the middle of the preplate divides this transient layer into the superficial [[marginal zone]], which will become layer one of the mature neocortex, and the [[subplate]], forming a middle layer called the [[cortical plate]]. These cells will form the deep layers of the mature cortex, layers five and six. Later born neurons migrate radially into the cortical plate past the deep layer neurons, and become the upper layers (two to four). Thus, the layers of the cortex are created in an inside-out order.
== Laminar pattern ==
The different cortical layers each contain a characteristic distribution of neuronal cell types and connections with other cortical and subcortical regions. One of the most clear examples of cortical layering is the [[Stria of Gennari]] in the primary visual cortex. This is a band of whiter tissue that can be observed with the naked eye in the fundus of the calcarine sulcus of the occipital lobe. The Stria of Gennari is composed of [[axon]]s bringing visual information from the [[thalamus]] into layer four of visual cortex.
Staining cross-sections of the cortex to reveal the position of the neuronal cell bodies and the intracortical axon tracts allowed neuroanatomists in the early 20th century to produced a detailed description of the laminar structure of the cortex in different species. After the work of [[Korbinian Brodmann]] (1909), the neurons of the cerebral cortex are grouped into six main layers, from outside (pial surface) to inside (white matter):
# The molecular layer I, which contains few scattered neurons and consists mainly of extensions of apical dendrites and horizontally-oriented axons; some [[Cajal-Retzius cell|Cajal-Retzius]] and spiny stellate neurons can be found
# The external granular layer II, which contains small [[Pyramidal cell|pyramidal neurons]] and numerous stellate neurons
# The external pyramidal layer III, which contains predominantly small and medium-size [[pyramidal neurons]], as well as non-pyramidal neurons with vertically-oriented intracortical axons; layers I through III are the main target of interhemispheric corticocortical [[afferents]], and layer III is the principal source of corticocortical [[wikt:efferent|efferents]]
# The internal granular layer IV, which contains different types of [[stellate cell|stellate]] and [[pyramidal neurons]], and is the main target of thalamocortical afferents as well as intra-hemispheric corticocortical afferents
# The internal pyramidal layer V, which contains large [[pyramidal neurons]] (such as the Betz cells in the primary motor cortex); it is the principal source of subcortical efferents
# The multiform layer VI, which contains few large [[pyramidal|pyramidal neurons]] and many small spindle-like pyramidal and [[multiform neurons]]; layer VI sends efferent fibers to the thalamus, establishing a very precise reciprocal interconnection between the cortex and the thalamus (Creutzfeldt, 1995).
It is important to note that the cortical layers are not simply stacked one over the other; there exist characteristic connections between different layers and neuronal types, which span all the thickness of the cortex. These cortical microcircuits are grouped into [[cortical column]]s and minicolumns, the latter of which have been proposed to be the basic functional units of cortex (Mountcastle, 1997). In 1957, Vernon Mountcastle showed that the functional properties of the cortex change abruptly between laterally adjacent points; however, they are continuous in the direction perpendicular to the surface. Later works have provided evidence of the presence of functionally distinct cortical columns in the visual cortex (Hubel and [[Torsten Wiesel|Wiesel]], 1959), auditory cortex
and associative cortex (Tanaka, 2003).
Cortical areas that lack a layer IV are called ''agranular''. Cortical areas that have only a rudimentary layer IV are called ''dysgranular''<ref> S.M. Dombrowski , C.C. Hilgetag , and H. Barbas. [http://cercor.oxfordjournals.org/cgi/content/full/11/10/975 Quantitative Architecture Distinguishes Prefrontal Cortical Systems in the Rhesus Monkey].Cereb. Cortex 11: 975-988. "...they either lack (agranular) or have only a rudimentary granular layer IV (dysgranular)."</ref>.
==Connections of the cerebral cortex==
The cerebral cortex is connected to various subcortical structures such as the [[thalamus]] and the [[basal ganglia]], sending information to them along efferent connections and receiving information from them via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus. Olfactory information, however, passes through the [[olfactory bulb]] to the olfactory cortex ([[piriform cortex]]). The vast majority of connections are from one area of the cortex to another rather than to subcortical areas; Braitenberg and Schüz (1991) put the figure as high as 99%.
The cortex is commonly described as comprising three parts: sensory, motor, and association areas.
===Sensory areas===
The sensory areas are the [[Cortical area|areas]] that receive and process information from the [[senses]]. Parts of the cortex that receive sensory inputs from the [[thalamus]] are called primary sensory areas. The senses of vision, audition, and touch are served by the primary [[visual cortex]], primary [[auditory cortex]] and primary [[somatosensory cortex]]. In general, the two hemispheres receive information from the opposite (contralateral) side of the [[body]]. For example the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left visual field. The organization of sensory maps in the cortex reflects that of the corresponding sensing organ, in what is known as a [[Topographic map (Neuroanatomy)|topographic map]]. Neighboring points in the primary [[visual cortex]], for example, correspond to neighboring points in the [[retina]]. This [[Topographic map (Neuroanatomy)|topographic map]] is called a [[retinotopy|retinotopic map]]. In the same way, there exists a [[tonotopy|tonotopic map]] in the primary auditory cortex and a [[somatotopy|somatotopic map]] in the primary sensory cortex. This last topographic map of the body onto the [[posterior central gyrus]] has been illustrated as a deformed human representation, the somatosensory [[homunculus]], where the size of different body parts reflects the relative density of their innervation. Areas with lots of sensory innervation, such as the fingertips and the lips, require more cortical area to process finer sensation.
===Association areas===
Association areas function to produce a meaningful [[perception|perceptual experience]] of the world, enable us to interact effectively, and support abstract thinking and language. The [[parietal lobe|parietal]], [[Temporal lobe|temporal]], and [[occipital lobe]]s - all located in the posterior part of the cortex - organize sensory information into a coherent perceptual model of our environment centered on our [[body image]]. The [[frontal lobe]] or ''prefrontal association complex'' is involved in planning actions and movement, as well as abstract thought. Our language abilities are localized to the association areas of the parietal-temporal-occipital complex, typically in the left hemisphere. [[Wernicke's area]] relates to understanding language while [[Broca's area]] relates to its use.
===Motor areas===
The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of the body, and vice versa.
Two areas of the cortex are commonly referred to as motor:
* ''[[Primary motor cortex]]'', which '''executes''' voluntary movements
* ''[[Supplementary motor area]]s'' and ''[[premotor cortex]]'', which '''select''' voluntary movements.
In addition, motor functions have been described for:
* ''[[Posterior parietal cortex]]'', which guides voluntary movements in space
* ''[[Dorsolateral prefrontal cortex]]'', which decides which voluntary movements to make according to higher-order instructions, rules, and self-generated thoughts.
== Classification ==
Based on the differences in lamination the cerebral cortex can be classified into two major groups:
* [[Isocortex]] (homotypical cortex), the part of the cortex with six layers
* [[Allocortex]] (heterotypical cortex) with variable number of layers, e.g., [[olfactory cortex]] and [[hippocampus]].
Auxiliary classes are:
* [[Mesocortex]], classification between isocortex and allocortex where layers 2, 3, and 4 are merged
* [[Proisocortex]], [[Brodmann area]]s [[Brodmann area 24|24]], [[Brodmann area 25|25]], [[Brodmann area 32|32]]
* [[Periallocortex]], comprising cortical areas adjacent to [[allocortex]].
Based on supposed developmental differences the following classification also appears:
* [[Archicortex]], which [[Phylogenetics|phylogenetically]] is the oldest cortex
* [[Paleocortex]]
* [[Neocortex]] or [[Neopallium]], which corresponds to the isocortex.
In addition, cortex may be classified on the basis of gross topographical conventions into four lobes:
* Occipital Cortex
* Temporal Cortex
* Parietal Cortex
* Frontal Cortex
== Cortical thickness ==
With magnetic resonance [[brain scanner]]s, it is possible to get a measure for the thickness of the human cerebral cortex and relate it to other measures.
One study has found some positive association between the cortical thickness and [[intelligence]].<ref>{{Cite journal
| author = Katherine L. Narr, [[Roger P. Woods]], [[Paul M. Thompson]], Philip Szeszko, Delbert Robinson, Teodora Dimtcheva, Mala Gurbani, [[Arthur W. Toga]] and Robert M. Bilder
| title = Relationships between IQ and Regional Cortical Grey Matter Thickness in Healthy Adults
| journal = [[Cerebral Cortex (journal)|Cerebral Cortex]]
| volume = 17
| issue = 9
| pages = 2163–2171
| year = 2007
| doi = 10.1093/cercor/bhl125
| pmid = 17118969
}}</ref>
Another study has found that the somatosensory cortex is thicker in [[migraine]] sufferers.<ref>{{Cite journal
| author = Alexandre F.M. DaSilva, Cristina Granziera, Josh Snyder and Nouchine Hadjikhani
| title = Thickening in the somatosensory cortex of patients with migraine
| journal = [[Neurology (journal)|Neurology]]
| year = 2007
| volume = 69
| pages = 1990–1995
| doi = 10.1212/01.wnl.0000291618.32247.2d
| pmid = 18025393
}} News report:
* {{Cite web
| author = Catharine Paddock
| title = Migraine Sufferers Have Thicker Brain Cortex
| publisher = Medical News Today
| url = http://www.medicalnewstoday.com/articles/89286.php
| date = 2007-11-20
}}</ref>
== See also ==
* [[Cortical column]]
* [[Frontal lobe]]
* [[Limbic system]]
* [[List of regions in the human brain]]
* [[Microgyrus]]
* [[Occipital lobe]]
* [[Parietal lobe]]
* [[Temporal lobe]]
* [[Cerebral hemisphere]]
* [[Neocortex]]
* [[Subplate]]
* [[Brain-computer interface]]
==Further reading==
* Kandel, E.R., Schwartz, J. H., and Jessell, T.M. ''Principles of Neural Science'' (Fourth Edition). 2000. New York, McGraw Hill. ISBN 0-8385-7701-6.
* Zigmond, M. J., Bloom, F. E., Landis, S.C., Roberts, J.L, and Squire, L.R. ''Fundamental Neuroscience''. 1999. San Diego, Academic Press. ISBN 0-12-780870-1.
==References==
{{reflist}}
* Angevine, J. and Sidman, R. 1961. Autoradiographic study of cell migration during histogenesis of cerebral cortex in the mouse. ''Nature'', 192:766-768
* Creutzfeldt, O. 1995. ''Cortex Cerebri''. Springer-Verlag.
* Marin-Padilla, M. 2001. Evolución de la estructura de la neocorteza del mamífero: Nueva teoría citoarquitectónica. ''Rev. Neurol'', 33(9):843-853
* Mountcastle, V. 1997. The columnar organization of the neocortex. ''Brain'', 120:701-722
* Noctor SC, Flint AC, Weissman TA, Dammerman RS, Kriegstein AR. (2001) Neurons derived from radial glial cells establish radial units in neocortex. "Nature" 409(6821):714-720. PMID 11217860
* Ogawa, M. ''et al.'' 1995. The reeler gene-associated antigen on Cajal-Retzius neurons is a crucial molecule for laminar organization of cortical neurones. ''Neuron'', 14:899-912
* Rakic, P. 1988. Specification of cerebral cortical areas. ''Science'', 241:170-176
* Friauf, J. 1991. Changing patterns of synaptic input to subplate and cortical plate during development of visual cortex.
* Braitenberg, V and Schüz, A 1991. "Anatomy of the Cortex: Statistics and Geometry" NY: Springer-Verlag
== External links ==
* {{BrainInfo|hier|20}}
* {{BrainMaps|cerebral%20cortex|cerebral cortex}}
* [http://webvision.med.utah.edu/VisualCortex.html Webvision - The primary visual cortex] Comprehensive article about the structure and function of the primary visual cortex.
* [http://webvision.med.utah.edu/imageswv/BasicCells.jpg Webvision - Basic cell types] Image of the basic cell types of the monkey cerebral cortex.
*[http://info.med.yale.edu/chldstdy/plomdevelop/development/Arch1.html Development of the Cerebral Cortex] Different topics on cortical development in the form of columns written by leading scientists.
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