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 cerebrum has a grey color, hence the name grey matter. Grey matter is formed by neurons and their unmyelinated fibers, whereas the white matter below the grey matter of the cortex is formed predominantly by myelinated axons 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 "sulci." The phylogenetically most recent part of the cerebral cortex, the neocortex, also called isocortex, is differentiated into 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 subfields. Relative variations in thickness or cell type (among other parameters) allow us to distinguish between different neocortical architectonic fields. The geometry of at least some 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) seem to be more clearly differentiated than in its deeper parts (Ref.: Welker, W. 1991. Why does the cerebral cortex fissures and folds? Cerebral Cortex, Vol 8b).
The cerebral cortex develops from the most anterior part of the neural plate, a specialized part of the embryonic 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 . 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, pial 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 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 glial cell or an ependymal cell. The migrating daughter cells become the pyramidal neurons of the cerebral cortex.
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.
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 axons bringing visual information from the thalamus into layer four of visual cortex.
Staining cross-sections of the cortex to reveal the position of 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 and spiny stellate neurons can be found
- The external granular layer II, which contains small 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 efferent
- The internal granular layer IV, which contains different types of 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 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 columns 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 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.
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.
The sensory areas are the 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
. Neighboring points in the primary visual cortex
, for example, correspond to neighboring points in the retina
. This topographic map
is called a retinotopic map
. In the same way, there exists a tonotopic map
in the primary auditory cortex and a 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 function to produce a meaningful perceptual experience
of the world, enable us to interact effectively, and support abstract thinking and language. The parietal
, and occipital lobes
- 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.
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:
In addition, motor functions have been described for:
Based on the differences in lamination the cerebral cortex can be classified into two major groups:
Auxiliary classes are:
Based on supposed developmental differences the following classification also appears:
In addition, cortex may be classified on the basis of gross topographical conventions into four lobes:
- Occipital Cortex
- Temporal Cortex
- Parietal Cortex
- Frontal Cortex
With magnetic resonance brain scanners
, 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
Another study has found that the somatosensory cortex is thicker in migraine
- 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.
- 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