Cerebral cortex

      The cerebral cortex is a structure within the vertebrate brain with distinct structural and functional properties. In non-living, 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, and plays a central role in many complex brain functions including memory, attention, perceptual awareness, thought, language, and consciousness.

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 more ancient part of the cerebral cortex, the hippocampus, is differentiated in five layers of neurons, whereas the more recent neo-cortex is differentiated in six basic layers. 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 sulcal "fundi").

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Development

The cerebral cortex develops from the neural plate, a specialised 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 glial cells. The most-frontal part of the neural tube, the telencephalon, gives rise to the cerebral hemispheres and the neocortex.

Most cortical neurons are generated within the ventricular zone close to the ventricles. At first, this zone contains "progenitor" cells, which divide to produce glial and neuronal cells ^[1]. The glial fibres produced in the first divisions of the progenitor cells are radially-oriented, spanning all the thickness of the cortex, and will provide scafolding for the future migration of neurones from the ventricular zone to the external surface of the brain. The first divisions of the progenitor cells will be 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 through the radial glia leaving the ventricular zone, and a daughter cell that continues to divide or that eventually dies. The migrating cells will become neurons.^[2]

After migration, neurons form efferents and receive afferent connections characteristic of their layer.

Laminar pattern

The layered structure of the cerebral cortex, formed during development, can be still observed in the adult vertebrate brain. The different cortical layers presents a characteristic distribution of 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 marks the layer where most visual connections arrive from the thalamus.

Staining the nervous tissue to reveal the position of the neuronal cell bodies of the intracortical myelin sheats allowed the 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 Brodman (1909), the different layers of the cerebral cortex are regrouped in six main layers, from outside to inward:

1. 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
2. The external granular layer II, which contains small pyramidal neurons and numerous stellate neurons
3. 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 efferents
4. 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
5. The internal pyramidal layer V, which contains large pyramidal neurons (as the Betz cells in the primary motor cortex), as well as interneurons; it is the principal source of efferent for all the motor-related subcortical structures
6. The multiform layer VI, which contains few large pyramidal neurons and many small spindle-like pyramidal and multiform neurons; the 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 functionally regrouped into cortical columns, which have been proposed to be the basic functional units of cortex (Mountcastle, 1997). In 1957, Vernon Mountcastle showed that the functional properties if the cortex change abruptly between adjacent points in the surface; 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 Wisel, 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^[3].

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 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 the information from the opposite sides 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 organisation of sensory maps in the cortex reflects that of the corresponding sensing organ, in which is known as a topographic map. Neighbouring points in the primary visual cortex, for example, correspond to neighbouring 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 deformed human representation, the somatosensory homunculus, where the size of different limbs reflects the importance of their innervation.

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 areas 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.

Association areas

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, temporal, and occipital lobes - all located in the posterior part of the brain - organise 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 localised 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.

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 areas 24, 25, 32
• Periallocortex, comprising cortical areas adjacent to allocortex.

Based on supposed developmental differences the following classification also appears:

• Archicortex, which 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 the following:

• Occipital Cortex
• Temporal Cortex
• Parietal Cortex
• Frontal Cortex.

Cortical thickness

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.^[4] Another study has found that the somatosensory cortex is thicker in migraine sufferers.^[5]

See also

• Cortical column
• Frontal lobe
• Limbic system
• List of regions in the human brain
• Microgyrus
• Occipital lobe
• Parietal lobe
• Temporal lobe
• Cerebral hemisphere
• 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

• 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