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 layers of the cerebrum has a grey color, hence the name "grey matter". Grey matter is formed by neurons and their unmyelinated fibers while 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, "thinking", language and consciousness.

The surface of the cerebral cortex is folded in large mammals where 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, while the more recent neo-cortex is differentiated in six basic layers. Relative variations in thickness or cell type (among other parameters) allows us to distinguish among 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").

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. Initially, progenitor cells in the ventricular zone divide symmetrically, producing two progenitor cells by mitotic cycle. Then, some progenitor cells begin to divide asymmetrically, producing one postmitotic cell that migrates radially and leaves the ventricular zone, and a daughter cell that continues to divide or that eventually dies. The migrating cells will become neurons. Recent work has identified radial glial cells (also here) as one population of progenitor cells.

During fetal and neonatal life, the immature cerebral cortex (the cortical plate) is sandwiched between two synaptic zones: the marginal zone above and an area just below the cortical plate, the subplate. The subplate is transient and disappears by approximately 2 months postnatal (Friauf, 1991).

Laminar pattern
The standard areas of cortex (isocortex) is characterized as having six distinct layers. From outside inward:


 * 1) Molecular layer
 * 2) External granular layer
 * 3) External pyramidal layer
 * 4) Internal granular layer
 * 5) Internal pyramidal layer
 * 6) Multiform layer.

After migration, neurons form efferents and receive afferent connections characteristic of their layer. It is interesting to note that during development, the inner layers are formed before the outer layers are.


 * 1) The molecular layer I contains few scattered neurons and consists mainly of extensions of apical dendrites and horizontally oriented axons, and some Cajal-Retzius and spiny stellate neurons can be found.
 * 2) The external granular layer II contains small pyramidal neurons and numerous stellate neurons.
 * 3) The external pyramidal layer III contains predominantly small and medium sized 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 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 contains large pyramidal neurons (as the Betz cells in the primary motor cortex), as well as interneurons, and it is the principal source of efferent for all the motor-related subcortical structures.
 * 6) The multiform layer VI 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).

During early development, there is an additional layer of neurons present in the future white matter. these are called subplate neurons and these neurons disappear during postnatal development.

The cortical layers are not simply stacked one over the other; they develop characteristic connections between different layers, which define the basic structure of the cortical columns in the mature cortex (Mountcastle, 1997).

There are no actual borders between the layers, and neurons cross layer boundaries with their dendrites and axons trees all over. The pyramidal cells (the majority of the neurons) span at least three layers, and in many cases all the layers. Thus, it is not obvious that the layers have any functional significance. However, the flow of current in the cortical layers is consistent and shows inputs principally in layer IV, and the spread of activity, and thus the flow of information, roughly follows the models put forth by Martin, Whitteridge, and Somogyi in 1985.

Connections of the cerebral cortex
The cerebral cortex sends connections (efferents) and receives connections (afferents) from many subcortical structures like the thalamus and basal ganglia. Most of the sensory stimulation arrives at the cerebral cortex indirectly through different thalamic nuclei. This is the case of touch, vision and sound but not of olfactory stimulation, which passes to the olfactory bulb and then to the olfactory (pyriform) cortex. The largest part of the connections arriving at the cerebral cortex do not come from subcortical structures however. The main source of cortical stimulation is the cerebral cortex itself: maybe 99% of the total connections (Braitenberg and Schüz, 1991).

Other areas receive impulses from the primary sensory areas and integrate the information coming in from different types of receptors (i.e., modalities). These are often called association areas and make up a great deal of the cortex in all primates, humans included. Thus, the cortex is commonly described as comprised of the primary sensory areas, the motor areas and the association area.

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 your 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.

Sensory areas
Areas that receive that particular information are called sensory areas. 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.

Association areas
Association areas comprise three major groups:


 * 1) Parietal, temporal, and occipital lobes - all located in the posterior part of the brain - are involved in producing our perceptions resulting from what our eyes see, ears hear, and other sensory organs inform us about the position of different parts of our body and relate them to the position of other objects in the environment
 * 2) Frontal lobe - called prefrontal association complex and involved in planning actions and movement, as well as abstract thought

In humans, the association areas of the left hemisphere, especially the parietal-temporal-occipital complex, are responsible for our understanding and use of language.

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 is cortical areas adjacent to allocortex.

Based on supposed developmental differences the following classification also appears:


 * Neocortex or Neopallium that corresponds to the isocortex.
 * Archicortex
 * Paleocortex

In addition, cortex may be classified on the basis of gross topographical conventions into the following:


 * Temporal Cortex
 * Parietal Cortex
 * Frontal Cortex
 * Occipital 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.