Cerebral cortex

The cerebral cortex is a brain structure in vertebrates, including humans. It is the outermost layer of the cerebrum and has a grey color. (Hence the name "grey matter". Grey matter is formed by neurons and their fibers, and white matter below the grey matter of the cortex is formed predominantly by nerve fibers interconnecting cortical areas with each other and with subcortical structures.) The human cerebral cortex is 2-4 mm (0.08-0.16 inches) thick and is folded.

In the "higher" animals (especially the higher mammals), the surface of the cerebral cortex becomes folded. This creates grooves on the surface of the brain called "sulci" (singular = "sulcus"). The bumps or ridges on the surface of the brain are called "gyri" (singular = "gyrus"). The folding of the cortex increases the cortical surface area. The cerebral cortex, made up of four lobes, is involved in many complex brain functions including memory, attention, perceptual awareness, "thinking", language and consciousness.

Input
The cerebral cortex receives input from the thalamus. This input includes sensory information that originates from many different sensory organs eg: eyes, ears, etc. and processes the information.

Areas that receive that particular information are called sensory areas. The two hemispheres receive the information from the opposite sides of the body.

Parts of the cortex that receive sensory inputs from the thalamus are called primary sensory areas.

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 areas.

Association areas
Association areas can be grouped the following way:


 * 1) in the parietal, temporal and occipital lobes. It is involved in producing our perceptions resulting from what our eyes see, ears hear and other sensory organs tell us about the position of different parts of our body and relate them to the position of other objects in the environment
 * 2) in the frontal lobe. Called prefrontal association complex and involved in planning actions and movement, as well as abstract thought
 * 3) in the limbic association area. Involved in emotion and memory

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

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: Executing voluntary movements
 * Supplementary motor areas and premotor cortex: Selecting voluntary movements

In addition, motor functions have been described for
 * Posterior Parietal Cortex: Guiding voluntary movements in space
 * Dorsolateral Prefrontal Cortex: Deciding which voluntary movements to make according to higher-order instructions, rules and self-generated thoughts

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 neurones and glial cells. The most frontal part of the neural tube, the telencephalon gives rise to the cerebral hemispheres and the neocortex.

Most cortical neurones 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 and leaves the ventricular zone, and a daughter cell that continues to divide or that eventually dies. The postmitotic cells will become neurones.

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 (interestingly, during development, the inner layers are formed before the outer layers are), neurons form efferents  and receive  afferent  connections characteristic of its layer.


 * 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--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 and many small spindle-like pyramidal and multiform neurons. The layer VI sends efferent fibres to the thalamus establishing a very precise reciprocal interconnection between the cortex and the thalamus (Creutzfeldt, 1995).

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.

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
 * Limbic Cortex
 * Insular Cortex