Cerebral Cortex

History of the study of the cortex

Antiquity and the Middle Ages

During high the Antiquity, the Cerveau was regarded as minor importance in the Human body. The Egyptian were not worried to preserve this body in the process of Momification, nevertheless the Papyrus Edwin Smith details cranial cases of Traumatisme S and medullary and their consequences. The seat of the thoughts and the emotions was located in the Cœur, belief which is found today in a certain number of expressions or of Symbole S. It is at the 5th century with the dissections of Alcmaéon that the seat of the Vision is localized in the brain. Thereafter Démocrite imagined the heart in the form of particles dispersed in all the body, but with a strong concentration in the brain. Aristote and Plato gave to the brain a role in the thought, the emotion S and the feelings, with equality with the heart.

The exact moment where the cortex was known antiques is unspecified, but Galien, eminent doctor of the 1st century knew it. He considered however under the influence of Hérophile that the seat of the thought was localized in the cerebral Ventricules, the white substance and gray being only one felting intended to protect these last. This idea will perdura during all the Moyen-âge. Under the influence of the Arab thought , but without one being able to allot this to a precise author, little by little, the idea was made that the cerebral cortex was at the origin of the advanced cognitive functions of the Mammifères.

Rebirth and the modern time

Knowledge on the cortex evolved/moved very little until the Renaissance for lack of an effective tool for exploration. Then the Microscope was outlined by the brothers Jansen, of the Dutch opticians in 1590 and became fully functional later less than one century thanks to Antoni van Leeuwenhoek. At that time, average the techniques were limited, but knowledge was it still more. It was enough to point its microscope to make exceptional discoveries anywhere. It is what Malpighi in the years 1660 did. It described the first cell cortex. But progress remained slow, because the cerebral cells are very dense and difficult to differentiate by traditional colorings. In spite of these difficulties, the Russian Vladimir Alexeïevitch Betz managed in 1874 to identify the pyramidal cells. It is of Camillo Golgi that the solution came, in 1873, when it developed a coloring with silver salts (the reazione will nera ) which marked only some cells in the whole of fabric. Those thus appeared fully differentiated from their neighbors, with all their quite visible tree structure. However, it is not him, but a contemporary, Santiago Ramon there Cajal which will launch out in the physiological exploration of the cortex. Using the technique of its fellow-member, it will describe the cellular types and the organization in six layers of the Néocortex. The two scientists were opposed a long time, Golgi supporting the reticular theory (the Neuron S constitute a Syncytium) nervous system, whereas Ramon Cajal was a partisan of the neuronal theory there (the neurons are independent cells connected between them by Synapse S). Finally, Golgi adopted the ideas of Ramon Cajal towards 1900et they jointly accepted there the Nobel Prize in 1906 for their histological work on the nervous system. The cellular theory was finally confirmed at the 20th century with the invention of the Electron microscope.

Parallel to the histological studies, systems analyzes were carried out. At the time, the only method was the study of the consequences of a lesion of the cerebral cortex on the human cognitive performances. The Egyptians had already noted that of the cranial traumatisms could involve visual problems. It is Paul Broca, which studying a patient aphasic, will highlight the bond between a cortical lesion and a cognitive deficit. Thereafter, the function of many cerebral zones was identified and at the dawn of the 20th century, the localization of the surfaces visual, auditive, somatosensorielles and driving was known. Beside the natural lesions, the neurobiologists studied the effects of the caused lesions, generally with an allegedly therapeutic aim: the principal one being the Lobotomy. A large wave of lobotomy, evoked in the film One Flew Over the Cuckoo's Nest , which will decline as from 1950 (date of its prohibition in the USSR) allowed a better knowledge of the nervous system, but with disastrous human costs. This technique is today prohibited in France, but continuous with being applied to the USA, in Northern Europe, India and some other countries in extreme cases.

Functional exploration by the cerebral imagery

In 1875, an English doctor, Richard Caton, are the first to measure the neuroelectric activity of the cerebral cortex while placing the electrode of a Galvanomètre directly in contact with the surface of the brain of craniotomized animals . It shows thus that the functional activity (for example vision) corresponds to the appearance of a negative polarization in a circumscribed zone of the cerebral cortex.

The recording of the neuroelectric activity at the human being will begin with the appearance of the electroencephalography (EEG) development by Hans Berger during the Années 1920. This technique makes it possible for the first time studied the neurophysiological corrélats cognitive activities in real-time with an excellent temporal resolution about the millisecond. This examination remains impossible to circumvent for the diagnosis and the classification of the epilepsy S.

It is in second half of the XXe century that a great revolution in the study of the cerebral cortex is, with the development of noninvasive methods of cerebral Imagerie. The doctor can see the brain functioning without opening cranium. Up to that point, radiographies standards gave only not exploitable images (the brain not being radiopaque) and the cerebral Angiographie made it possible to see only the cerebral vascular axes. “Thanks to the introduction of the techniques of neuro-imagery, the molar level of description could replace the molecular level dominating. ”

It was initially the scanner which made it possible to visualize, for the first time the brain and the cortical zones with a remarkable precision, then the nuclear Magnetic resonance (IRM) considerably modified, in its turn, the iconographic study of the cortical structures.

In 1938, the nuclear principle of magnetic resonance is discovered by Isidor Isaac Rabi. This discovery will succeed, in 1973, with the development of what was going to become the Imagerie by magnetic resonance or IRM, simultaneously by Paul Lauterbur and Peter Mansfield which jointly accepted a Nobel Prize (Nobel Prize of physiology or medicine) in 2003.

Evolution and development of the cortex

Phylogenic evolution

The cortex underwent a long evolution since its appearance at the first Craniates or their ancestors. One distinguishes in the mammals the neocortex, also called Néopallium and the allocortex itself subdivided in paléocortex (or Paléopallium) and archicortex (or Archipallium). The archicortex is oldest, it already exists in fish in the rhinencéphale, structure responsible for the treatment of the olfactive feelings. At the human being, one finds it in very old structures such as the Hippocampe (brain) and the gyrus notched. The paléocortex is more recent. It is well represented at the Reptile S where it reaches its maximum development, but it is still present at the Mammifères in old structures such as the central gray Noyaux or the Rhinencéphale. The neocortex is phylogéniquement most recent. Existing with the state of outline in the reptiles, it will be the subject of a growth in the mammals at the point to push back the allocortex in reduced zones. It is at the man that it reaches his maximum development, constituting 80% of the neurons of the central nervous system.

The modern cortex of the mammals derives from the rhinencéphale of the Poisson S. the fact that the old olfactive structure of fish gave rise to the cerebral hemispheres reflects the importance which smell in the first mammals had and which it has still today for many between-them. An abstract assumption suggests that could be with the absence of information carried by the olfactive feelings. Whereas the auditive signals can by themselves provide information on the size, the position and the movement of the other animals, it is not the case of the olfactive signals. To be useful, they must be associated with mnemic-traces which put them in relation to visual or auditive memories. The cerebral hemispheres developed thus and received visual, auditive and somatic afférences in seen to integrate all this information with the olfactive signals. This is only one assumption and there does not exist any means of checking it, the evolution making it possible to note the succession of the stages, but not to give the reasons of them, especially for a fossilizing body as badly as the Cerveau.

Development embryologic (ontogenetic evolution)

The cerebral cortex is formed in the former part of the Tube neural, itself drifting of the Plaque neurale, a differentiation of the dorsal Ectoderme under the influence of the Notochorde.

The first cerebral structure to be different in what will give the cerebral hemispheres is the ventricular system. The original cells of the neurons are located in the epithelium which borders the ventricles. Initially, the progéniteurs divide in a symmetrical way to multiply then in an asymmetrical way. One of the two cells migrates then outside the ventricular zone to reach the cortex. It is different then out of neuron. The other cell remains in the ventricular and continuous zone to be divided. The cells gliales multiply in a similar way, their progéniteurs are different from those of the neurons.

For the periods fetal and néonatale, the neurons of the immature cortex cerebral (the cortical plate) are sandwiched between the marginal zone outside and the under-plate located just in lower part at the interface with what will give the white substance. The under-plate has a transitory existence. It will disappear at the man, two months after the birth. The marginal zone will persist, becoming layer 1 of the neocortex.

Structure of the cerebral cortex

Histology

The histological study of the cortex started very early. The cellular types were highlighted as of the 19th century. Great names were associated with these discoveries, such as the Nobel Prize Santiago Ramon there Cajal and Camillo Golgi.

Under the microscope, the human neocortex appears subdivided in six layers. This number varies according to the species, for example five at the dolphin, three in the reptiles.

The layers are numbered from surface. One distinguishes in the order:

the molecular layer. It contains axons and dendrites. The neurons of the internal layers send to it dendrites courts and directed perpendicular to the surface of the cortex and axons long directed parallel to this surface. The neuronal prolongations have a structure similar to it to that found in the cérébelleux cortex, pointing out the toric memories of the computers of the years 1950. One finds there also some neurons of Cajal-Retzius and the spangled neurons.
the external granular layer contains granular neurons. It receives the afférences of other surfaces of the cortex. One speaks in what relates to it related cortico-cortical connections .
the external pyramidal layer. Composed of pyramidal cells, it emits connections towards other zones of the cerebral cortex. It is here about efferent cortico-cortical connections .
the internal granular layer. It contains spangled and pyramidal neurons. It is by this layer that information coming from the outside of the cortex (for example of the Thalamus) enters the cortex. It receives also the afférences coming from the other cerebral hemisphere.
the internal pyramidal layer. It is also a layer sending of connection efferent but which leaves the cortex. It is, for example, of this layer which the neurons leave which in the Motoneurone S.
the polymorphic layer, the internal layer in an adult state. It sends axonaux prolongations in direction of thalamus allowing a feedback on the entries of the cerebral cortex.

A seventh layer exists transitorily during embryogenesis. It disappears with cerebral maturation.

These cortical layers are not a simple stacking of neurons. The neurons are organized in functional units taking the shape of columns perpendicular to the surface of the cortex, each one providing a precise function. However it is not possible to distinguish them by histological methods; it is by functional studies of the visual Cortex that this structure was highlighted before being generalized with the whole of the neocortex.

The paléocortex and the archicortex, structurally simpler, are not laminated and do not have this structure in columns.

Anatomy

At the Man, the thickness of the cortex lies between 1 and 4,5 millimetres and its surface exceeds two square meters. In order to be able to place in the brain-pan, the cortex is folded up with edges called cerebral convolutions and furrows, the cracks. This kind of provision is known as gyrencéphale , in opposition to the lissencéphales existing for example in the rat, which presents a smooth cortex, deprived of convolution. This property is not in relation to the complexity of the brain, but is related to the size of the individual. When the size of a double individual, his volume is multiplied by eight. If the cortex remained smooth, its surface would be multiplied only by four. To preserve the proportion, the cortex must be folded according to the increase in size. To this, specificities related to the lines are added: for example, with equal size, the carnivorous are more intelligent than the Herbivore S, they have a larger cortex, therefore more folded.

The deepest cracks divide the cortex into lobes . According to their situation, one speaks about lobes frontal, parietal, occipital and temporal.

Under the cortex the Matière white made up of Axone S is which establishes connections between cellular bodies of the cortex and other parts of the brain.

Organization

The cerebral cortex is divided into functional zones, called surfaces , each one providing a precise cognitive function. In fact the studies of Paul Broca into 1861 for the first time suggested the existence of such an organization of the neocortex. These zones are about identical for all the individuals of the same species, but present small differences. This specialization is fuzzy in the primitive mammals and is specified as one approaches the man.

One distinguishes three great types of zones: sensory surfaces, driving surfaces and surfaces of association.

Sensory surfaces at the man

Three cerebral surfaces are specialized in the data processing sensory: the visual cortex located in the occipital Lobe, the auditive cortex in the temporal lobe and the cortex somatosensoriel in the parietal cortex.

The visual Cortex is divided into two zones: the visual cortex primary which is a direct projection of the retina and carries out a treatment of low level on the visual data (identification of the lines, the colors, the directions of displacements) and a visual cortex secondary which gathers these elements to obtain objects having a form, a color and a movement precise. As for driving information, the afférences of this surface are cross, but in different ways: the left hemisphere does not receive the data coming from the eye right, but coming from the right part of the field of vision of each eye.

The auditive Cortex is organized in a way similar to the visual cortex. It is located in the temporal Lobe. It includes/understands a primary auditive surface which identify the frequencies and a secondary auditive surface which reconstitutes the sounds.

The Cortex somatosensoriel is the exact reflection of the primary education driving cortex. Each body projects afférences somatotopic way there. The size of the zone assigned to each part of the body is proportional to the space discrimination of the zone: hand and face thus have the most important surfaces to with it. This provision is materialized by the concept of sensitive Homonculus. On the other hand, the various types of feeling do not seem separate at this stage.

Driving surfaces at the man

One distinguishes two zones from the cortex specialized in motricity, one and the other in the frontal Cortex.

The principal one is the driving surface primary which occupies all the posterior part of the frontal Lobe, just in front of the central furrow. It is organized in a somatotopic way (each zone of the body receiving a afférence of a precise part of this surface), surface associated with a muscle being proportional to the accuracy of mouvement of which it is able: the face and the hand thus are strongly represented. As for the somesthesy, there exists a driving Homoncule here. One finds there the Aire of Broca (cf image of right-hand side) for which the attack is responsible for the Aphasie of Broca, the auditive cortex in the temporal lobe and the cortex somatosensoriel in the parietal cortex. , a disease in which a person can express her opinion in the form of coherent sentences, but cannot pronounce them. Its role is to carry out the voluntary movements. The efférences of the neurons of this zone are cross: the left hemisphere orders the movements of the right part of the body and conversely.

There exists also an additional driving surface, located in the Cortex préfrontal, and which selects the voluntary movements.

Other driving surfaces were discovered in the parietal Cortex and préfrontal, taking part in the space integration of the movement and the relations between movement and thought.

Surfaces of association at the man

The surfaces of association represent the major part of the cerebral cortex at the man and are the independent factor of the size of its brain. In fact, this term indicates all the surfaces néocorticales which are neither driving, nor sensory; their functions are thus very diverse.

One finds of it in three cerebral lobes:

the temporal lobe have surfaces implied in memorizing. It contributes to functions evolved/moved such as the language or the identification of the faces.
the parietal Lobe contains the zone of sensory association which integrates the data coming from all the sensory systems to give an indication of the whole of the environment. It comprises also a small zone implied in the language, the surface of Wernicke, whose lesion involves the Aphasie of Wernicke which is different from the aphasia of Broca by the fact that the patient can form sentences grammatically and syntactically right, but without significance.
the lobe préfrontal is that which underwent the strongest development in the human line. It is here that one locates the seat of the human Intelligence. The fact is that it is almost absent at the majority of the mammals whereas it constitutes almost a quarter of the surface of the cortex at the man. It receives afférences of all the zones of the brain and ensures their integration to materialize the thought and to lead to a decision making.

Other zones

The zone of cortex of the median furrow, between the two hemispheres, is called the cortex cingulaire : it is a zone of paléocortex pertaining to the limbic system, a system implied in memorizing and the emotions. It is of great importance in the social relations.

Example of operation of the cortex

To analyze a scene and to provide an adapted answer, all the cerebral surfaces will collaborate. Let us take the example of a Chat which miaule to ask to eat.

The primary visual surface will identify a series of lines, curves and spots of gray, russet-red and white colors. The secondary visual surface will organize these disparate elements in a gray, russet-red and white object motionless. The zone of visual integration will recognize this object as being a sitted cat. In parallel, the primary auditive surface will collect a certain number of frequencies; the secondary auditive surface will organize these frequencies to obtain a sound with a precise stamp and a tessiture. The surface of auditive integration will recognize a mewing. The surface of parietal association will identify a cat which miaule. With the assistance of the temporal lobe, it will identify the cat and the nature of the mewing. All the elements of the cat are now identified.

From there, we will obtain a behavioral reaction. The limbic System located, partly between the two hemispheres, announces us the existence of an attachment for this Animal. It will push us to satisfy us of our action in progress to occupy us in priority of its needs. To note that if the individual acts because the mewings of the cat aggravate it, the result is the same one, it is the same system which manages these two emotions. The Cortex préfrontal will make the decision to nourish the cat (or to drive out it according to the cases). The additional driving surface will organize the unfolding of the act and the primary driving surface will order the movements necessary to its achievement. The primary somatosensorielle surface, in relation to the visual surface, will guide the course of operation by at every moment announcing the changes in the environment (the cat tends to put itself in our legs) and the position of the various segments of the body in space.

Finally, by the collaboration of all the cerebral surfaces, a coordinated behavior allowed the resolution of a problem. Let us announce in the passing that it is not the nature of the cortex which differentiates the man from the other animals, but the size of the cortex which allows reactions much more complex than those described here. In particular, the most advanced cognitive functions such as the language or the thought symbolic system are not concerned here. Contrary, the cortex does not do all. For example, the rotation movement of the head to look at the cat is not under the control of the cortex, but of the Quadrigeminate bodies, old structure of the Mésencéphale, which, at the Reptile S, had the function reserved today for the visual and auditive surfaces cortical at the Mammifère S.

Neuro-transmitters

The glutamate

The Glutamate is one of the Neurotransmetteur S most used in the brain since more of the third of the Neuron S appealed there. The neurons use it such as it is or by one of its metabolites, the GABA. Side postsynaptic, there exist three types of receivers: Receiving AMPA, Receiving NMDA and the Receiving kaïnate, thus named because of the pharmacological molecules able to selectively activate them in the absence of glutamate. The two first are implied in the phenomena of memorizing; lLe role of the third is less better included/understood.

These receivers are ionic channels: sodic as for receivers AMPA and kaïnate, calcic for the NMDA. Their effects all are exiting on the elements postsynaptic, which mean that they will support the emission of a Potentiel of action by the target neuron.

These receivers are the target of some Drogue S which will activate them uninterrupted, which will cause hallucinations, and for the NMDA, from the cytotoxicity of calcium, a death of the neuron by Apoptose.

The GABA

The acid gamma-amino butyric (GABA) is the principal inhibiting neuro-transmitter of the central nervous system. It is a neuromodulator who is recognized as being inhibiting at the adulthood, but which is exiting during the embryonic development. It in addition has a role neurotrophic, i.e. it supports the growth of certain neurons.

Acetylcholine

The Acétylcholine is one of the first neuro-transmitters discovered. Its fonctionnememt was especially studied on the level of the driving Plaque, but it is present everywhere in the nervous system. There exist two receivers of acetylcholine, both present in the cortex: the Receiving nicotinic, therefore the antagonist is the Nicotine and the Récepteur muscarinic sensitive to the Muscarine. Other drugs can distinguish from the sub-types within these two big families of receivers.

The receiver nicotinic is a receiver channel which lets pass the ions sodium when it is activated. On the other hand, the receiver muscarinic is a receiver of the metabotropic type, which means that it will not open an ionic channel, but to synthesize a molecule which will have an effect on operation of the neuron. This kind of receiver is implied more in the phenomena of regulation in the medium and long term than in the transmission of the potential of action. One speaks here rather about neuromodulation.

The cholinergic neurons are among the neurons more touched in the Maladie of Alzheimer and the first to be highlighted in this pathology.

Neurotransmitters

The neurotransmitters are molecules similar to the neuro-transmitters (sometimes a molecule can have the two roles), but which are emitted not in a synaptic slit, but in the cerebral environment. These molecules reach the neurons in a nonspecific way. Their role is not to propagate a potential of action through a synapse, but to create a molecular environment which will put the nervous system in a precise state.

In the cortex, one knows several molecules of this type:

the Noradrenalin which has the role of an emergency system, potentiating the attention, memorizing and the recall.
the Serotonin or the hydroxy-tryptamine implied in the cycles takes care/sleep, but also the food behavior, sexual and well of others. Certain antidepressants and psychotropic drugs act on recapture serotonin and thus potentiate its effect.
the Dopamine implied in the reward system.
the Mélatonine implied in the regulation of the circadian cycle, hormonal control and plays a part in the winter depression.

All these neurotransmitters are secreted by neurons whose cellular body is most of the time in the cerebral trunk. That makes it possible structures phylogéniquement older to exert a certain control on the cortex.

Pathologies of the cortex

Congenital malformations

The majority of the diseases which affect the cortex are not specific of this body. It is the case of the Anencéphalie, the Hydrocéphalie, the Macrocéphalie and of many other malformations of the Encéphale; other pathologies such as the Tumeur S are even more general. However, a class of diseases is really specific cortex, it acts of those which affect the good progress of the gyration, i.e. the formation of the cerebral convolutions during embryogenesis. One counts some several (Lissencéphalie, Polymicrogyrie, Pachygyrie), i.e. either a deficit of gyration leading to an insufficiently folded and thus too small cortex, or contrary an excessive gyration leading to furrows small and many or large but very few.

Epilepsies

Localized destruction of the cortex

The causes can be multiple:

intracranial Tumor (benign or malignant) compressive;
cranial Traumatism: accident or by weapon (firearm, generally), with hématome or tissue lesions;
Cerebral vascular accident;
Deterioration by neurotoxic products: alcohol, Ectasy, Lead, mercury…

The seat of the lesions and their extent will determine the importance and the type of central nervous system disorders.

The disease of Alzheimer

See too

the lobes of the brain
sensory Cortex and cerebral plasticity

External references and bonds

References

The majority of work on the cortex are old dating from second half of the 19th century and the beginning of XXeme. Only more recent work is quoted in this list of references. For the older results, to refer to the elements of the following chapter.

External bonds

Emission on the cerebral cortex of BioTV to the format Real Player accompanied by some illustrations captioned on the cortex.
the site of the university quebecquoise McGill on the operation of the brain, is not limited to the cerebral cortex. Different level from explanations (beginning, intermediary, advanced) are available.

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