Cerebral Cortex
The cerebral cortex (or History of study of the cortex During the high antiquity , the brain was considered of minor importance in the human body. The Egyptians did not care to keep that body in the process of mummification , however, the Edwin Smith Papyrus describes cases of traumatic brain and spinal cord and their consequences. The seat of thoughts and emotions was located in the heart , a belief that now finds itself in a number of expressions or symbols. This is the fifth century with the dissections of Alcmaeon the seat of vision is localized in the brain. Subsequently Democritus imagined the soul as particles dispersed throughout the body, but with a concentration in the brain. Aristotle and Plato gave the brain involved in thinking, emotions and sensations, tied with the heart. The exact moment of the cortex was known to ancient is undetermined, but Galen , the great physician first century knew. He believed, however, under the influence of Herophilus the seat of thought was located in the cerebral ventricles , substances white and gray being a felting to protect them. This idea lasted throughout the Middle Ages. Under the influence of thought Arabic , but that might be attributable to a particular author, little by little, the idea took the cerebral cortex was originally advanced cognitive functions of mammals. Knowledge of the cortex have changed very little until the Renaissance for lack of a tool for effective exploration. Then the microscope was drafted by the brothers Janssen , the Dutch spectacle-makers in 1590 and became fully operational less than a century later by Antoni van Leeuwenhoek. At that time, technical resources were limited, but knowledge were more. It was enough to point his microscope anywhere to make discoveries. This was done by Malpighi in the 1660s . He described the first cell of the cortex. But progress was slow, because brain cells are very dense and difficult to differentiate by traditional stains. Despite these difficulties, the Russian Vladimir Alekseevich Betz arrived in 1874 to identify the pyramidal cells . It is Camillo Golgi that the solution came in 1873 when he developed a staining with silver salts (the reazione nera ), which marked only a few cells throughout the tissue. So they appeared fully differentiated from their neighbors, with all their tree visible. However, it is not him, but a contemporary, Santiago Ramn y Cajal who will embark on the exploration of physiological cortex. Using the technique of his brother, he described the cell types and organization in six layers of neocortex. Both scholars long opposed, arguing the Golgi network theory (the neurons form a syncytium ) nervous system, while Ramn y Cajal was a supporter of the theory neuronal (nerve cells are independent cells connected together by synapses ). Finally, Golgi adopted the ideas of Ramon y Cajal about 1900 and together they received the Nobel Prize in 1906 for their histological work on the nervous system. The neuronal theory was finally confirmed in the twentieth century through the electron microscope. In parallel with histological studies were carried out functional analysis. At the time, the only way was to study the consequences of a lesion of the cerebral cortex on the human cognitive performance. The Egyptians had already found that head injuries could cause vision problems. That Paul Broca , a student who aphasic patient , highlight the link between cortical lesion and cognitive impairment. Subsequently, the function of many brain areas was identified and the dawn of the twentieth century , the location of visual areas, auditory, somatosensory and motor was known. Besides the natural lesions, neurobiologists studied the effects of resulting injuries, usually in a supposedly therapeutic purposes: the main one being the lobotomy. A great wave of lobotomy, mentioned in the film flying over a cuckoo's nest , which will decline from 1950 (date of its ban in the USSR) allowed a better understanding of the nervous system, but at a devastating human cost. This technique is now banned in France, but continues to be applied in the USA, Northern Europe, India and some other countries in extreme cases. In 1875 an English physician, Richard Caton was the first to measure the activity of the cerebral cortex neuroelectric by placing the electrode of a galvanometer in direct contact with the surface of the brain of animals craniotomiss. It shows that the functional activity (eg vision) corresponds to the appearance of a negative bias in a circumscribed area of the cerebral cortex. The activity recording neuroelectric humans begin with the appearance of electro-encephalography (EEG) developed by Hans Berger in the 1920's. This technique allows for the first time to study the neurophysiological correlates of cognitive activity in real time with excellent temporal resolution on the millisecond. This review remains essential for diagnosis and classification of epilepsies. It's in the second half of the twentieth century is a great revolution in the study of the cerebral cortex, with the development of methods of brain imaging noninvasive. The doctor can see the brain function without opening the skull. Until then, the standard radiographs provided only images unusable (the brain is not radiopaque) and angiography Cerebral only allowed to see that the axes strokes. "With the introduction of neuroimaging techniques, molecular level of description could replace the dominant molecular level. " First it was the scanner that allowed you to view, for the first time the brain and the cortical areas with remarkable precision, then the nuclear magnetic resonance (MRI) has significantly changed, in turn, the iconographic study of cortical structures. In 1938, the principle of nuclear magnetic resonance was discovered by Isidor Isaac Rabi. This discovery will lead, in 1973, the development of what would become the magnetic resonance imaging or MRI, simultaneously by Paul Lauterbur and Peter Mansfield , who received a joint Nobel Prize ( Nobel Prize in Physiology or Medicine ) in 2003. The cortex has undergone a long evolution since its first appearance in craniates or their ancestors . We distinguish the neocortex in mammals, also known neopallium and allocortex itself subdivided into paleocortex (or palopallium ) and archicortex (or archipallium ). The archicortex is the oldest, already exist in fish in the rhinencephalon, structure responsible for processing smells. In humans, it is found in ancient structures such as the hippocampus (brain) and the dentate gyrus. The paleocortex is more recent. It is well represented among reptiles when it reaches its maximum development, but it is still present in mammals in old structures such as basal ganglia or rhinencephalon. The neocortex is the phylogenetically more recent. Existing in draft form among reptiles. This is a smooth surface layer and poorly developed in birds , although it is the center of intelligence and learning . There will be growth in mammals as to repel allocortex in small areas. It is humans that reached its maximum development, constituting 80% of neurons in the central nervous system. The cortex is derived from modern mammals rhinencephalon of fish . The fact that the old structure of olfactory fish gave birth to the cerebral hemispheres reflects the importance that olfaction in the first mammals and it still possesses many of them. One hypothesis suggested informally that this could be due to the lack of information carried by olfactory sensations. While auditory signals by themselves can provide information on the size, position and movement of other animals, this is not the case of olfactory signals. To be useful, they must be associated with memory traces that connect them with visual or auditory memories, especially that early mammals were probably nocturnal animals of the size of a mouse. The cerebral hemispheres were then developed and received afferent visual, auditory and somatic to integrate all this information with olfactory signals. This is only a hypothesis and there is no way to verify it, evolution would identify the sequence of steps, but not giving reasons, especially for a fossilized organisms as badly as the brain. The cerebral cortex is formed in the anterior neural tube , itself derived from the neural plate , a differentiation of the ectoderm dorsal under the influence of the notochord. The first brain structure to differentiate themselves in what will the cerebral hemispheres is the ventricular system. Stem cells are neurons in the epithelium that lines the ventricles. Initially, the progenitors divide symmetrically to multiply and then asymmetrically. One of two cells then migrate outside the ventricular zone to reach the cortex . It differs then neuron . The other cell remains in the ventricular zone and continues to divide. Glial cells multiply in a similar way, their progenitors are different from those of neurons. During periods of fetal and newborn neurons in the immature cerebral cortex (cortical plate) are sandwiched between the marginal zone and outside the sub-plate located just below the interface which will give the white matter. The sub-plate has a transient existence. It will disappear in humans, two months after birth. Marginal zone persist, becoming the layer 1 of the neocortex. The histology of the cortex began very early. The cell types were identified from the nineteenth century. Big names have been associated with these discoveries, such as Nobel laureate Santiago Ramon y Cajal and Camillo Golgi. Under the microscope, the human neocortex is subdivided into six layers . This number varies among species, eg five in dolphins, three reptiles. The layers are numbered from the surface. A distinction is in order: A seventh layer exists transiently during embryogenesis. She disappears with brain maturation. These cortical layers are not just a stack of neurons. The neurons are organized into functional units in the form of columns perpendicular to the surface of the cortex, each providing a specific function. However it is not possible to distinguish by histological methods are, by functional studies of visual cortex that this structure has been demonstrated before being generalized to the entire neocortex . The paleocortex archicortex and structurally simple, are not stratified and does not show this column structure. In the man , the thickness of the cortex is between 1 and 4.5 mm and its surface is about two miles square centimeters , . To stay in the skull, the cortex is wrinkled by grooves (sulcus in scientific Latin) or fissures , variable depth, bounding ridges called gyri or cerebral convolutions. This kind of arrangement is said gyrencphale, versus lissencephaly existing example in the rat cortex, which has a smooth, free of convolution. This property is not related to the complexity of the brain, but is related to the size of the individual. When the size of an individual double its volume is roughly multiplied by eight (8 = 2 3). If the cortex remained smooth, its surface would be multiplied by four. To maintain the proportion, the cortex must creasing according to the size increase. To this, add the specificities related lineages: for example, the same size, the carnivores are more intelligent than herbivores , they have a cortex larger and therefore more wrinkled. The deepest fissures divide the cortex into lobes. Depending on their situation, it is called the frontal lobe , parietal , occipital and temporal. Beneath the cortex lies the white matter consists of axons that establish the connections between cell bodies in the cortex and other parts of the brain. The cerebral cortex is divided into functional areas, called areas, each providing a specific cognitive function. These are the studies of Paul Broca in 1861 who first suggested the existence of such an organization of the neocortex . These areas are almost identical for all individuals of the same species but have small differences. This specialization is blurred and primitive mammals are as precise and as we approach the man. There are three main types of zones: the sensory areas, motor areas and association areas. Three brain areas are specialized in processing sensory data: the auditory cortex in the temporal lobe , the visual cortex located in the occipital lobe , and somatosensory cortex in the parietal lobe. The auditory cortex is organized similarly to the visual cortex. It is located in the temporal lobe. It includes a primary auditory area, which identifies the frequencies and a secondary auditory area that recreates the sounds. The visual cortex is divided into two zones: the primary visual cortex is a direct projection of the retina and performs low level processing of visual data (identification of lines, colors, directions of travel) and a secondary visual cortex bringing together these elements for objects with shape, color and smooth movement. As for the information drive, afferents from this area are crossed, but in different ways: the left hemisphere does not receive data from the eye right, but from the right visual field of each eye. The somatosensory cortex is an accurate reflection of the primary motor cortex. Each organ afferents projected to somatotopically. The size of the area assigned to each body part is proportional to the spatial discrimination of the area: hand and face will therefore have the most important surfaces. This provision is embodied by the concept of sensory homunculus . However, different types of sensation does not seem to separate at this point. There are two areas of expertise in motor cortex, and the other one in the frontal cortex. The largest is the primary motor area occupies the entire posterior part of the frontal lobe , right in front of the central sulcus. It is organized somatotopically (each body area receiving afferent of a specific part of this area), the surface coupled with a muscle is proportional to the precision movements of which he is capable: the face and hand are strongly represented. As for the tactile, there is here a motor homunculus. It includes the Broca's area (see image right) which is responsible for achieving the Broca's aphasia , the auditory cortex in the temporal lobe and the somatosensory cortex in the parietal cortex. A disease in which a person can express his thoughts in the form of coherent sentences, but can not pronounce. Its role is to perform voluntary movements. The efferent neurons in this area are crossed: the left hemisphere controls the movements of the right side of the body and vice versa. There is also a supplementary motor area, located in the prefrontal cortex , which selects and voluntary movements. Other motor areas were found in the parietal cortex and prefrontal cortex, involved in spatial integration of motion and the relationship between movement and thought. Association areas represent the bulk of the cerebral cortex in humans and are the main factor in the size of his brain. In fact, this term refers to all neocortical areas that are neither motor or sensory, and their functions are very different. They are found in three cerebral lobes : Cortex area of the median groove between the two hemispheres is called the cingulate cortex: it is an area paleocortex belonging to the limbic system, a system involved in memory and emotions. It is of great importance in social relationships. To analyze a scene and provide an appropriate response, all areas of the brain will work. Take the example of a cat meowing to ask for food. The primary visual area will identify a series of lines, curves and spot color gray, red and white. The secondary visual cortex will organize these disparate elements into a stationary object gray, red and white. The integration area will recognize this visual object as a sitting cat. Meanwhile, the primary auditory area will attract a number of frequencies, the secondary auditory area will organize these frequencies to obtain a sound with a stamp and a specific range. The area of auditory integration will recognize a meow. The parietal association area will identify a cat that meows. With the help of the temporal lobe, it will identify the nature of the cat and meow. All elements of the cat are now identified. From there, we will obtain a behavioral response. The limbic system located partly between the two hemispheres, we reported the existence of an attachment to this animal. It will push us to interest ourselves in our ongoing efforts to deal with priority needs. Note that if the individual is acting because the cat meowing annoy him, the result is the same, the same system that manages these two emotions. The prefrontal cortex is the decision to feed the cat (or the hunt, as appropriate). The supplementary motor area will host the course of the act and the primary motor cortex will control the movements necessary for its accomplishment. The primary somatosensory area, in relation to the visual area, will guide the conduct of operations by pointing at each moment changes in the environment (the cat tends to put our legs) and positions of various segments body in space. Finally, through the collaboration of all brain areas, coordinated behavior helped solve a problem. Note in passing that this is not the nature of the cortex that distinguishes man from other animals, but the size of the cortex which allows reactions more complex than those described here. Notably, the most advanced cognitive functions such as language and symbolic thought are not brought into play here. In contrast, the cortex is not everything. For example, the rotational movement of the head to watch the cat is not under the control of the cortex, but quadrigemina , ancient structure of the midbrain , which, among reptiles , had the purpose assigned to visual areas today and auditory cortex in mammals. The glutamate is a neurotransmitter commonly used in the brain as more than a third of the neurons are used. Neurons use it as is or by one of its metabolites, GABA. Postsynaptic side, there are three types of receptors: the AMPA receptor , the NMDA receptor and kainate receptor , so named because of pharmacological molecules capable of selectively activating in the absence of glutamate. The first two are involved in the phenomena of memory, the role of the third is less well understood. These receptors are ion channels: sodium in affecting AMPA and kainate, NMDA channel blockers for. Their effects are all on excitatory postsynaptic elements, which means that they will favor the issuance of an action potential by the target neuron. These receptors are the target of some drugs that will enable them continuously, which will cause hallucinations, and NMDA, because of the cytotoxicity of calcium, neuron death by apoptosis. The gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system. It is a neuromodulator which is known as inhibitor in adulthood, but is excitatory during embryonic development. It also has a neurotrophic role, that is to say that it promotes the growth of certain neurons. The acetylcholine is one of the first neurotransmitter discovered . Its operation has been mainly studied at the neuromuscular junction , but is present throughout the nervous system. There are two receptors for acetylcholine, both in the cortex: the nicotinic receptor , which is the antagonist of nicotine and the muscarinic receptor sensitive to muscarine / A>. Other drugs may distinguish subtypes within these two families of receptors. The nicotinic receptor is a receptor channel that lets sodium ions when activated. In contrast, the muscarinic receptor is a metabotropic receptor subtype, which means it will not open an ion channel, but synthesize a molecule that will affect the functioning of the neuron. This type of receptor is more involved in regulatory phenomena in the medium and long term in the transmission of action potentials. We are talking instead of neuromodulation. Cholinergic neurons are among the most affected neurons in Alzheimer's disease and the first to have been highlighted in this pathology. The neurotransmitters are molecules similar to neurotransmitters (sometimes a molecule can have both roles), but are issued not in a cleft, but in the brain environment. These molecules reach the neurons in a nonspecific manner. Their role is not to propagate an action potential across a synapse, but to create an atmosphere that will molecular nervous system in a specific state. In the cortex, there are several known molecules of this type: All of these neurotransmitters are secreted by neurons whose cell body is mostly in the brainstem. This allows phylogenetically older structures to exercise some control over the cortex. Most diseases that affect the cortex are not specific to this organ. This is the case of anencephaly , the hydrocephalus , the macrocephaly and the many other malformations of the brain ; other pathologies such as tumors are more general. However, a class of diseases is very specific in the cortex, it is one which affects the smooth running of gyration, that is to say the formation of cerebral convolutions during embryogenesis. Many ignore ( lissencephaly , polymicrogyria , pachygyria ), that is to say a deficit of cortical gyration, leading to a poorly folded and therefore too small, or conversely leading to an excessive gyration of furrows and numerous small or big but small. Epilepsy are diseases of the cortex varied in their symptoms, but having a single source: an explosive activation of the cerebral cortex: depending on the area of the cortex disrupted, the results will be very different from the convulsive attack known as large evil until hallucinations, involuntary movements or sudden temporary absences of consciousness. This disease has a genetic component, but it can also result from infection or trauma. If there is no way to stop a crisis in progress, medication or surgery can prevent the occurrence of new attacks. The causes can be manifold: The location of the lesions and their extent will determine the amount and type of neurological disorders. This disease is characterized primarily by its symptoms (an early decline of cognitive abilities of the subject), corresponds to a degeneration of the cerebral cortex. Nerve tissue is gradually sprinkles of amyloid plaques, aggregates containing a protein from the cell membranes and incompletely degraded, causing disability by compressing surrounding neurons. Its prevalence has a genetic component, but environmental factors can affect its appearance and its evolution. Most work on the cortex are old from the second half of the nineteenth century and early nineteenth century. Only the more recent works are cited in the reference list. For earlier results, refer to the elements of the next chapter. Antiquity and Middle Age
Renaissance and Modern
Exploration by functional brain imaging
Evolution and development of the cortex
Evolution phylogenetic
embryological development (ontogenic evolution)
Structure of the cerebral cortex
Histology
In this picture, layers II and III and layers IV and V were grouped together. Anatomy
Organization
Primary motor area Supplementary motor area prefrontal Primary sensory area Sensory association area Primary visual area Primary auditory area Areas in humans sensory
motor areas in humans
Areas Association in humans
Other areas
Example of operation of the cortex
Neurotransmitters
Glutamate
GABA
Acetylcholine
Neuromediators
Pathologies cortex
Birth defects
Epilepsy
Destruction of localized cortical
Alzheimer's Disease
References
External Links
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