At present, it is known for certain that the higher functions of the nervous system, such as the ability to recognize signals received from the external environment, to think, to remember and think, are largely due to the way the cerebral cortex functions. Zones of the cerebral cortex, we will consider in this article.
The fact that a person is aware of his relationship with other people is associated with the excitation of neural networks. We are talking about those that are located in the cortex. It is the structural basis of intelligence and consciousness.
Neocortex
About 14 billion neurons have a cerebral cortex. Zones of the cerebral cortex, which will be discussed below, function thanks to them. The main part of neurons (about 90%) forms the neocortex. It belongs to the somatic nervous system, being its highest integrative department. The most important function of the neocortex is the processing and interpretation of information obtained using the senses (visual, somatosensory, gustatory, auditory). It is also important that he controls complex muscle movements. The neocortex houses centers that take part in the processes of speech, abstract thinking, and memory storage. The main part of the processes occurring in it is the neurophysiological basis of our consciousness.
Paleocortex
Paleocortex is another large and important division that has a cerebral cortex. Zones of the cerebral cortex related to it are also very important. This part has a simpler structure compared to the neocortex. The processes taking place here are not always reflected in consciousness. In the paleocortex, there are higher vegetative centers.
The relationship of the cortex with the underlying brain
It should be noted the connection of the cerebral cortex with the underlying parts of our brain (thalamus, basal nuclei, bridge and midbrain). It is carried out using large bundles of fibers that form the inner capsule. These bundles of fibers are wide layers composed of white matter. They contain many nerve fibers (millions). Some of these fibers (axons of the thalamic neurons) provide transmission of nerve signals to the cortex. The other part, namely the axons of cortical neurons, serves to transfer them to the nerve centers located below.
The structure of the cerebral cortex
Do you know which part of the brain is the largest? Some of you have probably guessed what this is about. This is the cerebral cortex. Zones of the cerebral cortex are just one type of parts that stand out in it. So, it is divided into the right and left hemisphere. They are connected to each other by beams of white matter, which forms the corpus callosum. The main function of the corpus callosum is to ensure coordination between the two hemispheres.
Zones of the cerebral cortex by location
Although there are many folds in the cerebral cortex, in general, the location of the most important furrows and convolutions is characterized by constancy. Therefore, the main ones serve as a guide when dividing the regions of the cortex. Its outer surface is divided into 4 parts by three furrows. These lobes (zones) are temporal, occipital, parietal and frontal. Although they are distinguished by location, each of them has its own specific functions.
The temporal zone of the cerebral cortex is the center where the cortical layer of the auditory analyzer is located. In case of damage, deafness occurs. The auditory area of the cerebral cortex also has a Wernicke speech center. If it is damaged, the ability to understand spoken language disappears. She begins to be perceived as noise. In addition, in the temporal lobe there are neural centers related to the vestibular apparatus. The sense of balance is disturbed if they are damaged.
Speech areas of the cerebral cortex are concentrated in the frontal lobe. This is where the speech-motor center is located. If in the right hemisphere it is damaged, the ability to change the intonation and timbre of speech will disappear. It becomes monotonous. If the damage refers to the left hemisphere, where there are also speech areas of the cerebral cortex, articulation disappears. The ability to sing and articulate speech also disappears.
The visual zone of the cerebral cortex corresponds to the occipital lobe. Here is the department that is responsible for our vision as such. The world we perceive is the brain, not the eyes. The occipital part is responsible for vision. Therefore, in the event of damage, complete or partial blindness develops.
The parietal lobe also has its own specific functions. She is responsible for the analysis of information regarding general sensitivity: tactile, temperature, pain. In the event of damage, the ability to recognize objects by touch, as well as some other abilities, is lost.
Motor zone
I would like to talk about it separately. The fact is that the motor zone of the cerebral cortex does not correlate with the lobes, which we described above. It is a part of the cortex that contains downward direct connections with the spinal cord, more precisely, with its motor neurons. So-called neurons that directly control the work of muscles.
The main motor zone of the cerebral cortex is located in the precentral gyrus. In many of its aspects, this gyrus is a mirror image of another sensory zone. Contralateral innervation is observed. In other words, innervation occurs in relation to the muscles located on the opposite side of the body. The exception is the facial area, in which there is bilateral control of the muscles of the jaw and lower face.
Another additional motor zone of the cerebral cortex is located in the area below the main zone. Scientists believe that she has independent functions associated with the output of motor impulses. This motor area of the cerebral cortex has also been studied by scientists. In experiments performed on animals, it was found that its stimulation leads to the occurrence of motor reactions. Moreover, this happens even if the main motor zone of the cerebral cortex was previously destroyed. In the dominant hemisphere, she is involved in speech motivation and movement planning. Scientists believe that its damage leads to dynamic aphasia.
Zones of the cerebral cortex by function and structure
As a result of clinical observations and physiological experiments carried out in the second half of the 19th century, the boundaries of the regions into which various receptor surfaces are projected were established. Among the latter, both sensory organs aimed at the external world (skin sensitivity, hearing, vision) and those that are embedded in the organs of motion themselves (kinetic, or motor analyzer) are distinguished.
The occipital region is the zone of the visual analyzer (fields 17 to 19), the upper temporal region is the auditory analyzer (fields 22, 41 and 42), the postcentral region is the skin-kinesthetic analyzer (fields 1, 2 and 3).
Cortical representatives of various analyzers by function and structure are divided into the following 3 zones of the cerebral cortex: primary, secondary and tertiary. In the early period, during the development of the embryo, the primary ones are laid, which are characterized by simple cytoarchitectonics. The last to develop tertiary. They have the most complex structure. An intermediate position from this point of view is occupied by the secondary zones of the cerebral hemispheres. We invite you to consider in more detail the functions and structure of each of them, as well as their relationship with the parts of the brain located below, in particular, with the thalamus.
Central fields
Scientists over many years of study have gained considerable experience in clinical research. As a result of observations, it was found, in particular, that damage to certain fields in the composition of cortical representatives of the analyzers affects the overall clinical picture is far from equivalent. Among the other fields in this respect, one is distinguished, which occupies a central position in the nuclear zone. It is called primary, or central. It is the field number 17 in the visual zone, in the auditory one - number 41, and in the kinesthetic one - 3. Their damage leads to very serious consequences. The ability to perceive or make the most subtle differentiation of stimuli of the corresponding analyzers is lost.
Primary zones
In the primary zone, the most developed complex of neurons, which is adapted to provide cortical-subcortical bilateral connections. It connects the cortex with one or another sense organ in the shortest and most direct way. Because of this, the primary zones of the cerebral cortex can give off enough irritants.
An important common feature of the functional and structural organization of these areas is that they all have a clear somatotopic projection. This means that individual points of the periphery (retina, skin surface, cochlea of the inner ear, skeletal muscles) are projected into the corresponding, strictly delimited points located in the primary zone of the cortex of the corresponding analyzer. For this reason, they began to be called projection.
Secondary zones
Otherwise, they are called peripheral, and this is no accident. They are located in the nuclear sites of the cortex, in their peripheral departments. Secondary zones differ from primary, or central, physiological manifestations, neural organization and features of architectonics.
What effects are observed with their electrical irritation or damage? These effects relate mainly to more complex types of mental processes. If the secondary zones are affected, then the elementary sensations are relatively preserved. Mainly upset is the ability to correctly reflect the mutual relationships and entire complexes of the constituent elements of various objects that we perceive. If the secondary zones of the auditory and visual cortex are irritated, then auditory and visual hallucinations are observed, deployed in a certain sequence (temporal and spatial).
These areas are very important for the implementation of the interconnection of stimuli, the allocation of which occurs using primary zones. In addition, they play a significant role in integrating the functions of the nuclear fields of various analyzers when combining receptions into complex complexes.
Secondary zones, therefore, are important for the implementation of more complex forms of mental processes that require coordination and are associated with a thorough analysis of the ratios of subject stimuli, as well as with orientation in time and in the surrounding space. In this case, connections are called associative. Afferent impulses, which are directed from the receptors of various surface sensory organs to the cortex, reach these fields through many additional switches in the associative nuclei of the thalamus (visual hillock). In contrast, afferent impulses that follow in the primary zones reach them in a shorter way through the relay core of the visual tubercle.
What is the thalamus
Fibers from thalamic nuclei (one or more) are suitable for each lobe of the hemispheres of our brain. The optic tubercle, or thalamus, is located in the forebrain, in its central region. It consists of many nuclei, with each of them transmitting momentum to a strictly defined area of the cortex.
All signals entering it (except for olfactory ones) pass through the relay and integrative nuclei of the thalamus. Then the fibers go from them to the sensory zones (in the parietal lobe - to the taste and somatosensory, in the temporal lobe - to the auditory in the occipital - to the visual). Impulses arrive respectively from the ventro-basal complex, the medial and lateral nuclei. As for the motor zones of the cortex, they are associated with the ventrolateral and anterior ventral nuclei of the thalamus.
EEG desynchronization
What happens if a person who is at rest is suddenly presented with a strong irritant? Of course, he would immediately be wary and concentrate his attention on this stimulus. The transition of mental activity, carried out from rest to a state of activity, corresponds to the replacement of the EEG alpha rhythm with a beta rhythm, as well as other more frequent fluctuations. This transition, called EEG desynchronization, appears as a result of sensory excitations coming to the cortex from nonspecific thalamic nuclei.
Activating reticular system
Nonspecific nuclei make up the diffuse nervous network located in the thalamus, in its medial sections. This is the anterior section of the APC (activating reticular system), which regulates the excitability of the cortex. Various sensory signals can activate APC. They can be visual, vestibular, somatosensory, olfactory and auditory. ARS is a channel through which these signals are transmitted to the surface layers of the cortex through non-specific nuclei located in the thalamus. Excitation of APC plays an important role. It is necessary to maintain an awake state. In experimental animals in which this system was destroyed, a coma-like state was observed.
Tertiary zones
The functional relationships that are traced between the analyzers are even more complex than described above. Morphologically, their further complication is expressed in the fact that during the growth of the nuclear fields of the analyzers on the hemisphere surface, these zones overlap. At the cortical ends of the analyzers, "overlapping zones" are formed, that is, tertiary zones. These formations are among the most complex types of combining the activities of skin-kinesthetic, auditory and visual analyzers. Tertiary zones are already located beyond the borders of their own nuclear fields. Therefore, their irritation and damage does not lead to pronounced precipitation. Also, with respect to the specific functions of the analyzer, significant effects are not observed.
Tertiary zones are special areas of the cortex. They can be called a collection of "scattered" elements of various analyzers. That is, these are elements that by themselves are no longer capable of producing any complex syntheses or analyzes of stimuli. The territory they occupy is quite extensive. It breaks up into a number of areas. We will briefly describe them.
The upper parietal region is important for integrating the movements of the whole body with visual analyzers, as well as for shaping the body diagram. As for the inferior parietal, it refers to the combination of abstract and generalized forms of signaling associated with complex and subtly differentiated speech and subject actions, the implementation of which is controlled by vision.
The temporoparietal-occipital region is also very important. She is responsible for the complex types of integration of visual and auditory analyzers with written and oral speech.
Note that the tertiary zones have the most complex communication chains in comparison with the primary and secondary. Bilateral connections are observed in them with a complex of thalamic nuclei, connected, in turn, with relay nuclei through a long chain of internal bonds located directly in the thalamus.
Based on the foregoing, it is clear that in humans, zones of primary, secondary and tertiary are areas of the cortex that are highly specialized. It is especially necessary to emphasize that the 3 groups of cortical zones described above, in a normally functioning brain, together with systems of connections and switching between themselves, as well as with subcortical formations, function as one complex, differentiated whole.