By origin, the cerebral cortex is divided into ancient (pleocortex), old (archecortex) and new (neocortex). The ancient cortex includes structures associated with the analysis of olfactory stimuli, it includes olfactory bulbs, tracts and tubercles. The old cortex includes the cingulate cortex, the hippocampal cortex, the dentate gyrus, and the amygdala. The ancient and old cortex forms the olfactory brain. In addition to the sense of smell, the olfactory brain provides reactions of alertness and attention, takes part in the regulation of vegetative functions, plays a role in the formation of sexual, nutritional, defensive instinctive behavior, and provides emotions.
All other structures of the cortex belong to the neocortex, which occupies about 96% of the total area of the entire cortex.
The arrangement of nerve cells in the cortex is designated by the term "cytoarchitectonics". And conductive fibers - "myeloarchitectonics".
The neocortex is composed of 6 cell layers, differing in cell composition, neural connections and functions. In the areas of the ancient cortex and the old cortex, only 2-3 layers of cells are revealed. Neurons in the upper four layers of the neocortex mainly process information from other regions. nervous system. The main centrifugal layer is the 5th layer. The axons of its cells form the main descending pathways of the cerebral cortex, they carry signals that control the work of the stem structures and the spinal cord.
1 layer - the outermost, molecular. It contains mainly nerve fibers of deeper located neurons. In addition, it contains a large number of small cells. The fibers of the molecular layer form connections between different areas of the cortex
Layer 2 - outer granular. It contains a large number of small multipolar neurons. In this layer, part of the ascending dendrites from the third layer ends.
3rd layer - outer pyramidal. It is the widest, contains mainly medium and less often small and large pyramidal neurons. Dendrites of neurons from this layer are sent to the second layer.
4 layer - internal granular. It consists of a large number of small granular, as well as medium and large stellate cells. They are divided into two sublayers: 4a and 4b.
Layer 5 - ganglionic, or internal pyramidal. It is characterized by the presence of large pyramidal neurons. Their upwardly directed dendrites reach the molecular layer, and the basal and collateral axons are distributed in the fifth layer.
6th layer - polymorphic. It contains, along with cells of other forms, spindle-shaped neurons. The shapes of other cells are very diverse: they have a triangular, pyramidal, oval and polygonal shape.
Bark big brain divided into ancient ( archicortex), old ( paleocortex) and new ( neocortex) on a phylogenetic basis, that is, in the order of occurrence in animals in the process of evolution. These areas of the cortex form extensive connections within the limbic system. In more phylogenetically ancient animals, the ancient and old cortex, like the entire Limbic system, was primarily responsible for smell. In humans, the limbic system performs much broader functions associated with the emotional and motivational sphere of behavior regulation. All three areas of the cortex are involved in the performance of these functions.
ancient bark along with other functions, it is related to the sense of smell and ensuring the interaction of brain systems. The ancient cortex includes the olfactory bulbs, which receive afferent fibers from the olfactory epithelium of the nasal mucosa; olfactory tracts located on the lower surface of the frontal lobe, olfactory tubercles, in which the secondary olfactory centers are located. This is the phylogenetically earliest part of the cortex, occupying adjacent areas of the frontal and temporal lobes on the lower and medial surfaces of the hemispheres.
old bark includes the cingulate gyrus, the hippocampus, and the amygdala.
Belt gyrus. It has numerous connections with the cortex and stem centers and acts as the main integrator of various brain systems that form emotions.
The tonsil also forms extensive connections with the olfactory bulb. Through these connections, the sense of smell in animals is involved in the control of reproductive behavior.
In primates, including humans, damage to the amygdala reduces the emotional coloring of reactions, in addition, aggressive affects completely disappear in them. Electrical stimulation of the amygdala causes predominantly negative emotions - anger, rage, fear. Bilateral removal of tonsils sharply reduces the aggressiveness of animals. Calm animals, on the contrary, can become uncontrollably aggressive. In such animals, the ability to evaluate incoming information and correlate it with emotional behavior is impaired. The amygdala is involved in the process of identifying dominant emotions and motivations and choosing behavior in accordance with them. The amygdala is a powerful emotion modifier.
The hippocampus is located in the medial part of the temporal lobe. The hippocampus receives afferent inputs from the hippocampal gyrus (receives inputs from almost all areas of the neocortex and other parts of the GM), from the visual, olfactory and auditory systems. Damage to the hippocampus leads to characteristic memory and learning disabilities. The activity of the hippocampus is to consolidate memory - the transition of short-term memory into long-term memory. Damage to the hippocampus causes a sharp violation of the assimilation of new information, the formation of short-term and long-term memory. Therefore, the hippocampus, as well as other structures of the limbic system, significantly affects the functions of the neocortex and the learning process. This influence is carried out primarily by creating an emotional background, which is largely reflected in the rate of formation of any conditioned reflex.
Pathways from the temporal cortex lead to the amygdala and hippocampus, transmitting information from the visual, auditory, and somatic sensory systems. Connections of the limbic system with the frontal lobes of the forebrain cortex have been established.
At new cortex greatest development size, differentiation of functions is observed in humans. The thickness of the new cortex ranges from 1.5 to 4.5 mm and is maximum in the anterior central gyrus. in the limbic system and in general nervous activity the cortex is engaged in the higher functions of the organization of activity.
Defeat frontal lobe causes emotional dullness, difficulty changing emotions. It is with the defeat of this area that the so-called frontal syndrome occurs. The prefrontal region and its associated subcortical structures (head of the caudate nucleus, mediodorsal nucleus of the thalamus) form the prefrontal system responsible for complex cognitive and behavioral functions. Pathways converge in the orbitofrontal cortex from the association areas of the cortex, the paralimbic areas of the cortex, and the limbic areas of the cortex. Thus, the prefrontal system and the limbic system intersect here. Such an organization determines the involvement of the prefrontal system in complex forms of behavior, where coordination of cognitive, emotional and motivational processes is necessary. Its integrity is necessary for assessing the current situation, possible actions and their consequences, and thus for making decisions and developing programs of behavior.
Removal temporal lobes causes hypersexuality in monkeys, and their sexual activity can be directed even to inanimate objects. Finally, the postoperative syndrome is accompanied by the so-called mental blindness. Animals lose the ability to correctly assess visual and auditory information, and this information has nothing to do with the monkeys' own emotional state.
The temporal lobes are closely related to the structures of the hippocampus and amygdala and are also responsible for information retention and long-term memory and play a key role in the process of converting short-term memory to long-term memory. The temporal cortex is also responsible for combining stored traces.
In this article, we will talk about the limbic system, the neocortex, their history of origin and main functions.
limbic system
The limbic system of the brain is a collection of complex neuroregulatory structures of the brain. This system is not limited to just a few functions - it performs a huge number of the most important tasks for a person. The purpose of the limbus is the regulation of higher mental functions and special processes of higher nervous activity, ranging from simple charm and wakefulness to cultural emotions, memory and sleep.
History of occurrence
The limbic system of the brain formed long before the neocortex began to form. This ancient hormonal-instinctive structure of the brain, which is responsible for the survival of the subject. For a long evolution, 3 main goals of the system for survival can be formed:
- Dominance - the manifestation of superiority in a variety of ways
- Food - subject's nutrition
- Reproduction - the transfer of one's genome to the next generation
Because a person has animal roots, a limbic system is present in the human brain. Initially, Homo sapiens had only affects that affect the physiological state of the body. Over time, communication was formed by the type of cry (vocalization). Individuals who knew how to convey their state with the help of emotions survived. Over time, an emotional perception of reality has been formed more and more. Such evolutionary stratification allowed people to unite into groups, groups into tribes, tribes into settlements, and the latter into entire peoples. The limbic system was first discovered by American researcher Paul McLean back in 1952.
System structure
Anatomically, the limbus includes areas of the paleocortex (ancient cortex), archicortex (old cortex), part of the neocortex ( new bark) and some subcortical structures (caudate nucleus, amygdala, globus pallidus). Titles listed various kinds crust denotes their formation at the indicated time of evolution.
Weight specialists in the field of neuroscience, they dealt with the question of which structures belong to the limbic system. The latter includes many structures:
In addition, the system is closely related to the system reticular formation(structure responsible for brain activation and wakefulness). The scheme of the anatomy of the limbic complex rests on the gradual layering of one part on another. So, on top lies the cingulate gyrus, and then descending:
- corpus callosum;
- vault;
- mamillary body;
- amygdala;
- hippocampus.
A distinctive feature of the visceral brain is its rich connection with other structures, consisting of complex pathways and two-way connections. Such a branched system of branches forms a complex vicious circles, which creates conditions for prolonged circulation of excitation in the limbus.
Functionality of the limbic system
The visceral brain actively receives and processes information from the outside world. What is the limbic system responsible for? Limbus- one of those structures that works in real time, allowing the body to effectively adapt to environmental conditions.
The human limbic system in the brain performs the following functions:
- Formation of emotions, feelings and experiences. Through the prism of emotions, a person subjectively evaluates objects and the phenomenon of the environment.
- Memory. This function is carried out by the hypocampus, located in the structure of the limbic system. Mnestic processes are provided by the processes of reverberation - a circular movement of excitation in the closed neural circuits of the sea horse.
- Selection and correction of a model of suitable behavior.
- Training, retraining, fear and aggression;
- Development of spatial skills.
- Defensive and foraging behavior.
- Expressiveness of speech.
- Acquisition and maintenance of various phobias.
- The work of the olfactory system.
- Reaction of caution, preparation for action.
- Regulation of sexual and social behavior. There is a concept of emotional intelligence - the ability to recognize the emotions of those around you.
At expression of emotions a reaction occurs, which manifests itself in the form of: changes in blood pressure, skin temperature, respiratory rate, pupil reaction, sweating, reaction of hormonal mechanisms, and much more.
Perhaps there is a question among women about how to turn on the limbic system in men. However answer simple: none. In all men, the limbus works to the full (with the exception of patients). This is justified by evolutionary processes, when a woman in almost all time periods of history was engaged in raising a child, which includes a deep emotional return, and, consequently, a deep development of the emotional brain. Unfortunately, men can no longer reach the level of development of a woman's limbus.
The development of the limbic system in infants largely depends on the type of upbringing and, in general, attitudes towards it. A stern look and a cold smile do not contribute to the development of the limbic complex, unlike a strong hug and a sincere smile.
Interaction with the neocortex
The neocortex and the limbic system are tightly connected by many pathways. Thanks to this unification, these two structures form one whole of the human mental sphere: they connect the mental component with the emotional one. The neocortex acts as a regulator of animal instincts: human thought usually goes through a series of cultural and moral inspections before taking any action spontaneously evoked by emotions. In addition to controlling emotions, the neocortex has an auxiliary effect. The feeling of hunger arises in the depths of the limbic system, and already the higher cortical centers that regulate behavior search for food.
The father of psychoanalysis, Sigmund Freud, did not bypass such brain structures in his time. The psychologist argued that every neurosis is formed under the yoke of the suppression of sexual and aggressive instincts. Of course, at the time of his work, there were no data on the limbus yet, but the great scientist guessed about such brain devices. So, the more cultural and moral layers (super ego - neocortex) an individual had, the more his primary animal instincts (Id - limbic system) are suppressed.
Violations and their consequences
Based on the fact that the limbic system is responsible for many functions, this very set can be susceptible to various damages. The limbus, like other structures of the brain, can be subject to injuries and other harmful factors, which include tumors with hemorrhages.
Syndromes of lesions of the limbic system are rich in number, the main ones are as follows:
dementia- dementia. The development of diseases such as Alzheimer's and Pick's syndrome is associated with atrophy of the systems of the limbic complex, and especially in the localization of the hippocampus.
Epilepsy. Organic disorders of the hippocampus lead to the development of epilepsy.
pathological anxiety and phobias. Violation of the activity of the amygdala leads to a mediator imbalance, which, in turn, is accompanied by a disorder of emotions, including anxiety. A phobia is an irrational fear of a harmless object. In addition, an imbalance of neurotransmitters provokes depression and mania.
Autism. At its core, autism is a deep and serious maladjustment in society. The inability of the limbic system to recognize the emotions of other people leads to dire consequences.
Reticular formation(or mesh formation) is a non-specific formation of the limbic system responsible for the activation of consciousness. After deep sleep, people wake up thanks to the work of this structure. In case of damage human brain undergoes various disorders of consciousness shutdown, including absences and syncope.
neocortex
The neocortex is the part of the brain found in higher mammals. The rudiments of the neocortex are also observed in lower animals that suckle milk, but they do not reach a high development. In humans, the isocortex is the lion's share of the common cerebral cortex, which has an average thickness of up to 4 millimeters. The area of the neocortex reaches 220 thousand square meters. mm.
History of occurrence
IN this moment neocortex - the highest level human evolution. Scientists managed to study the first manifestations of the new bark in representatives of reptiles. The last animals that do not have a new bark in the chain of development were birds. And only a developed person has.
Evolution is a complex and long process. Every kind of creature goes through a harsh evolutionary process. If an animal species could not adapt to a changing environment, the species would lose its existence. Why is a person was able to adapt and survive to this day?
Being in favorable living conditions (warm climate and protein food), the descendants of man (before the Neanderthals) had no choice but to eat and reproduce (thanks to the developed limbic system). Because of this, the mass of the brain, by the standards of the duration of evolution, gained a critical mass in a short period of time (several million years). By the way, the mass of the brain in those days was 20% more than that of a modern person.
However, all good things come to an end sooner or later. With climate change, the descendants had to change their place of residence, and with it, start looking for food. Having a huge brain, the descendants began to use it for searching for food, and then for social involvement, because. it turned out that by uniting in groups according to certain criteria of behavior, it was easier to survive. For example, in a group where everyone shared food with other members of the group, they were more likely to survive (Someone picked berries well, and someone hunted, etc.).
From that moment began separate evolution in the brain, separate from the evolution of the whole body. Since those times appearance man has not changed much, but the composition of the brain differs dramatically.
What does it consist of
The new cerebral cortex is an accumulation of nerve cells that form a complex. Anatomically, 4 types of cortex are divided, depending on its localization -, occipital,. Histologically, the cortex consists of six balls of cells:
- Molecular ball;
- external granular;
- pyramidal neurons;
- internal granular;
- ganglionic layer;
- multiform cells.
What functions does
The human neocortex is classified into three functional areas:
- touch. This zone is responsible for the highest processing of stimuli received from the external environment. So, ice becomes cold when information about the temperature enters the parietal region - there is no cold on the finger, but there is only an electrical impulse.
- association zone. This area of the cortex is responsible for the information connection between the motor and sensory cortex.
- motor zone. All conscious movements are formed in this part of the brain.
In addition to such functions, the new cortex provides the highest mental activity: intelligence, speech, memory and behavior.
Conclusion
Summing up, we can highlight the following:
- Due to two main, fundamentally different, structures of the brain, a person has a duality of consciousness. For every action, two different thoughts are formed in the brain:
- "I want" - the limbic system (instinctive behavior). The limbic system occupies 10% of the total mass of the brain, low energy consumption
- “Need” – neocortex ( social behavior). Neocortex occupies up to 80% of the total brain mass, high energy consumption and limited metabolic rate
Man is the only species on earth that is capable, in addition to satisfying the needs dictated by instincts, to carry out emotional, creative and mental activity. The uniqueness of people lies in the presence of vast, highly developed and complexly constructed areas of the brain, which have a generalized name neocortex. Therefore, in the study of man, as a species at the upper stage of evolution, the main directions are questions about the structure and functions of this part of the central nervous system.
General information
Neocortex (new cortex, isocortex or lat. neocortex) is a region of the cerebral cortex, occupying about 96% of the surface of the hemispheres and having a thickness of 1.5 - 4 mm, which are responsible for the perception of the world, motor skills, thinking and speech.
The neocortex is made up of three main types of neurons - pyramidal, stellate, and fusiform. The first, the most numerous group, which makes up about 70-80% of the total amount in the brain. The proportion of stellate neurons is at the level of 15-25%, and spindle-shaped - about 5%.
The structure of the neocortex is almost homogeneous and consists of 6 horizontal layers and vertical columns of the cortex. The layers of the new cortex have the following structure:
- Molecular, consisting of fibers and a small number of small stellate neurons. The fibers form a tangential plexus.
- Outer granular, formed by small neurons of various shapes, which are associated with the molecular layer over all areas. At the very end of the layer are small pyramidal cells.
- External pyramidal, consisting of small, medium and large pyramidal neurons. The processes of these cells can be associated with both layer 1 and white matter.
- Internal granular, which consists mainly of stellate cells. This layer is characterized by a non-dense arrangement of neurons in it.
- Internal pyramidal, formed by medium and large pyramidal cells, the processes of which are connected with all other layers.
- Polymorphic, which is based on spindle-shaped neurons connected by processes with the 5th layer and white matter.
In addition, the new cortex is divided into regions, which in turn are subdivided into Brodmann fields. The following areas are distinguished:
- Occipital (17,18 and 19 fields).
- Upper parietal (5 and 7).
- Lower parietal (39 and 40).
- Postcentral (1, 2, 3 and 43).
- Precentral (4 and 6).
- Frontal (5, 9, 10, 11, 12, 32, 44, 45, 46 and 47).
- Temporal (20, 21, 22, 37, 41 and 42).
- Limbic (23, 24, 25 and 31).
- Islet (13 and 14).
Cortex columns are a group of neurons that are perpendicular to the cerebral cortex. Within a small column, all cells perform the same task. But a hypercolumn, consisting of 50-100 minicolumns, can have either one or many functions.
neocortex functions
The new cortex is responsible for the performance of higher nervous functions (thinking, speech, processing information from the senses, creativity, etc.). Clinical trials have shown that each area of the cerebral cortex is responsible for strictly defined functions. For example, human speech is controlled by the left frontal gyrus. However, if any of the areas is damaged, the neighboring one can take over its function, although this requires a long period of time. Conventionally, there are three main groups of functions that the neocortex performs - sensory, motor and associative.
touch
This group includes a set of functions by which a person is able to perceive information from the senses.
Each feeling is analyzed by a separate area, but signals from others are also taken into account.
Signals from the skin are processed by the posterior central gyrus. Moreover, information from the lower extremities enters the upper part of the gyrus, from the body - to the middle, from the head and hands - to the lower. At the same time, only pain and temperature sensations are processed by the posterior central gyrus. The sense of touch is controlled by the upper parietal region.
Vision is controlled by the occipital region. Information is received in field 17, and in fields 18 and 19 it is processed, that is, color, size, shape and other parameters are analyzed.
Hearing is processed in the temporal region.
Charm and taste sensations are controlled by the hippocampal gyrus, which, unlike the general structure of the neocortex, has only 3 horizontal layers.
It should be noted that in addition to the zones of direct reception of information from the senses, there are secondary ones next to them, in which the received images are compared with those stored in memory. With damage to these areas of the brain, a person completely loses the ability to recognize incoming data.
Motor
This group includes the functions of the new cortex, with the help of which any movement of the human limbs is carried out. Motor skills are controlled and controlled by the precentral region. The lower limbs depend on the upper parts of the central gyrus, and the upper limbs depend on the lower ones. In addition to the precentral, the frontal, occipital and upper parietal regions are involved in the movement. An important feature performance of motor functions is that they cannot be performed without constant connections with sensory areas.
Associative
This group of neocortical functions is responsible for such complex elements of consciousness as thinking, planning, emotional control, memory, empathy, and many others.
Associative functions are performed by the frontal, temporal and parietal regions.
In these parts of the brain, a reaction is formed to the data coming from the sense organs and command signals are sent to the motor and sensory zones.
To receive and control, all sensory and motor areas of the cerebral cortex are surrounded by associative fields, in which the analysis of the received information takes place. But at the same time, it should be taken into account that the data coming into these fields are already initially processed in sensory and motor areas. For example, if there is a malfunction in the work of such a section in the visual area, a person sees and understands that there is an object, but cannot name it and, accordingly, make a decision about his further behavior.
In addition, the frontal lobe of the cortex is very tightly connected to the limbic system, which allows it to control and manage emotional messages and reflexes. This enables a person to take place as a person.
The performance of associative functions in the neocortex is possible due to the fact that the neurons of this part of the central nervous system are able to retain traces of excitation according to the principle feedback can persist for a long time (from several years to a lifetime). This ability is memory, with the help of which associative links of the received information are built.
The role of the neocortex in emotions and stereogenesis
Emotions in humans initially appear in the limbic system of the brain. But in this case, they are represented by primitive concepts, which, getting into the new cortex, are processed using the associative function. As a result, a person can operate with emotions on more high level, which makes it possible to introduce such concepts as joy, sadness, love, anger, etc.
Also, the neocortex has the ability to dampen strong bursts of emotion in the limbic system by sending calming signals to areas of high neuronal arousal. This leads to the fact that in a person the dominant role in behavior is played by the mind, and not by instinctive reflexes.
Differences from the old bark
The old cortex (archicortex) is an earlier emerging area of the cerebral cortex than the neocortex. But in the process of evolution, the new crust became more developed and extensive. In this regard, the archicortex ceased to play a dominant role and became one of the constituent parts.
If we compare the old one and according to the functions performed, then the first is assigned the role of fulfilling innate reflexes and motivation, and the second is the management of emotions and actions at a higher level.
In addition, the neocortex is much larger than the old cortex. So the first occupies about 96% of the total surface of the hemispheres, and the size of the second - no more than 3%. This ratio shows that the archicortex cannot perform higher nervous functions.
Neocortex - evolutionarily the youngest part of the cortex, occupying most of the surface of the hemispheres. Its thickness in humans is approximately 3 mm.
The cellular composition of the neocortex is very diverse, but approximately three-quarters of the neurons of the cortex are pyramidal neurons (pyramids), and therefore one of the main classifications of cortical neurons divides them into pyramidal and non-iramide (fusiform, stellate, granular, candelabra cells, Martinotti cells, etc. .). Another classification is related to the length of the axon (see section 2.4). The long-axon Golgi I cells are mainly pyramids and spindles, their axons can exit the cortex, the rest of the cells are short-axon Golgi II.
Cortical neurons also differ in the size of the cell body: the size of ultra-small neurons is 6x5 microns, the size of giant ones is more than 40 x 18. The largest neurons are the Betz pyramids, their size is 120 x 30-60 microns.
Pyramidal neurons (see Fig. 2.6, G) have the shape of a body in the form of a pyramid, the top of which is directed upwards. An apical dendrite extends from this apex and ascends into the overlying cortical layers. Basal dendrites extend from the rest of the soma. All dendrites have spines. A long axon departs from the base of the cell, forming numerous collaterals, including recurrent ones, which bend and rise upwards. Stellate cells do not have an apical dendrite; spinules on dendrites are absent in most cases. In fusiform cells, two large dendrites depart from opposite poles of the body, there are also small dendrites extending from the rest of the body. Dendrites have spines. The axon is long, slightly branching.
During embryonic development, the new cortex necessarily passes through the stage of a six-layer structure, with maturation in some areas the number of layers may decrease. The deep layers are phylogenetically older, the outer layers are younger. Each layer of the cortex is characterized by its neuronal composition and thickness, which can differ from each other in different areas of the cortex.
Let's list layers of neocortex(Fig. 9.8).
I layer - molecular- the outermost, contains a small number of neurons and mainly consists of fibers running parallel to the surface. Dendrites of neurons located in the underlying layers also rise here.
II layer - outer granular, or outer granular, - consists mainly of small pyramidal neurons and a small number of medium-sized stellate cells.
III layer - external pyramidal - the widest and thickest layer, contains mainly small and medium-sized pyramidal and stellate neurons. In the depths of the layer are large and giant pyramids.
IV layer - internal granular, or internal granular, - consists mainly of small neurons of all varieties, there are also a few large pyramids.
V layer - internal pyramidal, or ganglionic characteristic feature which is the presence of large and in some areas (mainly in fields 4 and 6; Fig. 9.9; subparagraph 9.3.4) - giant pyramidal neurons (Betz's pyramids). The apical dendrites of the pyramids, as a rule, reach the first layer.
VI layer - polymorphic, or multiform, - contains predominantly spindle-shaped neurons, as well as cells of all other forms. This layer is divided into two sublayers, which a number of researchers consider as independent layers, speaking in this case of a seven-layer bark.
Rice. 9.8.
A- Neurons are stained as a whole; b- only the bodies of neurons are painted; V- painted
only processes of neurons
Main functions each layer is also different. Layers I and II carry out connections between neurons of different layers of the cortex. Callosal and associative fibers mainly come from the pyramids of layer III and come to layer II. The main afferent fibers entering the cortex from the thalamus terminate on layer IV neurons. Layer V is mainly associated with the system of descending projection fibers. The axons of the pyramids of this layer form the main efferent pathways of the cerebral cortex.
In most cortical fields, all six layers are equally well expressed. Such a bark is called homotypic. However, in some fields, the severity of the layers may change during development. This bark is called heterotypic. It is of two types:
granular (zeros 3, 17, 41; Fig. 9.9), in which the number of neurons in the outer (II) and especially in the inner (IV) granular layers is greatly increased, as a result of which the IV layer is divided into three sublayers. Such a cortex is characteristic of primary sensory areas (see below);
Agranular (fields 4 and 6, or motor and premotor cortex; Fig. 9.9), in which, on the contrary, there is a very narrow II layer and practically no IV, but very wide pyramidal layers, especially the inner one (V).
