10. Cerebral cortex.
Structure.
The cortex is phylogenetically the youngest and at the same time complex part of the brain, designed for processing sensory information and forming behavioral reactions of the body.
The cerebral cortex is divided into ancient (olfactory bulb, olfactory tract, olfactory tubercle), old (part of the limbic system) and new cortex. The new bark occupies 95-96% of the total area and 4-5% is the share of ancient and old bark. The thickness of the bark ranges from 1.3 to 4.5 mm. The area of the cortex increases due to grooves and convolutions. In an adult it is 2200 cm²
The cortex consists of gray and white matter, as well as neuroglia. The number of neurons is 16-18 billion. Glial cells perform a trophic function.
Based on their functional characteristics, cortical neurons are divided into 3 types: afferent(sensory) - nerve fibers of the afferent pathways approach them, associative(intercalated) – within the brain and spinal cord, efferent(motor) - forms descending (efferent) pathways running from the cortex to different nuclei of the brain and spinal cord. Sensory cells include stellate cells, which are included in layers 3 and 4 of the sensory areas of the cortex. Efferent neurons include neurons of layer 5 of the motor zone, which are represented by giant pyramidal cells of Betz. Association cells include spindle-shaped and pyramidal cells of layer 3.
Due to the fact that the bodies and processes of the neurons described above have an ordered arrangement, the cortex is built according to the screen principle, i.e. the signal is focused not point to point, but on many neurons, which provides a complete analysis of the stimulus, as well as the ability to transmit the signal to other areas of the cortex that are interested in it.
The cortex consists of 7 layers.
Molecular layer– small neurons and fibers. Afferent thalamocortical fibers from the nonspecific nuclei of the thalamus come here, regulating the level of excitability of cortical neurons.
Outer granular layer formed by small granule-shaped neurons and small pyramidal cells.
Outer pyramidal layer consists of pyramidal cells of different sizes. Functionally, layers II and III of the cortex unite neurons, the processes of which provide cortico-cortical associative connections.
Inner granular layer formed by stellate cells. Afferent thalamocortical fibers coming from the projection nuclei of the thalamus end here.
Inner pyramidal layer includes large pyramidal cells - Betz cells, the axons of which go to the brain and spinal cord.
Polymorphic layer (multiform) – multiform neurons having a triangular and fusiform shape.
Fusiform neurons connect all layers of the bark, their fibers rise to 1 layer. Available only in certain areas of the cortex.
The functional unit of the cortex is a vertical column consisting of 7 cells, they together react to the same stimulus.
In the cortex, sensory, associative and motor zones are distinguished, based on the location of neurons:
Sensory areas are input areas of the cortex that receive sensory information from most of the body's receptors through ascending neural pathways.
Associative zones – 1) connect newly received sensory information with previously received and stored in memory blocks, due to which new stimuli are “recognized”, 2) information from some receptors are compared with sensory information from other receptors, 3) participate in the processes of memorization, learning and thinking.
Motor zones are output areas of the cortex. Motor impulses arise in them, going to voluntary muscles along descending pathways that are located in the white matter cerebral hemispheres.
Cytoarchitecture- This is the location of neurons in the cortex.
Myeloarchitecture- This is the distribution of fibers in the cerebral cortex.
The beginning of the different-quality structure of the cerebral cortex was laid in 1674 by the Kyiv anatomist A.A. Betsom. Later K. Brodman in 1903-09. identified 52 cytoarchitectonic fields. O. Vogt and C. Vogt identified 150 myeloarchitectonic fields in the cortex.
Localization of functions in the cerebral cortex.
I.P. Pavlov considered the cerebral cortex as a continuous perceptual surface, as a set of cortical ends of analyzers. The analyzer is a complex system that consists of a receptor - the perceiving apparatus, conductors of nerve impulses and the brain end, where higher analysis irritations. I.P. Pavlov showed that the cortex distinguishes between nuclei and scattered elements. The nucleus is the site of concentration of neurons, where all peripheral receptor structures are projected and important analysis and synthesis and integration of functions occur.
Scattered elements can be located along the periphery of the nucleus and at different distances from it. They provide simpler analysis and synthesis.
The cortical ends of the analyzer carry out the analysis and synthesis of signals.
Let's consider some localization of the nuclei of motor analyzers:
1. In the cortex of the postcentral gyrus (fields 1, 2, 3) and the superior parietal lobule (fields 5 and 7) lie nerve cells that form the core of the cortical analyzer of the general sensitivity(temperature, pain, tactile and proprioceptive). Sensory pathways leading to the cortex big brain, intersect either at the level of different segments of the spinal cord (pathways of pain, temperature sensitivity, touch and pressure), or at the level of the medulla oblongata (pathways of proprioceptive sensitivity of the cortical direction). As a result, the postcentral gyri of each hemisphere are connected to the opposite half of the body. In the postcentral gyrus, the receptor fields of various parts of the human body are projected in such a way that the cortical ends of the sensitivity analyzer of the lower torso and lower extremities are located most highly, and the receptor fields of the upper parts of the body, head and upper extremities are projected most low (closer to the lateral sulcus).
2. Core motor The analyzer is located mainly in the so-called motor cortex, which includes the procentral gyrus (fields 4 and 6) and the paracentral lobule on the medial surface of the hemisphere. In the 5th layer of the cortex of the precentral gyrus there are pyramidal neurons (Betz cells), which I.P. Pavlov classified them as intercalary cells, and noted that these cells, with their processes, are connected with the subcortical nuclei, motor cells of the nuclei of the cranial and spinal nerves. Moreover, in the upper parts of the precentral gyrus and in the paracentral lobule there are cells, impulses from which are sent to the muscles of the lowermost parts of the trunk and lower extremities. In the lower part of the precentral gyrus there are also motor centers that regulate the activity of the facial muscles.
Thus, all areas of the human body are projected “upside down” in the precentral gyrus. Due to the fact that the pyramidal tracts, originating from the gigantopyramidal cells, intersect either at the level of the brain stem (corticonuclear fibers) at the border with the spinal cord, or in segments of the spinal cord (corticospinal tract), the motor areas of each hemisphere connected to the skeletal muscles of the opposite side of the body. If the muscles of the limbs are connected in isolation to one of the hemispheres, the muscles of the trunk, larynx and pharynx are connected to the motor areas of both hemispheres.
3. Core visual The analyzer is located in the occipital lobe of the cerebral hemisphere (fields 17, 18, 19). The nucleus of the visual analyzer of the right hemisphere is connected by pathways to the lateral half of the retina of the right eye and the medial half of the retina of the left eye. In the cortex of the occipital lobe of the left hemisphere, the receptors of the lateral half of the retina of the left eye and the medial half of the retina of the right eye are projected, respectively. Only bilateral damage to the nuclei of the visual analyzer leads to complete cortical blindness. Damage to field 18, located slightly above field 17, is accompanied by loss of visual memory, but no loss of vision is noted. The highest in relation to the previous two in the occipital lobe cortex is field 19, the defeat of which is accompanied by a loss of the ability to navigate in an unfamiliar environment.
4. In the depth of the lateral sulcus, on the surface of the middle part of the superior temporal gyrus facing the insula, there is a nucleus auditory analyzer (fields 41, 42, 52). Conducting paths from receptors on both the left and right sides pass to the nerve cells that make up the core of the auditory analyzer of each hemisphere. In this regard, unilateral damage to this nucleus does not cause a complete loss of the ability to perceive sounds. Bilateral lesions are accompanied by cortical deafness, as in the case of complete cortical blindness.
5. Motor analyzer core speech articulation(speech motor analyzer) is located in the posterior parts of the inferior frontal gyrus (field 44). It borders on those parts of the precentral gyrus that analyze movements produced by contraction of the muscles of the head and neck. This is understandable, since the speech motor analyzer analyzes the movements of all muscles (lips, neck, tongue, larynx) that take part in the act of forming oral speech. Damage to the cortex of this area (field 44) leads to motor aphasia, i.e. loss of the ability to contract muscles involved in speech production. Moreover, if field 44 is damaged, the ability to pronounce sounds or sing is not lost.
In the central parts of the inferior frontal gyrus (field 45) there is the nucleus of the speech analyzer associated with singing. The defeat of field 45 is accompanied by vocal amusia - the inability to compose and reproduce musical phrases, and agrammatism, when the ability to compose meaningful sentences from individual words is lost. The speech of such people consists of a set of words that are not connected in semantic meaning.
6. Core auditory speech analyzer is closely interconnected with the cortical center of the auditory analyzer and is located, like the latter, in the region of the superior temporal gyrus. This nucleus is located in the posterior parts of the superior temporal gyrus, on its surface facing the lateral sulcus of the cerebral hemisphere (field 42).
Damage to the nucleus does not affect the auditory perception of sounds, but the ability to understand words and speech is lost. The function of this core is that a person does not hear or understand the speech of another person, but controls his own.
In the middle third of the superior temporal gyrus (field 22) there is a nucleus of the cortical analyzer, the defeat of which is accompanied by the onset of musical deafness, when musical phrases are perceived as a meaningless collection of various noises. This cortical end of the auditory analyzer belongs to the centers of the second signal system, which perceive the verbal designation of objects, actions, phenomena, i.e. signal-sensing signals.
7. In direct connection with the core of the visual analyzer there is a core visual analyzer of written speech(field 39), located in the angular gyrus of the inferior parietal lobule. Damage to this nucleus leads to loss of the ability to perceive written text and read.
There are 3 groups of fields in the cortex: primary, secondary and tertiary.
The primary field is associated with the sensory organs and organs of movement; it is formed earlier in ontogenesis and has the largest cells. These are the so-called nuclear zones of analyzers. They analyze stimuli entering the cortex from the corresponding receptors. If the nuclear zone is destroyed, cortical blindness, deafness, and motor paralysis will occur.
Secondary fields (peripheral zones of analyzers) are connected to individual organs only through primary fields. They serve to summarize and further process incoming information. If this field is destroyed, a person sees, hears, but does not understand the meaning.
Tertiary fields (areas of overlap of analyzers) occupy almost half of the cortex and have extensive connections with other parts of the cortex and nonspecific brain systems. Here, small and diverse (stellate) cells are mainly located and higher analysis and synthesis of information occurs, as a result of which goals and objectives of behavior are developed. According to them, motor activity is programmed. With congenital underdevelopment of the tertiary fields, a person is not able to master speech and even simple motor skills.
Humans and animals have primary and secondary fields, but only humans have a tertiary field. Tertiary fields mature in humans later than other cortical fields. For the development of fields, it is necessary that more information comes from visual, auditory, and muscle receptors.
Onto- and phylogeny of the cortex.
By the 30th day of intrauterine development, the cortex is formed. By 7–12 months of postnatal development, maturation of the brain systems occurs.
The newborn has developed phylogenetically old parts of the brain: the cerebellum, the pons, and the diencephalon. In newborns, the main sulci and gyri (central, lateral) are well expressed, and the branches of the sulci and gyri are weak. Myelination of afferent fibers begins at 2 months and ends by 4-5 years, and efferent fibers somewhat later - from 4-5 months to 7-8 years. The relationships of grooves, convolutions and sutures characteristic of an adult are established in children at 6-8 years of age.
11. Asymmetry of the cerebral hemispheres.
The forebrain, which represents the most massive part of the brain, is divided along the midline by a deep vertical fissure into the right and left hemispheres. Both of them are connected to each other by the corpus callosum. Each hemisphere has lobes: frontal, parietal, temporal, occipital and insula. Each lobe of the brain has a functional significance. The left and right hemispheres perform different functions, but together provide goal-directed behavior.
The doctrine of interhemispheric asymmetry arose more than 100 years ago. In the 1860s, French researcher P. Broca found that damage to a certain area of the cortex causes aphasia or speech disorder. This area is located at the edge of the frontal lobe of the left hemisphere, called Broca's area (area 1 ). It controls the implementation of speech reactions.
In 1874, the German researcher K. Wernicke discovered a sensory area in the left hemisphere 2 ) speech center, the defeat of which leads to a disorder in speech understanding. Wernicke's center is located in the temporal lobe. A person with a damaged center has fluent, meaningless speech, and the patient himself does not notice this defect.
After cutting the commissure connections of the two hemispheres, each of them functions independently, receiving information only from the right or left. If a patient with a split brain is presented with an object in the right half of the visual field, he can name it and pick it up with his right hand; the same with the word, i.e. the left hemisphere is used. In this case, he is no different from a normal person. The defect occurs when stimuli occur on the left side of the body or in the left half of the visual field. An object whose image is projected into right hemisphere, the patient cannot name. Although he chooses him correctly among others. Those. the right hemisphere cannot provide the function of naming an object, but it recognizes the object.
3 zone – motor, located in the anterior central gyrus of the right and left hemispheres. This area controls the muscles of the face, limbs and torso.
The right hemisphere controls and regulates the sensorimotor and motor functions of the left half of the body, and the left hemisphere controls and regulates the right half of the body. Musical abilities are associated with the right hemisphere. Left hemisphere– speech, processes information analytically and sequentially, right – simultaneously and holistically. A person with a predominance of the left hemisphere function gravitates toward theory, has a large vocabulary, and is characterized by motor activity and purposefulness. A right-hemisphere person gravitates towards specific types of activity, is slow, taciturn, but has a subtle feeling and worry.
The asymmetry of the functions of the cerebral hemispheres is genetically predetermined. It is expressed in the predominant participation of the left and right halves of the brain in a qualitatively different analysis of external stimuli.
Functional asymmetry can increase with targeted intervention due to the formation of a stable dominant attitude, but the relearning of genetically programmed asymmetric forms of movements.
A study of functional brain asymmetry in children showed that speech signals are initially processed by both hemispheres, and the dominance of the left hemisphere is formed later. If a child who has learned to speak develops damage to the speech area of the left hemisphere, then he develops aphasia, and after a year speech is restored. And then the center of speech moves to the zone of the right hemisphere. Such a transfer of speech function from the left hemisphere to the right is possible only up to 10 years.
Specialization of the right hemisphere in the function of orientation in space also does not appear immediately: in boys from the age of 6 years, and in girls after 13 years.
D. Kimura believes that in evolutionary terms, it was the development of the hand as an organ of sign language and its manipulative abilities that led to the development of the left hemisphere. Later, this function of the hand was transferred to the vocal muscles.
The left hemisphere is also superior to the right in the ability to understand speech, although these differences are less pronounced. According to the motor theory of perception, the main component of speech sound recognition is kinesthetic signals arising from the muscles of the speech apparatus when perceiving speech signals. In this, a special role belongs to the motor systems of the left hemisphere.
Speech functions in right-handed people are predominantly localized in the left hemisphere. And only 5% of people have speech centers in the right hemisphere. 70% of left-handed people have a speech center, just like right-handed people, in the left hemisphere. In 15% of left-handed people, the speech center is in the right hemisphere.
Rice. 7. Human projection in the central gyri.
Functional asymmetry is not found in all people; in approximately one third it is not expressed, i.e. hemispheres do not have a clear functional specialization.
There are several types of functional asymmetries:
Motor asymmetry is the difference in the motor activity of the arms, legs, face, and halves of the body, controlled by each hemisphere of the brain.
Sensory asymmetry is the unequal perception by each hemisphere of objects located to the left and right of the midplane of the body.
Mental asymmetry is the specialization of the brain hemispheres in relation to various forms mental activity.
The ratio of activity of the two hemispheres may be different. On this basis I.P. Pavlov identified specific human types of external nervous activity (ENA): artistic, mental and average.
The artistic type of people is characterized by a predominance of the activity of the first signaling system over the second. “Right-hemisphere” imaginative thinking predominates in them. In a person of the thinking type, the second signaling system prevails over the first, i.e. “left-hemisphere” abstract thinking dominates.
When the corpus callosum is cut, a split personality occurs. Two models of behavior are built for the same situation. In the process of evolution, a separation of the functions of the right and left hemispheres occurs in humans. Morphologically, the hemispheres are not fundamentally different. The right hemisphere is only 5 g larger than the left, but the left hemisphere has more gray matter. The left hemisphere is responsible for speech, writing, reading, counting, and conscious abstract thinking. Right - recognizing objects, color, shape, distinguishing voices.
Human consciousness is based on the joint activity of two hemispheres, although one of them is dominant. The left hemisphere receives processed information, analytical and consistent, involving facts and logic. The right hemisphere processes information simultaneously and holistically, without considering individual parts or elements that make up an object or phenomenon. The right hemisphere monitors all changes in the environment, changes in mood, and the left hemisphere analyzes these changes; it is responsible for choosing the goal that we set for ourselves for the future. The right hemisphere integrates all the information coming from the somatosensory area, informing the relative position of the body in space. It connects with information coming from the visual and auditory cortex, thanks to which we have an accurate understanding of our own body as it moves in space. In the left hemisphere, this information is combined with memory, which allows you to meaningfully interpret visual, auditory and tactile sensations (messages from receptors of the skin, muscles, joints) and develop a certain line of behavior.
There are two centers in the left hemisphere: Broca's center and Wernicke's center. Broca's center is located in the frontal lobe of the left hemisphere. It is adjacent to the motor cortex. This is the motor center of speech, which controls the muscles of the tongue, jaws and pharynx, due to which sounds are pronounced. If this center is damaged, aphasia occurs and motor acts become difficult. After strokes, this center is paralyzed, while understanding speech, reading and writing are not impaired, and the patient is aware of his defect.
Wernicke's center is located in the superior posterior region of the left temporal lobe. This is the sensory center of speech, which is responsible for understanding speech and comprehending it. It is in this zone that the main nervous substrate is located, which determines the construction of oral speech, its form, meaning and content. When it is damaged, Wernicke's aphasia occurs, when understanding speech is very difficult, speech is fluent, meaningless, reading and writing are impaired, and the patient does not realize the meaninglessness of his speech.
Broca's center and Wernicke's center are connected by nerve fibers, forming an arcuate fascicle. First, in Broca's area, under the influence of incoming impulses, a detailed and coordinated localization program is built - how and in what sequence the muscles of the lips, uvula and pharynx should act. From here, impulses enter the motor cortex, which controls the activity of all muscles. From the motor zone, impulses travel to the corresponding muscles. The sound of a word is captured by the auditory cortex, but to understand the meaning, it is necessary that the signals pass through Wernicke's area, which is adjacent to the auditory zone of the temporal region. Here sounds are interpreted as speech. If a word is perceived not through sound, but through the eyes (when reading), then in this case information from the primary visual cortex should also flow to Wernicke’s center. Since oral speech arises in the process of evolution earlier than writing; children begin to speak and understand speech before they learn to read and write.
Dyslexia is a disorder in children's reading ability. This may be the result of previous trauma, especially before the 1st year of life, and as a result of impaired visuospatial perception. They cannot perceive words as a whole. They cannot distinguish between similar words and are lost if they are asked to pronounce an unfamiliar word. Most often, such children have instability of the ocular dominant. For most people, one eye, like the hand, is dominant. Instability of ocular dominance can lead to impaired eye movement and then it is very difficult for a person to follow the order of letters and words on the page.
Ocular-dominant instability may result from instability of control on the part of the cerebral hemispheres, when neither hemisphere takes on the dominant role of controlling eye movements.
Girls begin to speak and read earlier than boys. In men, damage to the left hemisphere causes aphasia 3 times more often than in women and leads to a significant deterioration in verbal ability. In women, the specialization of the hemispheres is less pronounced. This is laid down in the prenatal (antenatal) period. Already in the 3rd month of intrauterine development in a male fetus, the concentration of the male sex hormone testosterone increases significantly. In the female fetus it is formed in lower concentrations, since it comes from the mother’s body. Testosterone affects the rate of development of the hemispheres, as if slowing down the growth of the left hemisphere and promoting the rapid development of the right hemisphere, responsible for spatial ability.
Pathological changesIAndIIsignaling system.
In humans, unlike animals, there may be specific forms of manifestation of neuroses, and this depends on which signaling system is involved in the pathological process.
Hysteria – signal system I predominates. Everything around is annoying, there is an increased sensitivity to the external environment, to stimuli that are subthreshold for everyone around, and for the patient - threshold or superthreshold, when a strong process of excitation is formed, reaching hysteria. As a result, an overvoltage of the excitation force may occur. All this negatively affects mental and physical performance, all types of internal inhibition.
Psychosthenia – the II signaling system predominates. People are disappointed in life, poor in emotions, prone to empty philosophizing. Their plans are fruitless, unrealistic, divorced from life.
brain Document
TO laboratory work By special course“Electrophysiology”: Topic 1. Electrocardiography... -t, Med. Fak., Dept. normal and pathological anatomy/ Compiled by: A. X. Urusmambetov, I. I. Novikova, ... bioelectric activity of the brain brain patients with...
Previously, it was believed that the higher functions of the human brain are carried out by the cerebral cortex. Back in the last century, it was established that when the cortex is removed from animals, they lose the ability to perform complex acts of behavior due to acquired life experience. It has now been established that the cortex is not the highest distributor of all functions. Many of its neurons are part of the middle-level sensory and motor systems. The substrate of higher mental functions is the distribution systems of the central nervous system, which include both subcortical structures and cortical neurons. The role of any cortical area depends on internal organization its synaptic connections, as well as its connections with other formations of the central nervous system. At the same time, in the process of evolution, humans have undergone corticolization of all functions, including vital visceral functions. Those. their subordination to the cortex. It has become the main integrating system of the entire central nervous system. Therefore, in the event of the death of a significant part of the cortical neurons in a person, his body becomes non-viable and dies as a result of a violation of homeostasis (brain hypothermia).
Cerebral cortex consists of six layers:
1. Molecular layer, the most superficial. Formed by many ascending dendrites of pyramidal neurons. There are few neuron bodies in it. This layer is pierced by axons of nonspecific thalamic nuclei belonging to the reticular formation. Due to this structure, the layer ensures activation of the entire cortex.
2. Outer granular layer. It is formed by densely located small neurons that have numerous synaptic contacts with each other. Due to this, a long-term circulation of nerve impulses is observed. This is one of the memory mechanisms.
3. Outer pyramidal layer. Consists of small pyramidal cells. With the help of them and the cells of the second layer, intercortical connections are formed, i.e. connections between different areas of the cortex.
4. Inner granular layer. Contains stellate cells on which the axons of switching and associative neurons of the thalamus form synapses. All information from peripheral receptors comes here.
5. Inner pyramidal layer. Formed by large pyramidal neurons, the axons of which form descending pyramidal tracts leading to the medulla oblongata and spinal cord.
6. Layer of polymorphic cells. The axons of its neurons go to the thalamus.
Cortical neurons form neural networks, including three main components:
1. afferent (input) fibers;
2. interneurons;
3. efferent (output) neurons.
These components form several neural network levels .
1. Microgrids. The lowest level. These are individual interneuron synapses with their pre- and postsynaptic structures. The synapse is complex functional element having internal self-regulatory mechanisms. Cortical neurons have highly branched dendrites. On them is huge amount drumstick-shaped spines. These spines serve to form input synapses. Cortical synapses are extremely sensitive to external influences. For example, deprivation of visual stimulation by keeping growing animals in the dark leads to a significant reduction in synapses in the visual cortex. In Down's disease, there are also fewer synapses in the cortex than normal. Each spine, forming a synapse, acts as a transducer of signals going to the neuron.
2. Local networks. The neocortex is a layered structure, the layers of which are formed by local neural networks. Impulses from all peripheral receptors can come to it through the thalamus and olfactory brain. Input fibers pass through all layers, forming synapses with their neurons. In turn, collaterals of input fibers and interneurons of these layers form local networks at every level of the cortex. This structure of the cortex provides the ability to process, store and interact with various information. In addition, there are several types of output neurons in the cortex. Almost every layer of it produces output fibers that go to other layers or distant areas of the cortex.
3. Cortical columns. Input and output elements with interneurons form vertical cortical columns or local modules. They pass through all layers of the cortex. Their diameter is 300-500 microns. The neurons that form these columns are concentrated around the thalamo-cortical fiber, which carries a certain type of signal. The columns contain numerous interneuron connections. Neurons of layers 1-5 of the columns provide perception and processing of incoming information. Neurons of layers 5-6 form the efferent pathways of the cortex. Adjacent columns are also interconnected. In this case, the excitation of one is accompanied by inhibition of the neighboring ones.
Columns performing the same type of function are concentrated in certain areas of the cortex. These areas are called cytoarchitectonic fields . There are 53 of them in the human cortex. The fields are divided into primary, secondary and tertiary. Primary provide processing of certain sensory information, and secondary and tertiary– interaction of signals from different sensory systems. In particular, primary somatosensory field, to which impulses from all skin receptors (tactile, temperature, pain) go, is located in the region of the posterior central gyrus. The most space in the cortex is occupied by the lips, face, and hands. Therefore, when this area is damaged, the sensitivity of the corresponding areas of the skin changes.
Representation of proprioceptors of muscles and tendons, i.e. motor cortex occupies the anterior central gyrus. Impulses from the proprioceptors of the lower extremities go to the upper part of the gyrus. From the muscles of the torso to the middle part. From the muscles of the head and neck to its lower part. The largest area of this field is also occupied by the muscles of the lips, tongue, hands and face.
Impulses from receptors eyes enter the occipital cortex around calcarine groove. Damage to the primary fields leads to cortical blindness, and damage to the secondary and tertiary fields leads to loss of visual memory.
Auditory the cortex area is located in superior temporal gyrus and transverse gyrus of Heschl. When the primary fields of the zone are damaged, cortical deafness develops, while the peripheral fields develop difficulties in distinguishing sounds. In the posterior third of the superior temporal gyrus of the left hemisphere there is sensory speech center Wernicke Center. With its pathological changes, the ability to understand speech is lost.
Motor speech center- center Broca, located in the inferior frontal gyrus of the left hemisphere. Abnormalities in this part of the cortex lead to loss of the ability to pronounce words.
Functional asymmetry of the hemispheres
The forebrain is formed by two hemispheres, which consist of identical lobes. However, they play different functional roles. The differences between the hemispheres were first described in 1863 by the neurologist Paul Broca, who discovered that with tumors of the left frontal lobe, the ability to pronounce speech is lost. In the 50s of the 20th century, R. Sperry and M. Gazzaniga studied patients in whom the corpus callosum was transected in order to stop epileptic seizures. It contains commissural fibers connecting the hemispheres. Mental abilities do not change in people with split brains. But with the help of special tests it was discovered that the functions of the hemispheres are different. For example, if an object is in the field of view of the right eye, i.e. visual information enters the left hemisphere, then such a patient can name it, describe its properties, read or write a text.
If an object falls into the field of vision of the left eye, then the patient cannot even name it and talk about it. He can't read with this eye. Thus, the left hemisphere is dominant in relation to consciousness, speech, counting, writing, abstract thinking, and complex voluntary movements. On the other hand, although the right hemisphere does not have pronounced speech functions, it is to a certain extent capable of understanding speech and thinking abstractly. But to a much greater extent than the left, it has the mechanisms of sensory recognition of objects and figurative memory. The perception of music is entirely a function of the right hemisphere. Those. the right hemisphere is responsible for non-speech functions, such as the analysis of complex visual and auditory images, the perception of space and shape. Each hemisphere separately receives, processes and stores information. They have their own feelings, thoughts, and emotional assessments of events. The left hemisphere processes information analytically, i.e. sequentially, and the right one simultaneously, intuitively. Thus, the hemispheres use different ways knowledge. The entire education system in the world is aimed at developing the left hemisphere, i.e. abstract thinking rather than intuitive. Despite functional asymmetry, normally the hemispheres work together, providing all the processes of the human psyche.
Cortical plasticity
Some tissues retain the ability to form new cells from progenitor cells throughout life. These are liver cells, skin cells, enterocytes. Nerve cells do not have this ability. However, they retain the ability to form new processes and synapses. Those. Each neuron is capable of forming new ones when the process is damaged. Restoration of processes can occur in two ways: through the formation of a new growth cone and the formation of collaterals. Typically, new axon growth is prevented by the formation of a glial scar. But despite this, new synaptic contacts are formed by collaterals of the damaged axon. The plasticity of cortical neurons is highest. Any of its neurons is programmed to actively try to restore lost connections when damaged. Each neuron is involved in competition with others for the formation of synaptic contacts. This serves as the basis for the plasticity of neural cortical networks. It has been established that when the cerebellum is removed, the nerve pathways leading to it begin to grow into the cortex. If a section of the brain of another animal is transplanted into an intact brain, the neurons of this piece of tissue form numerous contacts with the neurons of the recipient’s brain.
Plasticity of the cortex manifests itself both under normal conditions, for example, during the formation of new intercortical connections during the learning process, and under pathology. In particular, the functions lost due to damage to a part of the cortex are taken over by its neighboring fields or another hemisphere. Even when large areas of the cortex are damaged due to hemorrhage, their functions begin to be performed by the corresponding areas of the opposite hemisphere.
The cerebral cortex is the highest and most complexly organized nerve center of the screen type, the activity of which ensures the regulation of various body functions and complex forms of behavior.
The cortex is formed by a layer of gray matter 3-5 mm thick. Gray matter contains nerve cells (more than 10 billion neurons), nerve fibers and neuroglial cells. Its various sections, which differ from each other in certain features of the location and structure of cells, the location of fibers and functional significance, are called fields, which were first described by the German physician and scientist K. Brodmann.
Cytoarchitecture
Among the multipolar neurons of the cortex, pyramidal, stellate, spindle-shaped, arachnid, horizontal, “candelabra” cells, cells with a double bouquet of dendrites and some other types of neurons are distinguished.
Pyramidal neurons constitute the basic and most specific form for the cerebral cortex. They have an elongated cone-shaped body, the apex of which faces the surface of the cortex. Dendrites extend from the apex and lateral surfaces of the body. Axons originate from the base of the pyramidal cells.
Pyramidal cells of different layers of the cortex differ in size and have different functional significance. Small cells are interneurons. Axons of large pyramids take part in the formation motor pyramid paths.
The neurons of the cortex are located in vaguely delimited layers, which are designated by Roman numerals and numbered from the outside to the inside. Each layer is characterized by the predominance of one type of cell. There are six main layers in the cerebral cortex:
- I - molecular;
- II - external granular;
- III - pyramidal;
- IV - internal granular;
- V - ganglionic;
- VI - layer of polymorphic cells.
I - Molecular the cortex layer contains a small number of small associative horizontal cells of Cajal. Their axons run parallel to the surface of the brain as part of the tangential plexus of nerve fibers of the molecular layer. However, the bulk of the fibers of this plexus are represented by the branching of dendrites of the underlying layers.
II - External granular the layer is formed by numerous small pyramidal and stellate neurons. The dendrites of these cells rise into the molecular layer, and the axons either go into white matter, or, forming arcs, also enter the tangential plexus of fibers of the molecular layer.
III - The widest layer of the cerebral cortex - pyramidal. It contains pyramidal neurons, Martinotti cells and spindle cells with a double bouquet of dendrites. The apical dendrites of the pyramids extend into the molecular layer, and the lateral dendrites form synapses with adjacent cells of this layer. The axon of a pyramidal cell always extends from its base. In small cells it remains within the cortex, in large cells it forms a myelin fiber that goes into the white matter of the brain. The axons of small polygonal Martinotti cells are directed into the molecular layer. The pyramidal layer performs primarily associative functions.
IV - Internal grainy the layer in some fields of the cortex is very developed (for example, in the visual and auditory areas of the cortex), while in others it may be almost absent (for example, in the precentral gyrus). This layer is formed by small stellate neurons. It contains a large number of horizontal fibers.
V- Ganglionic layer of the cortex is formed by large pyramids, and the area of the motor cortex (precentral gyrus) contains giant pyramids, which were first described by the Kiev anatomist V. A. Bets. The apical dendrites of the pyramids reach the first layer. The axons of the pyramids project to the motor nuclei of the brain and spinal cord. The longest axons of Betz cells in the pyramidal tracts reach the caudal segments of the spinal cord.
In addition to pyramidal neurons, the ganglion layer of the cortex contains vertical spindle-shaped cells, the axons of which rise into the 1st layer cortex, as well as basket cells.
VI - Layer polymorphic cells formed by neurons of various shapes (fusiform, stellate, Martinotti cells). The axons of these cells extend into the white matter as part of the efferent pathways, and the dendrites reach the molecular layer.
Myeloarchitecture
Among the nerve fibers of the cerebral cortex we can distinguish associative fibers connecting individual areas of the cortex of one hemisphere, commissural, connecting the cortex different hemispheres, And projection fibers, both afferent and efferent, that connect the cortex with the nuclei of the lower parts of the central nervous system. Projection fibers in the cerebral cortex form radial rays ending in the third pyramidal layer. In addition to the already described tangential plexus of the I - molecular layer, at the level of IV - internal granular and V - ganglion layers there are two tangential layers of myelin nerve fibers - respectively, the outer strip of Baillarger and the internal strip of Baillarger. The last two systems are plexuses formed by the terminal sections of afferent fibers.
Modular principle of cortex organization
In the cerebral cortex, repeating blocks, or modules, of neurons are described, which are considered as its morphofunctional units capable of relatively autonomous activity. They have the shape of cylinders, or columns, running vertically through the entire thickness of the bark.
Each module includes afferent pathways, a system of local connections and efferent pathways.
TO afferent pathways include cortico-cortical and thalamo-cortical fibers.
The module is organized around cortico-cortical fibers, which are axons of pyramidal cells of either the same hemisphere or the opposite one. Cortico-cortical fibers form endings in all layers of the cortex of a given module.
The module also includes thalamo-cortical fibers ending in layer IV of the cortex on spiny stellate neurons and basal dendrites of pyramidal neurons.
Efferent pathways are formed by axons of large and giant pyramidal neurons, as well as axons of fusiform and some other cells of layer VI of the cortex.
Local connections system is formed by interneurons of the module, which include more than a dozen cell types. Most of them are inhibitory and regulate the activity of predominantly pyramidal neurons.
From inhibitory neurons of the module highest value have:
- axo-axonal cells;
- "candelabra" cells;
- basket cells;
- cells with a double bouquet of dendrites;
- cells with an axonal brush.
The system of inhibitory neurons plays the role of a filter, inhibiting part of the pyramidal neurons of the cortex.
Meninges of the brain and spinal cord
The brain and spinal cord are covered with three connective tissue membranes: soft, directly adjacent to the brain tissue, arachnoid and hard, which borders the bone tissue of the skull and spine. The meninges provide protection, incl. shock-absorbing function, ensure the production and absorption of cerebrospinal fluid.
Pia mater directly adjacent to the brain tissue and delimited from it by the marginal glial membrane. The loose fibrous connective tissue of the membrane contains a large number of blood vessels that supply the brain, numerous nerve fibers, terminal apparatus and single nerve cells. The pia mater surrounds the vessels penetrating the brain, forming a perivascular pial membrane around them. In the ventricles of the brain, the pia mater, together with the ependyma, takes part in the formation of choroid plexuses that produce cerebrospinal fluid.
Arachnoid represented by a thin layer of loose fibrous connective tissue. Between it and the pia mater lies a network of crossbars consisting of thin bundles of collagen and thin elastic fibers. This network connects the shells with each other. Between the pia mater, which follows the relief of brain tissue, and the arachnoid, which runs along elevated areas without going into the recesses, there is a subarachnoid (subarachnoid) space, permeated with thin collagen and elastic fibers that connect the membranes to each other. The subarachnoid space communicates with the ventricles of the brain and contains cerebrospinal fluid. Large blood vessels pass through this space, the branches of which supply the brain.
The villi of the arachnoid membrane (the largest are called Pachyon granulations) serve as sites through which substances from the cerebrospinal fluid return to the blood. They are avascular outgrowths of the arachnoid membrane, containing a network of slit-like spaces, and protruding into the lumen of the sinuses of the dura mater.
Dura mater formed by dense fibrous connective tissue containing many elastic fibers. In the cranial cavity it is tightly fused with the periosteum. In the spinal canal, the dura mater is delimited from the vertebral periosteum by the epidural space, filled with a layer of loose fibrous connective tissue, which provides it with some mobility. Between the dura mater and the arachnoid membrane is the subdural space. The subdural space contains a small amount of fluid.
The membranes on the side of the subdural and subarachnoid space are covered with a layer of flat cells of glial nature.
Age-related changes
Changes in the central nervous system in old age are associated primarily with sclerotic changes in the blood vessels of the brain. In old age, the pia mater and arachnoid membrane of the brain thicken. Lime deposits may appear in them. There is atrophy of the cerebral cortex, primarily the frontal and parietal lobes. The number of neurons per unit volume of the cortex decreases, this depends mainly on cell death. Neurons decrease in size, partially lose their basophilic substance, the nuclei become denser, and their outline becomes uneven. The pyramids of layer V of the motor cortex and the pyriform cells of the cerebellar cortex change faster than others. Lipofuscin granules accumulate in neurons of various parts of the nervous system.
Depending on the nature of the functions performed, the new cortex is divided into motor, sensory and associative areas.
Motor cortex in primates and humans they are located in the precentral region (anterior central gyrus and posterior parts of the superior and middle frontal gyri), cytoarchitectonic areas 4 and 6 (see rice. 8.2). In addition to them, an additional motor area is located on the medial surface of the cortex. Electrical stimulation of various parts of the motor cortex leads to movements of individual muscles of the opposite half of the body. Various groups muscles are represented by groups of neurons of the motor cortex, located in a strictly defined sequence. The representation of the muscles of the lower extremities is located in the medial part of the precentral gyrus, near the longitudinal fissure; representation of the muscles of the head and neck - in the dorsolateral areas. That is, the zones of the motor cortex are organized strictly according to the somatotopic principle. In mammals with a lower level of organization of the motor cortex (dogs, cats), the spatial representation of muscles is less differentiated. Unlike humans and primates, in carnivores, stimulation of the motor cortex causes movements that involve large groups rather than individual muscles. The most discrete movements occur when field 4 is stimulated; here are the lowest thresholds for causing movement. Neuronal groups of the motor cortex associated with the movement of different muscles occupy different areas and
unevenly distributed. Disproportionally large areas are associated with the movement of fingers, hands, tongue, facial muscles, and much smaller areas are associated with the large muscles of the back and lower extremities. The map of muscle representation in the motor cortex looks like a “homunculus” - a little man with a huge head, tongue, hand and a very small torso and legs. The uneven distribution is due to the fact that the axons of pyramidal neurons of the motor cortex make the largest number of synaptic contacts on those motor neurons of the spinal cord that innervate the muscles of the fingers, tongue, and face. This organization provides the most subtle and precise control of the movement of these particular muscles. Removal of areas of the motor cortex causes disruption of the corresponding movement. Damage to the motor cortex in humans leads to paralysis, especially of the muscles of the hands, muscles associated with speech, and facial muscles. With unilateral lesions of the motor cortex, a gradual restoration of motor functions occurs; with bilateral lesions, recovery does not occur. This indicates the importance of bilateral connections between the motor cortex of both hemispheres to compensate for motor impairments.
Neurons in the motor cortex are grouped into vertical columns that control small groups of muscle fibers. There are separate columns of the motor cortex associated with fast (phasic) and slow (tonic) movements. The efferent outputs of the columns are giant pyramidal neurons located in layer V of the motor cortex. The axons of the giant pyramids form a pyramidal tract ending with excitatory synapses on motor neurons different levels spinal cord of the opposite side. Axons
The pyramidal spinothalamic tract has the highest rate of excitation transmission and serves to evoke voluntary movements. Along with the pyramidal system, the extrapyramidal system begins in the motor cortex. This system is distinguished by the fact that the axons of its cortical cells go to the basal ganglia, cerebellum, red nucleus, reticular nuclei and other structures of the brain stem, which through rubrospinal, reticulospinal, olivospinal and other descending pathways affect spinal neurons.
The motor cortex receives sensory afferentation from the somatosensory, visual and auditory zones of both hemispheres and associative areas of the cortex. The most spatially clearly organized afferent inputs to the motor cortex are from the ventrolateral nucleus of the thalamus. The afferentation from muscle and tendon receptors entering the motor cortex is of great importance. The topically organized input to the motor cortex from muscle afferents is so specific that it allows reflexes to be closed through the neurons of the motor cortex to the motor neurons of the spinal cord. This structuring underlies a high degree of self-organization and improvement of movement by correcting movement control based on feedback signals.
Sensory areas bark carry out the highest level of sensory analysis. They receive afferentation from specific relay nuclei of the thalamus and, spatially distributing it on the screen projection, have a topical principle of organization. That's why they are called projection zones. Along with complex analysis, integration and critical assessment of information that comes here through specific afferents takes place in sensory areas.
ny inputs. Sensory afferentation entering the cortex has multiple representations: each of the sensory zones includes a zone of primary projection, secondary and tertiary. The main sensory areas are the visual, auditory and somatic sensory systems of the cortex.
Visual sensory system The cortex is represented by the first visual area (field 17) and the second visual area (fields 18 and 19) (see. rice. 8.2). In the first visual area there is a clear spatial distribution of the retina - retinotopic organization. So, above the calcarine sulcus is the upper half of the retina (lower half of the visual field), in the lower part of field 17 is the lower part of the retina, and in the posterior part is the area of the macula. With local destruction of sections 17 of the field, the corresponding sections of the field of view fall out. In the first visual area, layer IV is divided into two sublayers. This division of the IV layer of the cortex is associated both with the arrival here of afferentation from the retinas of both eyes, and with the presence of two types of photoreceptors in the retina. The vertical columns 17 of the field combine neurons with the same receptive fields. As a rule, the feature that groups neurons into columns is the feature that dominates visual awareness in a given animal species. For example, in squirrels - by selectivity to the direction of movement, in cats - by the orientation of receptive fields. The ability of neurons in the visual cortex to detect lines and contours of a certain orientation is determined by the spatial organization of their receptive fields. The receptive fields of neurons in the visual cortex do not have a concentric shape, but a parallel arrangement of antagonistic zones in the form of elongated stripes, oriented in a certain way
field of view. In addition to such simple receptive fields, there are complex receptive fields without antagonistic zones, covering most of the visual field. Neurons with highly complex receptive fields are selective for several stimulus characteristics (orientation, size, direction of movement, etc.). In the visual cortex, there is not a continuous description, as in the underlying subcortical levels of analysis of visual information, but the detection of individual features of the visual image, which makes it possible to reduce the redundancy of transmitted information. Recognition of different features of a visual image in the visual cortex is carried out in parallel, which makes it possible to assign a visual image to a certain class and identify it. Secondary visual areas (fields 18 and 19) carry out the association of visual information with tactile, proprioceptive and auditory information. Such synthesis provides a more complete assessment of visual information and gives an idea of its significance. If damage to field 17 leads to loss of vision, then damage to fields 19 and 18 leads to impaired assessment of what is seen; the ability to understand the meaning of a written text is lost. If electrical stimulation of the 17th field causes light sensations in a person, then stimulation of the 19th field causes visual hallucinations. When field 37 is damaged, visual object agnosia occurs, and when field 39 is damaged, disturbances in the perception of spatial relationships occur.
Auditory sensory system The cortex consists of primary auditory areas (fields 41 and 42), located mainly in the lateral sulcus, and secondary auditory areas (fields 52, 22, 21). Axons of relay cells of the medial geniculate body, carrying auditory and vestibular signals, arrive in the primary auditory cortex.
information. Here there is a clear spatial distribution of the representation of hair cells of different parts of the cochlea, perceiving sounds of different pitches - tonotopic organization. The inner part of Heschl's gyrus is associated with the analysis of high tones, and the outer part is associated with the perception of low tones. All neurons in one vertical column respond to sound of the same frequency. At the cortical level of auditory analysis, the tone, volume and nature of short (less than 100 ms) sound signals are distinguished; perception of longer sounds is also possible at the subcortical level of sensory analysis. Neurons in the auditory cortex have a clear dependence of the response threshold on sound frequency: different neurons have their own optimal frequency, corresponding to the minimum threshold. There are groups of neurons that respond to changes in the tone and volume of sounds. Electrical stimulation of the primary auditory cortex causes a sensation of noise and ringing in the ears, and a unilateral lesion causes hearing impairment, but does not lead to complete deafness due to the preservation of the auditory cortex in the opposite hemisphere. Secondary auditory zones have associative connections with other sensory systems; their activity is associated with the assessment of the specific and individual significance of sound signals. Certain areas of the secondary auditory fields in the human left hemisphere are associated with understanding sounding words and the ability to pronounce them. Thus, when field 22 is damaged, Wernicke's sensory aphasia occurs.
Somatic sensory system The cortex analyzes signals from cutaneous, muscular and visceral sensitivity and consists of primary and secondary somatosensory areas. The first somatosensory zone occupies the posterior central gyrus (fields 1, 2 and
3) and is highest level analysis of information from skin and muscle receptors. The axons of the relay cells of the posterior ventral nucleus of the thalamus come here. Different parts of the body have a clear spatial representation here - the somatotopic principle of organization. The projection of the lower extremities is in the upper part of the posterior central gyrus, and the projection of the head and upper body is in the lower part of the posterior central gyrus. Different areas of the body occupy unevenly different areas of the somatosensory cortex: the hands, face, and vocal apparatus have a particularly large representation, the trunk and lower limbs have a minimal representation. This uneven representation is due to the fact that the number of tactile receptors only on the tips of the fingers, lips, and tongue is approximately the same as on the skin of the entire body. Most neurons in the first somatosensory area have local receptive fields; As the stimulus moves from the center of the receptive field to the periphery, the probability of a response decreases. Vertical columns combine neurons that respond to any one of the modalities of musculocutaneous sensitivity. When electrically stimulating local areas of the somatosensory cortex, a person experiences sensations of pressure, touch or heat in the corresponding area on the opposite side of the body. When this area is affected, a fine gradation of tactile sensations is lost and awkwardness of movement appears in the corresponding part of the body. The second somatosensory zone is located in the lateral sulcus, ventral to the first sensory zone, and occupies a significantly smaller area. The axons of the relay cells of the posterior central nucleus of the thalamus enter here, largely carrying information from the viscerore-
receptors. In the area of the postcentral gyrus, where tactile and temperature reception of the oral cavity is represented, taste sensory signals are analyzed, along with the hippocampus.
Association cortical areas unlike projection zones, they do not have specialized inputs or specific manifestations upon stimulation or damage, and in this regard they are “silent” zones. However, in the course of phylogenetic development, they acquire an increasingly important role in complex forms of behavior and occupy a significant part of the neocortex in primates. The main associative areas are the parietal (fields 5, 7, 39, 40) and frontal association areas (fields 8, 9, 10, 11, 12). Parietal association area ensures the reconstruction of complete images of objects or phenomena. Here the integration of afferent streams of different sensory systems takes place, which is necessary for the implementation of adaptive behavior. On the neural groups of the parietal region, convergence of afferent flows of different sensory systems occurs, which creates optimal opportunities for afferent synthesis, which underlies the perception of a holistic image of an object and its spatio-temporal relationships with other objects. Most neurons in the parietal cortex respond to stimuli from two or three sensory modalities. There are nerve cells that are excited only by a complex (association) of multisensory stimuli. The parietal cortex does not have such a clear topical organization as in the projection areas, but in areas of the parietal cortex adjacent to the occipital areas there are more neurons responding to light stimuli, to the temporal areas - to auditory stimuli, to postcentral areas - to tactile stimuli. The parietal cortex receives
afferentation from the projection areas of the cortex along associative fibers and from the associative nuclei of the thalamus (cushion and ventrobasal complex). Axons of thalamic neurons form axodendritic contacts not only in layer IV, but more evenly in all layers of the cortex. Large number Efferent outputs from the parietal cortex go to the motor cortex, where the formation of a voluntary action command on the basis of afferent synthesis occurs.
The integrative function of the parietal cortex in humans is especially pronounced. If it is damaged, the ability to comprehensively perceive objects in the totality of their qualitative characteristics, differentiate objects, and spatial discrimination is impaired. First of all, the ability to synthesize individual components into a complex system of purposeful behavior is lost. Stimulation of areas of the parietal region leads to changes in memory processes: it is possible to evoke in a person memories of events in which he previously participated. The memories were accompanied by the same emotional experiences as during previous events. Probably, the processes occurring in the parietal region are related to memory mechanisms.
Frontal association areas The cortex is fully formed only in primates and humans. They are also characterized by the absence of specialized afferent inputs, the multisensory nature of neural reactions, the abundance and complexity of connections with cortical areas and deep structures of the brain. In humans, the anterior areas of the frontal region are involved in the implementation of the most complex processes associated with the preservation of personality, the formation social relations. It is assumed that these areas of the frontal cortex are associated with the mechanisms of organizing goal-oriented
leveled activities, formation of a program of action and decision-making. The frontal areas of the cortex in humans are directly involved in the activity of the second signaling system - the speech signaling system. Irritation or damage to the lower parts of the frontal cortex of the left hemisphere leads to various disorders of speech function. When Broca's center is damaged, motor aphasia occurs when speech movements are impaired. The patient is able to publish individual sounds, but not can't utter a single word. In other localizations of the lesion, the ability to write is impaired - agraphia, the ability to read aloud or the ability to perceive speech (Wernicke's sensory aphasia). Defeat The frontal and parietal areas of the association cortex especially affect the most complex forms of brain activity, but damage to many other brain structures can lead to similar lesions. Based on many experimental facts and clinical observations, we can assume that mental activity is not localized in any individual structures and is a function of the entire brain.
9. Principles of regulation motor functions