Transmission and reception of information in the brain. What does the human brain consist of? Complex neural networks and higher brain functions
At the same time, despite a fraction of a second of delay, the brain-computer-Internet-computer-brain interface implemented by scientists allowed one person to control the movements of another person. Given that this work is being conducted under the auspices of the US Army Research Office, it is not at all surprising that the latest demonstration used a shooting game and simulated actions with explosive devices. The US military sees this technology as an opportunity to use direct information transmission to bypass language barriers and differences in experience between two people who need to jointly perform some, possibly dangerous, work.
The first demonstration of the functionality of this system was carried out last year. And the current demonstration not only confirmed the functionality of the idea itself, but also showed some of its expanded capabilities. As before, one of the participants, the one who remotely controls the actions of another person, wears EEG sensors, with the help of which the computer reads patterns of brain activity in certain areas of the brain. This data is digitized and transmitted over the Internet to another computer, which performs the entire sequence in reverse. The second person, the performer, is exposed to a magnetic field induced by a coil aimed at the area of the brain that controls hand movements. A human operator can send a command to another person and for this he does not even need to move, he just needs to imagine that he is moving his hand. The human performer receives commands from the outside using transcranial magnetic excitation technology and his hands move independently of his consciousness.
In their experiments, the researchers tested the performance of the system on three pairs of participants. The operator and performer were always located in two buildings, the distance between which was 1.5 kilometers and between which only one digital communication line was laid. “The first operator was involved in a computer game in which he had to defend a city from attack by using various types of weapons and shooting down missiles launched by the enemy. At the same time, he was completely deprived of the possibility of physical influence on the gameplay. The only way the operator could play the game was by mentally controlling the movements of his hands and fingers, write the Washington researchers. - The accuracy of the game varied greatly from pair to pair and ranged from 25 to 83 percent. And the highest level of errors was due to the error in executing the “fire” command.”
The researchers have now received a million-dollar grant from the W. M. Keck Foundation, which will allow them to continue and expand their research. As part of the new stage, researchers are going to learn how to decipher and transmit more complex brain processes, expand the number of types of transmitted information, which will allow the transfer of concepts, thoughts and rules. Thanks to this, at least this is what scientists are counting on, it will become possible in the near future to implement such fantastic technologies, with the help of which, for example, brilliant scientists will be able to transfer their knowledge directly to students, or virtuoso musicians or surgeons will be able to perform operations remotely, using the hands of others of people.
Principles of information transmission and structural organization of the brain
Plan
Introduction
Principles of information transmission and structural organization of the brain
Interconnections in simple nervous systems
Complex neural networks and higher brain functions
Structure of the retina
Neuron patterns and connections
Cell body, dendrites, axons
Methods for identifying neurons and tracing their connections. Non-nervous elements of the brain
Grouping cells according to function
Cell subtypes and function
Convergence and divergence of connections
Literature
Introduction
The terms “neurobiology” and “neurosciences” came into use in the 60s of the 20th century, when Stephen Kuffler created the first department at Harvard Medical School, whose staff included physiologists, anatomists and biochemists. Working together, they solved problems of the functioning and development of the nervous system and explored the molecular mechanisms of brain function.
The central nervous system is a continuously working conglomerate of cells that constantly receive information, analyze it, process it and make decisions. The brain is also able to take the initiative and produce coordinated, efficient muscle contractions for walking, swallowing, or singing. To regulate many aspects of behavior and to directly or indirectly control the entire body, the nervous system has a huge number of lines of communication provided by nerve cells (neurons). Neurons are the basic unit, or building block, of the brain
Interconnections in simple nervous systems
Events that occur during the implementation of simple reflexes can be traced and analyzed in detail. For example, when the knee ligament is struck with a small hammer, the muscles and tendons of the thigh are stretched and electrical impulses travel along sensory nerve fibers to the spinal cord, where motor cells are excited, producing impulses and activating muscle contractions. The end result is straightening of the leg at the knee joint. Such simplified circuits are very important for regulating muscle contractions that control limb movements. In such a simple reflex, in which a stimulus leads to a specific output, the role of signals and interactions of just two types of cells can be successfully analyzed.
Complex neural networks and higher brain functions
Analyzing the interaction of neurons in complex pathways involving literally millions of neurons is significantly more difficult than analyzing simple reflexes. Re-
Providing information to the brain for the perception of sound, touch, smell or sight requires the sequential engagement of neuron by neuron, just as when performing a simple voluntary movement. A major challenge in analyzing neuronal interactions and network structure arises from the dense packing of nerve cells, the complexity of their interconnections, and the abundance of cell types. The brain is structured differently from the liver, which is made up of similar populations of cells. If you have discovered how one area of the liver works, then you know a lot about the liver as a whole. Knowing about the cerebellum, however, does not tell you anything about the functioning of the retina or any other part of the central nervous system.
Despite the enormous complexity of the nervous system, it is now possible to analyze the many ways in which neurons interact during perception. For example, by recording the activity of neurons along the path from the eye to the brain, it is possible to trace signals first in cells that specifically respond to light, and then, step by step, through successive switches, to higher centers of the brain.
An interesting feature of the visual system is its ability to distinguish contrasting images, colors and movements over a huge range of color intensities. As you read this page, signals within the eye make it possible for black letters to stand out on a white page in a dimly lit room or in bright sunlight. Specific connections in the brain form a single picture, even though the two eyes are located separately and scan different areas of the outside world. Moreover, there are mechanisms that ensure the constancy of the image (even though our eyes are constantly moving) and provide accurate information about the distance to the page.
How do nerve cell connections provide such phenomena? Although we are not yet able to provide a complete explanation, much is now known about how these properties of vision are mediated by simple neural networks in the eye and early switching stages in the brain. Of course, many questions remain about what the connections are between neuronal properties and behavior. So, in order to read a page, you must maintain a certain position of your body, head and hands. Further, the brain must ensure constant hydration of the eyeball, constant breathing and many other involuntary and uncontrolled functions.
The functioning of the retina is a good example of the basic principles of the nervous system.
Rice. 1.1. Pathways from the eye to the brain via the optic nerve and optic tract.
Structure of the retina
Analysis of the visual world depends on information coming from the retina, where the first stage of processing occurs, setting the limits for our perception. In Fig. Figure 1.1 shows the paths from the eye to the higher centers of the brain. The image that enters the retina is inverted, but in all other respects it represents a bona fide representation of the external world. How can this picture be transmitted to our brain through electrical signals that originate in the retina and then travel along the optic nerves?
Neuron patterns and connections
In Fig. Figure 1.2 shows the different types of cells and their location in the retina. Light entering the eye passes through layers of transparent cells and reaches the photoreceptors. Signals transmitted from the eye along the fibers of the optic nerve are the only information signals on which our vision is based.
The scheme for the passage of information through the retina (Fig. 1.2A) was proposed by Santiago Ramon y Cahal1) at the end of the 19th century. He was one of the greatest researchers of the nervous system and conducted experiments on a wide variety of animals. He made a significant generalization that the shape and arrangement of neurons, as well as the region of origin and final target of neuronal signals in a network, provide critical information about the functioning of the nervous system.
In Fig. Figure 1.2 clearly shows that the cells in the retina, as in other parts of the central nervous system (CNS), are very densely packed. At first, morphologists had to tear apart nervous tissue to see individual nerve cells. Techniques that stain entire neurons are virtually useless for examining cell shape and connectivity because structures such as the retina appear as a dark patch of intertwined cells and processes. Electron micrograph in Fig. Figure 1.3 shows that the extracellular space around neurons and supporting cells is only 25 nanometers wide. Most of the drawings of Ramón y Cajal were made using the Golgi staining method, which stains, by an unknown mechanism, only a few random neurons from the entire population, but these few neurons are completely stained.
Rice. 1.2. Structure and connections of cells in the mammalian retina. (A) Scheme of the signal direction from the receptor to the optic nerve according to Ramon y Cajal. (B) Ramon y Cajal distribution of retinal cellular elements. (C) Drawings of rods and cones of the human retina.
Rice. 1.3. Dense packing of neurons in the monkey retina. One rod (R) and one cone (C) are labeled.
Scheme in Fig. Figure 1.2 shows the principle of the orderly arrangement of neurons in the retina. It is easy to distinguish between photoreceptors, bipolar cells and ganglion cells. The direction of transmission is from input to output, from photoreceptors to ganglion cells. In addition, two other types of cells, horizontal and amacrine, form connections that connect different pathways. One of the goals of the neurobiology present in Ramon y Cajal's drawings is the desire to understand how each cell participates in creating the picture of the world that we observe.
Cell body, dendrites, axons
The ganglion cell shown in Fig. 1.4 illustrates the structural features of nerve cells inherent in all neurons of the central and peripheral nervous system. The cell body contains the nucleus and other intracellular organelles common to all cells. The long extension that leaves the cell body and forms a connection with the target cell is called an axon. The terms dendrite, cell body, and axon are applied to processes at which incoming fibers form contacts that act as receiving stations for excitation or inhibition. In addition to the ganglion cell, in Fig. Figure 1.4 shows other types of neurons. The terms used to describe the structure of a neuron, particularly dendrites, are somewhat controversial, but nevertheless they are convenient and widely used.
Not all neurons conform to the simple cell structure shown in Fig. 1.4. Some neurons do not have axons; others have axons on which connections are formed. There are cells whose dendrites can conduct impulses and form connections with target cells. While a ganglion cell conforms to the blueprint of a standard neuron with dendrites, cell body, and axon, other cells do not conform to this standard. For example, photoreceptors (Fig. 1.2C) have no obvious dendrites. The activity of photoreceptors is not caused by other neurons, but is activated by external stimuli, lighting. Another exception in the retina is the absence of photoreceptor axons.
Methods for identifying neurons and tracing their connections
Although the Golgi technique is still widely used, many new approaches have facilitated the functional identification of neurons and synaptic connections. Molecules that stain the entire neuron can be injected through a micropipette, which simultaneously records the electrical signal. Fluorescent markers such as Lucifer yellow reveal the finest processes in a living cell. Intracellular markers such as the enzyme horseradish peroxidase (HRP) or biocytin can be introduced; once fixed, they form a dense product or glow brightly under fluorescent light. Neurons can be stained with horseradish peroxidase and with extracellular application; the enzyme is captured and transported into the cell body. Fluorescent carbocyanic dyes, upon contact with the neuron membrane, dissolve and diffuse over the entire surface of the cell.
Rice. 1.4. Shapes and sizes of neurons.
Rice. 1.5. A group of bipolar cells stained with an antibody for the enzyme phosphokinase C. Only cells containing the enzyme stained.
These techniques are very important for tracing the passage of axons from one part of the nervous system to another.
Antibodies are used to characterize specific neurons, dendrites, and synapses by selectively labeling intracellular or membrane components. Antibodies are successfully used to trace the migration and differentiation of nerve cells during ontogenesis. An additional approach to characterize neurons is hybridization in situ: specifically labeled probes label neuronal mRNA that encodes the synthesis of a channel, receptor, transmitter, or structural element.
Non-nervous elements of the brain
Glial cells. Unlike neurons, they do not have axons or dendrites and are not directly connected to nerve cells. There are a lot of glial cells in the nervous system. They perform many different functions related to signal transmission. For example, the axons of the retinal ganglion cells that make up the optic nerve conduct impulses very quickly because they are surrounded by an insulating lipid sheath called myelin. Myelin is formed by glial cells that wrap around axons during ontogenetic development. Glial cells in the retina are known as Müller cells.
Grouping cells according to function
A remarkable property of the retina is the arrangement of cells according to function. The cell bodies of photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells are arranged in distinct layers. Similar layering is observed throughout the brain. For example, the structure where the fibers of the optic nerve terminate (the lateral geniculate body) consists of 6 layers of cells that are easy to distinguish even with the naked eye. In many areas of the nervous system, cells with similar functions are grouped into distinct spherical structures known as nuclei (not to be confused with the cell nucleus) or ganglia (not to be confused with retinal ganglion cells).
Cell subtypes and function
There are several distinct types of ganglion, horizontal, bipolar, and amacrine cells, each with characteristic morphology, transmitter specificity, and physiological properties. For example, photoreceptors are divided into two easily distinguishable classes - rods and cones - which perform different functions. Elongated rods are extremely sensitive to the slightest changes in lighting. As you read this page, the ambient light is too bright for the wands, which only function in low light after a long period in the dark. Cones respond to visual stimuli in bright light. Moreover, cones are further classified into photoreceptor subtypes that are sensitive to red, green, or blue light. Amacrine cells are a striking example of cellular diversity: more than 20 types can be distinguished according to structural and physiological criteria.
The retina thus illustrates the deepest problems of modern neurobiology. It is not known why so many types of amacrine cells are needed and what different functions each of these cell types has. It is sobering to realize that the function of the vast majority of nerve cells in the central, peripheral and visceral nervous systems is unknown. At the same time, this ignorance suggests that many of the basic principles of the robotic brain are not yet understood.
Convergence and divergence of connections
For example, there is a strong decrease in the number of cells involved along the path from receptors to ganglion cells. The outputs of more than 100 million receptors converge on 1 million ganglion cells, the axons of which make up the optic nerve. Thus, many (but not all) ganglion cells receive input from a large number of photoreceptors (convergence) through intercalary cells. In turn, one ganglion cell intensively branches and ends on many target cells.
In addition, unlike the simplified diagram, the arrows should point outward to indicate interactions between cells in the same layer (lateral connections) and even in opposite directions - for example, back from horizontal cells to photoreceptors (reciprocal connections). Such convergent, divergent, lateral, and recurrent influences are constant properties of most neural pathways throughout the nervous system. Thus, simple step-by-step signal processing is complicated by parallel and reverse interactions.
Cellular and molecular biology of neurons
Like other types of cells in the body, neurons fully possess the cellular mechanisms of metabolic activity and the synthesis of membrane proteins (for example, ion channel proteins and receptors). Moreover, proteins of ion channels and receptors are directedly transported to localization sites in the cell membrane. Sodium- or potassium-specific channels are located on the membrane of ganglion cell axons in discrete groups (clusters). These channels are involved in the initiation and conduct of PD.
Presynaptic terminals, formed by processes of photoreceptors, bipolar cells and other neurons, contain specific channels in their membrane through which calcium ions can pass. The entry of calcium triggers the release of the transmitter. Each type of neuron synthesizes, stores and releases a specific type of transmitter(s). Unlike many other membrane proteins, receptors for specific neurotransmitters are located in precisely defined locations - postsynaptic membranes. Among membrane proteins, pump proteins or transport proteins are also known, the role of which is to maintain the constancy of the internal contents of the cell.
The main difference between nerve cells and other types of cells in the body is the presence of a long axon. Since axons do not have the biochemical "kitchen" for protein synthesis, all essential molecules must be transported to the terminals by a process called axonal transport, often over very long distances. All molecules needed to maintain structure and function, as well as membrane channel molecules, travel away from the cell body via this pathway. In the same way, molecules captured by the terminal membrane make their way back to the cell body using axonal transport.
Neurons also differ from most cells in that, with a few exceptions, they cannot divide. This means that in adult animals, dead neurons cannot be replaced.
Regulation of nervous system development
The high degree of organization of a structure such as the retina poses new problems. If a human brain is needed to build a computer, then no one controls the brain as it develops and makes connections. It is still a mystery how the correct “assembly” of parts of the brain leads to the appearance of its unique properties.
In the mature retina, each cell type is located in a corresponding layer or sublayer and forms strictly defined connections with the corresponding target cells. Such a device is a necessary condition for proper functioning. For example, for normal ganglion cells to develop, the precursor cell must divide, migrate to a specific location, differentiate into a specific shape, and form specific synaptic connections.
The axons of this cell must find, through a considerable distance (optic nerve), a certain layer of target cells in the next link of synaptic switching. Similar processes occur in all parts of the nervous system, resulting in the formation of complex structures with specific functions.
The study of the mechanisms of formation of such complex structures as the retina is one of the key problems of modern neurobiology. Understanding how the complex interconnections of neurons are formed during individual development (ontogenesis) can help describe the properties and origins of functional brain disorders. Some molecules may play key roles in neuronal differentiation, growth, migration, synapse formation, and survival. Such molecules are now being described more and more often. It is interesting to note that electrical signals regulate molecular signals that trigger axon growth and connection formation. Activity plays a role in establishing the pattern of connections.
Genetic approaches allow the identification of genes that control the differentiation of entire organs, such as the eye as a whole. Hering and colleagues studied gene expression eyeless in a fruit fly Drosophila, which controls eye development. Removing this gene from the genome results in eyes not developing. Homologous genes in mice and humans (known as small eye And aniridia) similar in structure. If a homologous gene eyeless mammals is artificially integrated and expressed in the fly, then this animal develops additional (fly-like in structure) eyes on the antennae, wings and legs. This suggests that this gene controls eye formation in the same way in a fly or mouse, despite the completely different structure and properties of insect and mammalian eyes.
Regeneration of the nervous system after injury
The nervous system not only makes connections during development, but can repair some connections after damage (your computer cannot do this). For example, axons in the hand can sprout after injury and establish connections; the hand can again move and feel touch. Similarly, in a frog, fish or invertebrate animal, following destruction in the nervous system, axonal regeneration and restoration of function are observed. After cutting the optic nerve in a frog or fish, the fibers grow back and the animal can see. However, this ability is not inherent in the central nervous system of adult vertebrates - regeneration does not occur in them. The molecular signals that block regeneration and their biological significance for nervous system function are unknown
conclusions
∙ Neurons are connected to each other in a strictly defined way.
∙ Information is transmitted from cell to cell through synapses.
∙ In relatively simple systems, such as the retina, it is possible to trace all the connections and understand the meaning of intercellular signals.
∙ Nerve cells of the brain are the material elements of perception.
∙ Signals in neurons are highly stereotyped and are the same for all animals.
∙ Action potentials can travel long distances without loss.
∙ Local gradual potentials depend on the passive electrical properties of neurons and propagate only over short distances.
∙ The special structure of nerve cells requires a specialized mechanism for the axonal transport of proteins and organelles to and from the cell body.
∙ During individual development, neurons migrate to their final locations and establish connections with targets.
∙ Molecular signals control axon growth.
Bibliography
Penrose R. THE NEW MIND OF THE KING. About computers, thinking and the laws of physics.
Gregory R. L. Intelligent Eye.
Lekah V. A. The key to understanding physiology.
Gamow G., Ichas M. Mr. Tompkins inside himself: Adventures in new biology.
Kozhedub R. G. Membrane and synoptic modifications in manifestations of the basic principles of brain function.
A team of scientists from Spain, France and England announced the completion of the first ever experiment on transmitting a signal between the minds of two people using exclusively non-invasive technologies. A signal consisting of 140 bits of information was transmitted from India to France via the Internet. The work was published in PLOS One.
General scheme of the experiment. Image: PLOS one article |
The experiment was based on brain-computer interfaces (BCI) and computer-brain interfaces (CBI), the signal was transmitted via the Internet. The message was ultimately the word "hola" - "hello" in Spanish (and Catalan). The Bacon cipher, which uses 5 bits per letter, was used for encoding. The word was transmitted 7 times to collect sufficient statistics, so the final message was 140 bits long.
The scientists modeled the brain-computer interface as follows: to encode “0,” the human “transmitter” moved his foot, and to encode “1,” he moved his palm. By taking an electroencephalogram from the areas of the cerebral cortex responsible for these movements, the computer received the transmitted message in the form of binary bits.
With the computer-brain interface, things were more complicated. On the head of the human “receiver” they found the visual center of the cerebral cortex, upon stimulation of which the phenomenon of phosphenes arose - visual sensations that arise without information from the eye. The presence of such a feeling was coded “1”, the absence - “0”.
Four volunteers aged 28-50 years acted as transmitters and receivers. For the final experiment, the signal was transmitted from India to France. In order to eliminate interference from the senses, the “receiver” person was wearing a light-proof mask over his eyes, and plugs were placed in his ears. To eliminate the possibility of guessing the encoded word, the sequence was first further encoded to obtain a pseudo-random code, which, after transmission, was decrypted to restore the original message.
As a result of the experiment, it was possible to transmit 140 bits of information with an error rate of 4%. For comparison, to make sure that this result is statistically significant: the probability of guessing all 140 characters in a row is less than 10 -22 , and to guess at least 80% of 140 characters is less than 10 -13 . Thus, according to scientists, there actually was a direct transmission of the signal from brain to brain.
The novelty and significance of this work stem from the fact that so far all such experiments have either been limited to one of two interfaces, or have been carried out on laboratory animals, or have involved invasive procedures for implanting sensors into a living organism. In this work, scientists for the first time managed to realize non-invasive transmission from person to person.
From the retina, signals are sent to the central part of the analyzer along the optic nerve, which consists of almost a million nerve fibers. At the level of the optic chiasm, about half of the fibers go to the opposite hemisphere of the brain, the remaining half goes to the same (ipsilateral) hemisphere. The first switching of optic nerve fibers occurs in the lateral geniculate body of the thalamus. From here, new fibers are sent through the brain to the visual cortex (Fig. 5.17).
Compared to the retina, the geniculate body is a relatively simple formation. There is only one synapse here, since the incoming fibers of the optic nerve end on cells that send their impulses to the cortex. The geniculate body contains six layers of cells, each of which receives input from only one eye. The top four are small cell, the bottom two are large cell, so the top layers are called parvocellular(parvo - small, cellula - cell, lat.) and the lower ones - magnocellular(magnus - big, lat.)(Fig. 5.18).
These two types of layers receive information from different ganglion cells associated with different types of bipolar cells and receptors. Each cell of the geniculate body is activated from the receptive field of the retina and has “on” or “ofrV” centers and a periphery of the opposite sign. However, between the cells of the geniculate body and the ganglion cells of the retina there is
Rice. 5 17 Transmission of visual information to the brain. 1- eye; 2 - retina; 3 - optic nerve; 4 - visual chiasm; 5 - external geniculate body, 6 - visual radiation; 7 - visual cortex; 8 - occipital lobes (Lindsney, Norman, 1974)
The brain is the physical basis of vision. Most of the pathways leading from the retina to the visual cortex in the posterior hemispheres pass through the lateral geniculate body. A cross-section of this subcortical structure reveals six cell layers, two of which correspond to magnocellular connections (M), and four to parvocellular connections (P) (Zeki, 1992).
There are differences, of which the most significant is the much more pronounced ability of the periphery of the receptive field of the cells of the geniculate body to suppress the effect of the center, i.e. they are more specialized (Hubel, 1974).
Neurons of the lateral geniculate body send their axons to the primary visual cortex, also called zoneVI (visual - visual, English). Primary visual (striatal) the cortex consists of two parallel and largely independent systems - magnocellular and parvocellular, named according to the layers of the geniculate bodies of the thalamus (Zeki and Shopp, 1988). The magnocellular system is found in all mammals and therefore has an earlier origin. The parvocellular system is present only in primates, which indicates its later evolutionary origin (Carlson, 1992). The magnocellular system is included in the analysis of shape, movement and depth of visual space. The parvocellular system is involved in visual functions developed in primates, such as color perception and fine detail detection (Merigan, 1989).
The connection between the geniculate bodies and the striate cortex is carried out with high topographic accuracy: zone VI actually contains a “map” of the entire surface of the retina. Damage to any part of the nerve pathway connecting the retina with zone VI leads to the appearance fields of absolute blindness, the dimensions and position of which exactly correspond to the length and lo-
localization of damage in zone VI. S. Henschen named this zone cortical retina (Zeki, 1992).
Fibers coming from the lateral geniculate bodies are in contact with the cells of the fourth layer of the cortex. From here, information eventually spreads to all layers. Cells of the third and fifth layers of the cortex send their axons to deeper structures of the brain. Most of the connections between the cells of the striate cortex run perpendicular to the surface, the lateral connections are predominantly short. This suggests the presence of locality in information processing in this area.
The area of the retina that affects the simple cell of the cortex (the receptive field of the cell), like the fields of the neurons of the retina and geniculate bodies, is divided into “on” and “offr” regions. However, these fields are far from a perfect circle. In a typical case, the receptive field consists of a very long and narrow “op” area, which is adjacent on both sides by wider “o!G” areas (Hubel, 1974).
The composition of the human brain includes structural and functionally interconnected neurons. This organ of mammals, depending on the species, contains from 100 million to 100 billion neurons.
Each mammalian neuron consists of a cell - an elementary structural unit, dendrites (short process) and an axon (long process). The body of the elementary structural unit contains the nucleus and cytoplasm.
Axon exits the cell body and often gives rise to many small branches before reaching the nerve endings.
Dendrites extend from the nerve cell body and receive messages from other units of the nervous system.
Synapses– these are contacts where one neuron connects to another. Dendrites are covered with synapses that are formed by the ends of axons from other structural and functional units of the system.
The composition of the human brain is 86 billion neurons, consisting of 80% water and consuming about 20% of the oxygen intended for the entire body, although its mass is only 2% of body weight.
How signals are transmitted in the brain
When the units of a functional system, neurons, receive and send messages, they transmit electrical impulses along their axons, which can vary in length from a centimeter to one meter or more. it is clear that it is very complex.
Many axons are covered in a multilayer myelin sheath, which speeds the transmission of electrical signals along the axon. This shell is formed with the help of specialized elementary structural units of glia. In the central nervous system, glia are called oligodendrocytes, and in the peripheral nervous system they are called Schwann cells. The medulla contains at least ten times more glia than nervous system units. Glia perform many functions. The importance of glia in the transport of nutrients to neurons, cleansing, processing of some dead neurons.
To transmit signals, the functional units of any mammal's body system do not work alone. In a neural circuit, the activity of one elementary unit directly affects many others. To understand how these interactions control brain function, neuroscientists study the connections between nerve cells and how they transmit signals in the brain and change over time. This study could lead scientists to a better understanding of how the nervous system develops, becomes susceptible to disease or injury, and disrupts the natural rhythms of brain connections. Thanks to new imaging technology, scientists are now better able to visualize the circuits connecting the regions and composition of the human brain. 
Advances in techniques, microscopy and computing technology are allowing scientists to begin to map the connections between individual nerve cells in animals better than ever before.
By studying the composition of the human brain in detail, scientists can shed light on brain disorders and errors in the development of the nervous network, including autism and schizophrenia.