Nerve cells or neurons are electrically excitable cells that process and transmit information using electrical impulses. Such signals are transmitted between neurons through synapses . Neurons can communicate with each other in neural networks. Neurons are the main material of the brain and spinal cord of the human central nervous system, as well as ganglia of the human peripheral nervous system.
Neurons come in several types depending on their functions:
- Sensory neurons that respond to stimuli such as light, sound, touch, as well as other stimuli that affect the cells of the sensory organs.
- Motor neurons that send signals to muscles.
- Interneurons connect one neuron to another in the brain, spinal cord, or neural networks.
Neuron structure
A typical neuron consists of a cell body ( soma ), dendrites , and an axon . Dendrites are thin structures extending from the cell body; they have multiple branching and are several hundred micrometers in size. An axon, which in its myelinated form is also called a nerve fiber, is a specialized cellular extension that originates from the cell body at a place called the axon hillock (hillock) and extends over a distance of up to one meter. Often, nerve fibers are bundled into bundles and into the peripheral nervous system, forming nerve filaments.
The cytoplasmic part of the cell containing the nucleus is called the cell body or soma. Typically, the body of each cell has dimensions from 4 to 100 microns in diameter and can be of various shapes: spindle-shaped, pear-shaped, pyramidal, and also much less often star-shaped. The nerve cell body contains a large spherical central nucleus with many Nissl granules containing a cytoplasmic matrix (neuroplasm). Nissl granules contain ribonucleoprotein and take part in protein synthesis. Neuroplasm also contains mitochondria and Golgi bodies, melanin and lipochrome pigment granules. The number of these cellular organelles depends on the functional characteristics of the cell. It should be noted that the cell body exists with a non-functional centrosome, which prevents neurons from dividing. This is why the number of neurons in an adult is equal to the number of neurons at birth. Along the entire length of the axon and dendrites there are fragile cytoplasmic filaments called neurofibrils, originating from the cell body. The cell body and its appendages are surrounded by a thin membrane called the neural membrane. The cell bodies described above are present in the gray matter of the brain and spinal cord.
The short cytoplasmic appendages of the cell body that receive impulses from other neurons are called dendrites. Dendrites conduct nerve impulses into the cell body. Dendrites have an initial thickness of 5 to 10 microns, but gradually their thickness decreases and they continue to branch abundantly. Dendrites receive an impulse from the axon of a neighboring neuron through the synapse and conduct the impulse to the cell body, which is why they are called receptive organs.
A long cytoplasmic appendage of the cell body that transmits impulses from the cell body to a neighboring neuron is called an axon. The axon is significantly larger than the dendrites. The axon originates at a conical height of the cell body called the axon hillock, which is devoid of Nissl granules. The length of the axon is variable and depends on the functional connection of the neuron. The axon cytoplasm or axoplasm contains neurofibrils, mitochondria, but does not contain Nissl granules. The membrane that covers the axon is called the axolemma. The axon can produce processes called accessory along its direction, and towards the end the axon has intensive branching ending in a brush, its last part has an increase to form a bulb. Axons are present in the white matter of the central and peripheral nervous systems. Nerve fibers (axons) are covered with a thin membrane that is rich in lipids called the myelin sheath. The myelin sheath is formed by Schwann cells that cover nerve fibers. The part of the axon that is not covered by the myelin sheath is a node of adjacent myelinated segments called the node of Ranvier. The function of the axon is to transmit an impulse from the cell body of one neuron to the dendron of another neuron through the synapse. Neurons are specifically designed to transmit intercellular signals. The diversity of neurons is associated with the functions they perform; the size of the neuron soma varies from 4 to 100 μm in diameter. The soma nucleus has dimensions from 3 to 18 microns. The dendrites of a neuron are cellular appendages that form entire dendritic branches.
The axon is the thinnest structure of a neuron, but its length can exceed the diameter of the soma by several hundred and thousand times. The axon carries nerve signals from the soma. The place where the axon emerges from the soma is called the axon hillock. The length of the axons can vary and in some parts of the body reaches a length of more than 1 meter (for example, from the base of the spine to the tip of the toe).
There are some structural differences between axons and dendrites. Thus, typical axons almost never contain ribosomes, with the exception of some in the initial segment. Dendrites contain granular endoplasmic reticulum or ribosomes, which decrease in size with distance from the cell body.
The human brain has a very large number of synapses. Thus, each of 100 billion neurons contains on average 7,000 synaptic connections with other neurons. It has been established that the brain of a three-year-old child has about 1 quadrillion synapses. The number of these synapses decreases with age and stabilizes in adults. In an adult, the number of synapses ranges from 100 to 500 trillion. According to research, the human brain contains about 100 billion neurons and 100 trillion synapses.
Structural classification of neurons
Based on the number and arrangement of dendrites and axons, neurons are divided into axonless neurons, unipolar neurons, pseudounipolar neurons, bipolar neurons, and multipolar (many dendritic arbors, usually efferent) neurons.
· Axonless neurons are small cells grouped near the spinal cord in the intervertebral ganglia, which do not have anatomical signs of division of processes into dendrites and axons. All processes of the cell are very similar. The functional purpose of axonless neurons is poorly understood.
· Unipolar neurons - neurons with one process, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain.
· Bipolar neurons - neurons with one axon and one dendrite, located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
· Multipolar neurons - Neurons with one axon and several dendrites. This type of nerve cells predominates in the central nervous system
· Pseudounipolar neurons are unique in their kind. One process extends from the body, which immediately divides in a T-shape. This entire single tract is covered with a myelin sheath and is structurally an axon, although along one of the branches the excitation goes not from, but to the body of the neuron. Structurally, dendrites are branches at the end of this (peripheral) process. The trigger zone is the beginning of this branching (i.e., it is located outside the cell body). Such neurons are found in the spinal ganglia.
Functional classification of neurons Based on their position in the reflex arc, afferent neurons (sensitive neurons), efferent neurons (some of them are called motor neurons, sometimes this not very accurate name applies to the entire group of efferents) and interneurons (interneurons) are distinguished.
Afferent neurons (sensory, sensory or receptor). Neurons of this type include primary cells of the sensory organs and pseudounipolar cells, whose dendrites have free endings.
Efferent neurons (effector, motor or motor). Neurons of this type include the final neurons - ultimatum and penultimate - non-ultimatum.
Associative neurons (interneurons or interneurons) - this group of neurons communicates between efferent and afferent, they are divided into commissural and projection (brain).
Morphological classification of neurons The morphological structure of neurons is diverse. In this regard, several principles are used when classifying neurons:
1. take into account the size and shape of the neuron body,
2. the number and nature of branching of processes,
3. the length of the neuron and the presence of specialized shells.
According to the shape of the cell, neurons can be spherical, granular, stellate, pyramidal, pear-shaped, fusiform, irregular, etc. The size of the neuron body varies from 5 μm in small granular cells to 120-150 μm in giant pyramidal neurons. The length of a neuron in humans ranges from 150 µm to 120 cm. Based on the number of processes, the following morphological types of neurons are distinguished: - unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain; - pseudounipolar cells grouped near the spinal cord in the intervertebral ganglia; - bipolar neurons (have one axon and one dendrite), located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia; - multipolar neurons (have one axon and several dendrites), predominant in the central nervous system.
Development and growth of a neuron A neuron develops from a small precursor cell, which stops dividing even before it releases its processes. (However, the issue of neuronal division currently remains controversial.) Typically, the axon begins to grow first, and dendrites form later. At the end of the developing process of the nerve cell, an irregularly shaped thickening appears, which, apparently, makes its way through the surrounding tissue. This thickening is called the growth cone of the nerve cell. It consists of a flattened part of the nerve cell process with many thin spines. The microspinuses are 0.1 to 0.2 µm thick and can reach 50 µm in length; the wide and flat region of the growth cone is about 5 µm in width and length, although its shape can vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspikes are in constant motion - some are retracted into the growth cone, others elongate, deviate in different directions, touch the substrate and can stick to it. The growth cone is filled with small, sometimes connected to each other, membrane vesicles of irregular shape. Directly below the folded areas of the membrane and in the spines is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules and neurofilaments found in the body of the neuron. It is likely that microtubules and neurofilaments elongate mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axonal transport in a mature neuron.
Since the average speed of advancement of the growth cone is approximately the same, it is possible that during the growth of the neuron process, neither the assembly nor destruction of microtubules and neurofilaments occurs at its far end. New membrane material is added, apparently, at the end. The growth cone is an area of rapid exocytosis and endocytosis, as evidenced by the many vesicles present there. Small membrane vesicles are transported along the neuron process from the cell body to the growth cone with a stream of fast axonal transport. The membrane material is apparently synthesized in the body of the neuron, transported to the growth cone in the form of vesicles and incorporated here into the plasma membrane by exocytosis, thus lengthening the process of the nerve cell. The growth of axons and dendrites is usually preceded by a phase of neuronal migration, when immature neurons disperse and find a permanent home.
Neuroglia. Unlike nerve cells, glial cells have greater diversity. Their number is tens of times greater than the number of nerve cells. Unlike nerve cells, glial cells are capable of dividing; their diameter is significantly smaller than the diameter of a nerve cell and is 1.5-4 microns.
For a long time it was believed that the function of gliocytes is unimportant, and they perform only a supporting function in the nervous system. Thanks to modern research methods, it has been established that gliocytes perform a number of functions important for the nervous system: supporting, delimiting, trophic, secretory, protective.
Among gliocytes, according to their morphological organization, a number of types are distinguished: ependymocytes, astrocytes.
Ependymocytes form a dense layer of cells, elements lining the spinal canal and ventricles of the brain. During ontogenesis, ependymocytes were formed from spongioblasts. Ependymocytes are slightly elongated cells with branching processes. Some ependymocytes perform a secretory function, releasing biologically active substances into the blood and into the ventricles of the brain. Ependymocytes form clusters on the capillary chain of the brain ventricles; When a dye is introduced into the blood, it accumulates in ependymocytes, which indicates that the latter perform the function of the blood-brain barrier.
Astrocytes perform a supporting function. This is a huge number of glial cells with many short processes. Among astrocytes there are 2 groups:
o plasma cells
o fibrous astrocytes
Oligodendrocytes are large glial cells, often concentrated around a nerve cell and are therefore called satellite gliocytes. Their function is very important for the trophism of the nerve cell. During functional overstrain of a nerve cell, gliocytes are able to abstract substances entering the nerve cell by pinocytosis. Under functional loads, first the synthetic apparatus of glial cells is depleted, and then nerve cells. During restoration (reparation), the functions of neurons are first restored, and then the functions of glial cells. Thus, gliocytes take part in ensuring the functions of neurons. Glial cells are significantly capable of influencing the trophism of the brain, as well as the functional status of the nerve cell. Nerves (nervi) are anatomical formations in the form of cords, built primarily from nerve fibers and providing communication between the central nervous system and innervated organs, blood vessels and the skin of the body.
Nerves arise in pairs (left and right) from the brain and spinal cord. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves; the totality of N. and their derivatives makes up the peripheral nervous system ( Fig. ), which, depending on the characteristics of the structure, functioning and origin, is divided into two parts: the somatic nervous system, innervating the skeletal muscles and skin of the body, and the autonomic nervous system,
innervating internal organs, glands, circulatory system, etc.
Nerve fibers are processes of nerve cells (neurons) that have a membrane and are capable of conducting nerve impulses. The main component of the nerve fiber is the process of the neuron, which forms, as it were, the axis of the fiber. Mostly this is an axon. The nerve process is surrounded by a membrane of complex structure, together with which it forms a fiber. The thickness of the nerve fiber in the human body, as a rule, does not exceed 30 micrometers. Nerve fibers are divided into pulpy (myelinated) and non-myelinated (non-myelinated). The former have a myelin sheath covering the axon, the latter lack a myelin sheath.
Myelin fibers predominate in both the peripheral and central nervous systems. Nerve fibers lacking myelin are located predominantly in the sympathetic division of the autonomic nervous system. At the point where the nerve fiber departs from the cell and in the area of its transition into the final branches, the nerve fibers can be devoid of any membranes, and then they are called bare axial cylinders.
Depending on the nature of the signal carried through them, nerve fibers are divided into motor autonomic, sensory and motor somatic.
Structure of nerve fibers:
The myelin nerve fiber contains the following elements (structures): 1) the axial cylinder, located in the very center of the nerve fiber, 2) the myelin sheath covering the axial cylinder, 3) the Schwann sheath.
The axial cylinder consists of neurofibrils. The pulpy membrane contains a large amount of lipoid substances known as myelin. Myelin ensures the speed of nerve impulses. The myelin sheath does not cover the entire axial cylinder, forming gaps called nodes of Ranvier. In the area of the nodes of Ranvier, the axial cylinder of the nerve fiber is adjacent to the superior Schwann membrane.
The fiber space located between two nodes of Ranvier is called a fiber segment. In each such segment, the nucleus of the Schwann membrane can be seen on stained preparations. It lies approximately in the middle of the segment and is surrounded by the protoplasm of the Schwann cell, the loops of which contain myelin. Between the nodes of Ranvier, the myelin sheath is also not continuous. In its thickness, so-called Schmidt-Lanterman notches are found, running in an oblique direction.
Schwann membrane cells, as well as neurons with processes, develop from the ectoderm. They cover the axial cylinder of the nerve fiber of the peripheral nervous system, similar to how glial cells cover the nerve fiber in the central nervous system. As a result, they may be called peripheral glial cells.
In the central nervous system, nerve fibers do not have Schwann sheaths. The role of Schwann cells here is performed by elements of oligodendroglia. An unmyelinated (unmyelinated) nerve fiber is devoid of a myelin sheath and consists only of an axial cylinder and a Schwann sheath.
Function of nerve fibers.
The main function of nerve fibers is the transmission of nerve impulses. Currently, two types of nerve transmission have been studied: pulsed and non-pulse. Impulse transmission is provided by electrolyte and neurotransmitter mechanisms. The speed of nerve impulse transmission in myelinated fibers is much higher than in nonmyelinated fibers. In its implementation, the most important role is played by myelin. This substance is capable of isolating a nerve impulse, as a result of which signal transmission along the nerve fiber occurs spasmodically, from one node of Ranvier to another. Pulseless transmission is carried out by axoplasmic current along special axon microtubules containing trophogens - substances that have a trophic effect on the innervated organ.
Ganglion (ancient Greek γανγλιον - node) or nerve ganglion is a collection of nerve cells consisting of bodies, dendrites and axonal nerve cells and glial cells. Typically, the ganglion also has a sheath of connective tissue. Found in many invertebrates and all vertebrates. They often connect with each other, forming various structures (nerve plexuses, nerve chains, etc.).
TICKET No. 13
1. Bones of the facial skull. Eye socket. Nasal cavity. Messages.
2. Large intestine: sections, their topography, structure, relationship to the peritoneum, blood supply and innervation.
3. Medulla oblongata. External and internal structure. Topography of gray and white matter.
Types of neurons
Neurons come in several shapes and sizes and are classified according to their morphology and function. For example, anatomist Camillo Golgi divided neurons into two groups. He included neurons with long axons that transmit signals over long distances into the first group. He included neurons with short axons, which could be confused with dendrites, in the second group.
Neurons are classified according to their structure into the following groups:
- Unipolar . The axon and dendrites emerge from the same appendage.
- Bipolar . The axon and single dendrite are located on opposite sides of the soma.
- Multipolar . At least two dendrites are located separately from the axon.
- Golgi type I. A neuron has a long axon.
- Golgi type II . Neurons whose axons are located locally.
- Anaxon neurons . When the axon is indistinguishable from dendrites.
- Basket cells are interneurons that form densely woven endings throughout the soma of target cells. Present in the cerebral cortex and cerebellum.
- Betz cells . They are large motor neurons.
- Lugaro cells are interneurons of the cerebellum.
- Medium spiky neurons . Present in the striatum.
- Purkinje cells . They are large multipolar cerebellar neurons of the Golgi type I.
- Pyramidal cells . Neurons with a triangular soma of Golgi type II.
- Renshaw cells . Neurons connected at both ends to alpha motor neurons.
- Unipolar racemose cells . Interneurons that have unique brush-shaped dendritic endings.
- Cells of the anterior horny process . They are motor neurons located in the spinal cord.
- Spindle cells . Interneurons connecting distant areas of the brain.
- Afferent neurons . Neurons that transmit signals from tissues and organs to the central nervous system.
- Efferent neurons . Neurons that transmit signals from the central nervous system to effector cells.
- Interneurons , connecting neurons in specific areas of the central nervous system.
INTRODUCTION TO COGNITIVE NEUROSCIENCE. Chapter 3. Neurons and connections between them.
INTRODUCTION TO COGNITIVE NEUROSCIENCE. From the textbook BRAIN. COGNITION. INTELLIGENCE. E-book https://t.me/kudaidem/1879
Chapter 3. Neurons and connections between them.
What do we know about the processes occurring at the neuronal level? Can we now construct a consistent theory regarding events at this level?
The main cells of the brain are neurons, highly conserved from an evolutionary point of view . They have persisted relatively unchanged for many hundreds of millions of years, and even very different animal species have the same types of neurons . From many points of view, neurons are no different from other cells, but there is one thing that sets them apart from the rest: a specialization in electrochemical signaling, thanks to which they are able to receive an incoming signal at the dendrites and send an electrochemical signal along the axon. The entire brain can be considered as a highly complex structure consisting of interconnected neurons.
(Fig. 3.1).
Dendrites and axons are extensions of the neuron body; one neuron can have up to ten thousand dendrites and one axon.
(Fig. 3.2 and 3.3).
The action potential (AP) travels along the axon much more slowly than the electric current in a computer, but our brain performs many tasks much better than modern computers. Currently, computers are far behind humans in tasks of perception, language communication, semantic memory, motor control and creativity.
Neuroscience focuses on the connections and interactions of neurons. It is convenient to start considering such connections with a generalized neuron .
Classic neurons connect via synapses, which can be excitatory or inhibitory.
(Fig. 3.6)
Neuron activity is mediated by dozens of factors—the sleep-wake cycle, the availability of neurotransmitter precursors, and many others. All of these factors influence the probability of a signal passing between two neurons and can be represented as synaptic weights. Thus, the entire diversity of neurons can be successfully represented in the form of an integrative neuron, and all methods of interneuron communication can be successfully represented in the form of the probability of signal transmission between neurons.
There are at least six major neurotransmitters and at least thirty "less important" ones, mostly neuropeptides .
Even the dendrites of a single cell appear to be capable of information processing . There is also evidence that neuroglia , the supporting tissue of the nervous system, can also take part in information processing.
It is now known that stem cells exist in some parts of the adult brain. The formation of new synapses occurs throughout life; To form new synapses, dendritic processes can form within a few minutes.
(Fig. 3.7).
1.3. Information processing by neurons.
Artificial neural networks have been used to model many brain functions—recognizing image elements, controlling robots, learning, and improving functioning based on experience.
In many cases, such networks performed tasks better than computer programs based on logic and mathematics.
They also help us understand the principles of operation of real neural networks in the brain.
Neural networks help us understand the functioning of the nervous system.
Thus, artificial neural networks can serve as models for studying real structures in the brain.
We will limit ourselves to considering synapses of only two types - excitatory (increasing the probability of the passage of AP (Action Potential) on the postsynaptic neuron) and inhibitory (reducing this probability).
Glutamate, the most common neurotransmitter in the central nervous system, is an excitatory.
GABA (gamma aminobutyric acid) is the most common inhibitory neurotransmitter .
Arrays of neurons, often referred to as maps, are common throughout the nervous system.
2.1. Simplified case: receptors, pathways and circuits.
Each sensory nerve may contain several parallel channels, each carrying slightly different information. Thus, the visual tract has a channel for transmitting color, called small-cell, and a channel for transmitting the shape and size of an object, called large-cell.
In the same way, somatosensory pathways combine channels for transmitting touch, pressure, pain and some others.
Most sensory fibers terminate in the thalamus, where they transmit signals to neurons that terminate in the cortex.
(Fig. 3.10 and 3.11)
Thus, in most signaling pathways there are feedback loops, such as in a neural network with two or more layers.
From this point of view, the brain appears to be a system of arrays and networks influencing each other .
A neuron array is a two-dimensional network of neurons .
When arrays correspond to the spatial organization of a particular structure, they are called maps.
Both temporal and spatial coding take place in the brain, along with many other ways of encoding and processing information.
Spatial maps are the most visual form of spatial coding.
(Fig. 3.15 and 3.16).
Somatosensory information
such as touch and pain is also processed by cortical maps. Other senses, such as hearing, taste, and smell, are much less associated with receptor location, but the auditory cortex has a map-like wedge-shaped region.
Thus, even information from sense organs not associated with space is processed by arrays and maps of neurons.
Our brain organizes huge amounts of incoming information to reflect the position of surrounding objects. The motor cortex, as you might guess, also looks like a disproportionate map of the body's skeletal muscles.
The main question regarding sensory technology today is how high-level processing of perceived information occurs. And the neural network model provides one possible answer.
The brain continually adjusts motor systems based on sensory information and adapts sensory systems through motor activity.
Sensory systems can be thought of as hierarchical systems, consisting of lower-order hierarchical systems, starting with receptors and gradually moving to increasingly complex objects.
Motor systems can be represented as a hierarchical structure of the opposite direction, ending in motoneurons .
There is a continuous exchange of information between the two systems in a cycle from perception to action, from the lowest to the highest levels of planning, thinking and analysis of possible developments.
(Fig. 3.20 Hierarchical system from the field of architecture).
In the diagram of a hierarchical information processing system, each array of neurons is called a map; cards exist at different levels, and the signal can go up, down, and to another card at the same level.
When considering the electrical activity of tens of billions of neurons, the brain inevitably begins to seem like a huge orchestra, rather than a single instrument. Over hundreds of millions of years of evolution, neurons with a variety of types of temporal and spatial coding have appeared in the brain (block 3.1).
Information paths have many choice points from which it can be directed along several different paths or transferred to a higher or lower level.
If we return to the step pyramid, then such a branched path is similar to a person’s path to the top: he can reach it in a direct or roundabout way.
The visual picture of the world is subject to constant changes. However, the brain still processes such changes. An animal cannot afford not to notice a predator hiding in the grass just because it is sunset or because a shadow falls on it.
In order to survive, we had to have a superior visual system.
For example, a cat stalking a prey can examine a tree with only one eye, while the other eye remains passive. This leads to the phenomenon of binocular competition —competition between visual inputs from different eyes.
Many animals receive completely different inputs from different eyes - animals such as rabbits and deer have no areas of visual field overlap at all, so for them the phenomenon of binocular competition is impossible.
The brain constantly has some expectations about the external conditions it encounters. When going down the stairs in the dark, we expect that there will be a step under our feet.
When analyzing ambiguously interpreted objects, expectation determines the choice of the most acceptable interpretation option. Many words in a language have more than one meaning, so even as you read this you are forced to deal with ambiguities. The brain relies not only on incoming information - it has many reasons for choosing one option or another , based on predicting the outcome and expectation.
Selective attention allows us to dynamically change our sensory preferences, and long-term memory increases the strength of the synapses responsible for accurate perception.
Many scientists believe that the entire cortex, along with associated areas such as the thalamus, should be considered a single functional unit. It is often called the thalamocortical system .
(Plastic brain).
One of the main properties of animal behavior is the ability to adapt.
The main property of the brain , therefore, is adaptability . However, what changes in the structure of the brain itself lead to such adaptability?
For these purposes, methods for visualizing brain structures, which have received intensive development in the past two decades, are much better suited.
Although most imaging techniques are domain-specific, thereby emphasizing functional separation rather than integration, attempts have been made to study learning as a systemic process involving global changes in brain structure and function.
The development of magnetic resonance imaging technology has made it possible to begin to study changes in the structural components of communication - white matter tracts - under the influence of learning.
Learning to juggle has been shown to cause changes in both the gray and white matter of the brain.
These results were truly revolutionary, since for many years it was believed that the structure of the brain is unchanged .
Such discoveries, which allow us to consider the brain as a functionally and structurally labile organ , are without a doubt a step forward in our understanding of the learning process.
(4.0. Adaptation and training of neuron arrays).
The most well-known rule for training neural networks, expressed in the slogan “ neurons that fire together, wire together.”
Neurons that fire together wire together.
(Hebb Training).
Donald Hebb postulated in 1949 that assemblies of neurons can learn by strengthening connections between neurons that fire simultaneously when stimulated .
The basis of learning and memory is the effectiveness of synaptic connections.
There are many ways to influence the efficiency of synaptic transmission. Thus, two neurons can form more synapses, more neurotransmitter can be produced in the synapses themselves, and the receptors of the postsynaptic neuron can become more effective.
two types of changes involved in learning ; they can be thought of as increased excitation and increased inhibition .
A long-term increase in the excitability of a single neuron is called long-term potentiation .
A long-term decline is long-term depression . Both events take place in the hippocampus.
Visually, Hebbian learning can be represented as thickening of the lines between network nodes, as in a simple collection of cells.
Models with a third, hidden layer allow the neural network to change the strength of connections.
A classic three-layer forward network with a hidden layer and tunable interaction strength can be trained efficiently by mapping the neural network output to the desired output and adjusting the strength of the connections to achieve the desired result.
The process is called backpropagation and is in many ways similar to negative feedback .
Networks of this type are the most common today.
In a self-organizing auto-associative network, the output is matched to the input.
This strategy is useful when recognizing patterns, such as the sound of a familiar voice.
Self-organizing systems are used in nature to solve many problems.
The organisms themselves and their nervous systems can be considered as self-organizing systems.
The self-organizing network is able to cope with the fundamental problem of recognizing human faces.
A person learns to respond to normal, undeformed faces very early in life and soon becomes able to distinguish familiar from unfamiliar faces.
The problem solved by the network is much simpler than that solved by humans, since in the model only the formation of the circuit occurs.
The network is able to learn to predict the location of the mouth at the bottom of the picture and the two eyes at the top.
4.2. Darwinian approach in the nervous system: the cells and synapses best adapted to the task survive.
Neural Darwinism suggests that neurons develop and connect to each other according to Darwinian principles.
Selectionism is an effective way of adaptation.
The selection of neurons leads to the formation of long-lived neural aggregates that perform tasks of adaptation, learning, pattern recognition, and the like.
Neural networks are characterized by a high level of parallelism (which means the ability to perform many different calculations at the same time) and distribution (the ability to process information in different places using different mechanisms).
This indicates the greater proximity of neural networks to biological methods of information processing.
Neural networks are quite easy to translate into mathematical expressions.
Neural networks are capable of processing symbolic information, and symbols can be translated into neural networks.
Neural network learning occurs as the network recognizes the input and cuts off alternative options.
There are many ways to coordinate the work of neurons . One of them is large-scale rhythms , which coordinate the work of large groups of neurons in the same way that a conductor coordinates the playing of a symphony orchestra. If a large mass of neurons is activated simultaneously, then their activity, as a rule, sums up.
Current evidence favors much faster gamma and theta correlations at the frequencies at which the brain does most of this work.
Encephalogram rhythms are now considered to signal different but coordinated processes.
For example, high-density gamma rhythms are thought to be associated with conscious visual perception and the process of solving simple equivalence problems.
Alpha rhythms are traditionally associated with the absence of tasks requiring focused attention, while theta rhythms are currently believed to control the hippocampal region and frontal cortex during long-term memory retrieval. Delta rhythms - signals of deep sleep - group fast neuronal activity in order to consolidate the received data.
When designing an aircraft, engineers build some functional redundancy into its design in case critical systems fail. So, if one engine fails, most of the aircraft will be able to reach the runway on the remaining ones.
Humans and animals also have a certain amount of functional redundancy .
This rule also applies to the brain. The brain is able to work even after receiving very significant damage.
6.0. Conclusion.
Lateral inhibition is a common strategy for highlighting differences between two homogeneous signal regions, such as dark spots on a light background.
Cells in sensory systems have so-called receptive fields that are tuned to certain input parameters, such as line orientation, color, movement, shape and type of object. As the level of visual maps increases, their resolution decreases, while the ability to integrate information increases.
Because sensory and motor systems are studied separately, the brain appears to be a huge sensorimotor organ that allows continuous high-level interactions between input and output.
Spatial arrays of neurons make spatial coding possible, but we should not forget that the nervous system also has temporal coding. The fundamental rhythms of the encephalogram are believed to be responsible for the temporal coordination of the activity of large groups of neurons.
Recent research suggests that the gamma rhythm is responsible for the integration of sensory information into conscious sensations, and the theta rhythm is responsible for retrieving information from long-term memory.
Test assignments for this chapter.
1. Describe the main functions of an integrative neuron.
2. What is lateral inhibition and what role does it play in sensory systems?
3. How can sensory and motor systems be viewed as hierarchical structures?
4. Describe the role of bidirectional interactions in brain function.
5. What is the Darwinian approach to the nervous system and what aspects of brain processes does it address?
6. Name the three most common properties of sensory systems.
Action of neurons
All neurons are electrically excitable and maintain voltage across their membranes using metabolically conductive ion pumps coupled with ion channels that are embedded in the membrane to generate ion differentials such as sodium, chloride, calcium, and potassium. Changes in voltage in the cross-membrane lead to changes in the functions of voltage-dependent ionic cells. When the voltage changes at a sufficiently large level, the electrochemical impulse causes the generation of an active potential, which quickly moves along the axon cells, activating synaptic connections with other cells.
Most nerve cells are the basic type. A certain stimulus causes an electrical discharge in the cell, a discharge similar to the discharge of a capacitor. This produces an electrical impulse of approximately 50-70 millivolts, which is called the active potential. The electrical impulse propagates along the fiber, along the axons. The speed of propagation of the pulse depends on the fiber; it is approximately on average tens of meters per second, which is noticeably lower than the speed of propagation of electricity, which is equal to the speed of light. Once the impulse reaches the axon bundle, it is transmitted to neighboring nerve cells under the influence of a chemical transmitter.
A neuron acts on other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect of a postsynaptic neuron is determined not by the presynaptic neuron or neurotransmitter, but by the type of receptor activated. The neurotransmitter is like a key, and the receptor is a lock. In this case, one key can be used to open different types of “locks”. Receptors, in turn, are classified into excitatory (increasing the rate of transmission), inhibitory (slowing down the rate of transmission) and modulating (causing long-lasting effects).
Communication between neurons is carried out through synapses, at this point the end of the axon (axon terminal) is located. Neurons such as Purkinje cells in the cerebellum can have more than a thousand dendritic junctions, communicating with tens of thousands of other neurons. Other neurons (large neuron cells of the supraoptic nucleus) have only one or two dendrites, each of which receives thousands of synapses. Synapses can be either excitatory or inhibitory. Some neurons communicate with each other through electrical synapses, which are direct electrical connections between cells.
At a chemical synapse, when the action potential reaches the axon, voltage opens in the calcium channel, allowing calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with neurotransmitter molecules to penetrate the membrane, releasing the contents into the synaptic cleft. The process of transmitters diffusing through the synaptic cleft occurs, which in turn activate receptors on the postsynaptic neuron. In addition, high cytosolic calcium at the axon terminal induces mitochondrial calcium uptake, which in turn activates mitochondrial energy metabolism to produce ATP, which supports ongoing neurotransmission.
How do they work together and how are they different?
Afferent neurons typically have two axons that carry electrochemical signals to the spinal column or brain. Once there, the signal travels through a network of interneurons and through the efferent neuron. Afferent-efferent pairs of neurons that pass through the spine control reflexes (such as the knee-jerk reaction).
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Afferent neurons are designed to respond to various stimuli. For example, an afferent neuron designed to respond to heat detects excess heat and sends an impulse through the central nervous system. The efferent neuron then causes the muscles to contract to move the body away from the heat. The skin has sensory receptors for heat, cold, pleasure, pain and pressure.
Afferent neurons have round and smooth cell bodies, while efferent neurons have satellite bodies. Afferent neurons are found in the peripheral nervous system, and efferent neurons are located in the central nervous system. Axons in afferent neurons move from the ganglia (clusters of nerve cells that contain afferent and efferent neurons) to the spinal cord. The long axon is actually connected to the efferent neuron.
Afferent neurons have a single long myelinated dendrite, whereas efferent neurons have shorter dendrites. The dendrite in an afferent neuron is what is responsible for transmitting nerve impulses from the receptors to the cell body, while in an efferent neuron the impulses travel through the dendrite and exit through the neuromuscular junction that is formed between the effectors and the axon.
