Embryonic source of formation of the peripheral nervous system. Maturation of the nervous system in embryogenesis

MAIN QUESTIONS OF THE TOPIC:

1. General morphofunctional characteristics of the nervous tissue.

2. Embryonic histogenesis. Differentiation of neuroblasts and glioblasts. The concept of regeneration of the structural components of the nervous tissue.

3. Neurocytes (neurons): sources of development, classification, structure, regeneration.

4. Neuroglia. General characteristics. Sources of development of gliocytes. Classification. Macroglia (oligodendroglia, astroglia and ependymal glia). Microglia.

5. Nerve fibers: general characteristics, classification, structure and functions of non-myelinated and myelinated nerve fibers, degeneration and regeneration of nerve fibers.

6. Synapses: classifications, structure of a chemical synapse, structure and mechanisms of excitation transmission.

7. Reflex arcs, their sensitive, motor and associative links.

MAIN THEORETICAL PROVISIONS

NERVE TISSUE

nervous tissue performs the functions of perception, conduction and transmission of excitation received from the external environment and internal organs, as well as analysis, preservation of the information received, integration of organs and systems, interaction of the organism with the external environment.

The main structural elements of the nervous tissue are cells And neuroglia.

Neurons

Neurons consist of a body pericarion) and processes, among which are distinguished dendrites And axon(neuritis). There can be many dendrites, but there is always one axon.

A neuron, like any cell, consists of 3 components: nucleus, cytoplasm and cytolemma. The bulk of the cell falls on the processes.

Core occupies a central position in pericarion. One or more nucleoli are well developed in the nucleus.

plasmalemma takes part in the reception, generation and conduction of a nerve impulse.

Cytoplasm The neuron has a different structure in the perikaryon and in the processes.

In the cytoplasm of the perikaryon there are well-developed organelles: ER, Golgi complex, mitochondria, lysosomes. The structures of the cytoplasm specific for the neuron at the light-optical level are chromatophilic substance of the cytoplasm and neurofibrils.

chromatophilic substance cytoplasm (Nissl substance, tigroid, basophilic substance) appears when nerve cells are stained with basic dyes (methylene blue, toluidine blue, hematoxylin, etc.) in the form of granularity - these are clusters of GREPs cisterns. These organelles are absent in the axon and in the axon hillock, but are present in the initial segments of the dendrites. The process of destruction or disintegration of clumps of basophilic substance is called tigrolysis and is observed during reactive changes in neurons (for example, when they are damaged) or during their degeneration.

neurofibrils- This is a cytoskeleton consisting of neurofilaments and neurotubules that form the framework of a nerve cell. Neurofilaments represent intermediate filaments 8-10 nm in diameter, formed by fibrillar proteins. The main function of these elements of the cytoskeleton is the support - to ensure a stable shape of the neuron. A similar role is played by subtle microfilaments(transverse diameter 6-8 nm) containing actin proteins. Unlike microfilaments in other tissues and cells, they do not bind to micromyosins, which makes active contractile functions impossible in mature nerve cells.

Neurotubules according to the basic principles of their structure, they do not actually differ from microtubules. They, like all microtubules, have a transverse diameter of about 24 nm, the rings are closed by 13 molecules of the globular protein tubulin. In the nervous tissue, microtubules play a very important, if not unique, role. As elsewhere, they carry a frame (support) function, provide cyclosis processes. Microtubes are polar. It is the polarity of the microtube, which has negatively and positively charged ends, that makes it possible to control diffusion-transport flows in the axon (the so-called fast and slow axotok). Their detailed description is given below.

In addition, lipid inclusions (lipofuscin granules) can often be seen in neurons. They are characteristic of senile age and often appear during dystrophic processes. In some neurons, pigment inclusions are normally found (for example, with melanin), which causes staining of the nerve centers containing such cells (black substance, bluish spot).

Neurons are energetically highly dependent on aerobic phosphorylation and in adulthood are virtually incapable of anaerobic glycolysis. In this regard, nerve cells are in a pronounced dependence on the supply of oxygen and glucose, and if blood flow is disturbed, nerve cells almost immediately cease their vital activity. The moment of cessation of blood flow in the brain means the beginning of clinical death. With instant death, at room temperature, and normal body temperature, the processes of self-destruction in neurons are reversible within 5-7 minutes. This is the period of clinical death, when the revival of the organism is possible. Irreversible changes in the nervous tissue lead to the transition from clinical death to biological.

In the body of neurons, one can also see transport vesicles, some of which contain mediators and modulators. They are surrounded by a membrane. Their size and structure depend on the content of a particular substance.

Dendrites- short shoots, often strongly branched. The dendrites in the initial segments contain organelles like the body of a neuron. The cytoskeleton is well developed.

axon(neuritis) most often long, weakly branching or not branching. It lacks GREPS. Microtubules and microfilaments are ordered. In the cytoplasm of the axon, mitochondria and transport vesicles are visible. Axons are mostly myelinated and surrounded by processes of oligodendrocytes in the CNS, or lemmocytes in the peripheral nervous system. The initial segment of the axon is often expanded and is called the axon hillock, where the summation of the signals entering the nerve cell occurs, and if the excitatory signals are of sufficient intensity, then an action potential is formed in the axon and the excitation is directed along the axon, being transmitted to other cells (action potential).

Axotok (axoplasmic transport of substances). Nerve fibers have a peculiar structural apparatus - microtubules, through which substances move from the cell body to the periphery ( anterograde axotok) and from the periphery to the center ( retrograde axotok).

There are fast (at a rate of 100-1000 mm/day) and slow (at a rate of 1-10 mm/day) axotok. Quick axotok– the same for different fibers; requires a significant concentration of ATP; occurs with the participation of transport bubbles. It transports mediators and modulators. Slow axotok- due to it, biologically active substances, as well as components of cell membranes and proteins, spread from the center to the periphery.

nerve impulse is transmitted along the neuron membrane in a certain sequence: dendrite - perikaryon - axon.

Classification of neurons

1. According to morphology (by the number of processes), there are:

- multipolar neurons (d) - with many processes (most of them in humans),

- unipolar neurons (a) - with one axon,

- bipolar neurons (b) — with one axon and one dendrite (retina, spiral ganglion).

- false- (pseudo-) unipolar neurons (c) - the dendrite and axon depart from the neuron in the form of a single process, and then separate (in the spinal ganglion). This is a variant of bipolar neurons.

2. By function (by location in the reflex arc) they distinguish:

- afferent (sensory)) neurons (arrow on the left) - perceive information and transmit it to the nerve centers. Typical sensitive are false unipolar and bipolar neurons of the spinal and cranial nodes;

- associative (insert) neurons interact between neurons, most of them in the central nervous system;

- efferent (motor)) neurons (arrow on the right) generate a nerve impulse and transmit excitation to other neurons or cells of other types of tissues: muscle, secretory cells.

synapses

synapses - these are specific contacts of neurons that ensure the transfer of excitation from one nerve cell to another. Depending on the methods of transmission of excitation, chemical and electrical synapses are distinguished.

Evolutionary more ancient and primitive are electrical synaptic contacts . They are similar in structure to slot-like junctions (nexuses). It is believed that the exchange occurs in both directions, but there are cases when excitation is transmitted in one direction. Such contacts are often found in lower invertebrates and chordates. In mammals, electrical contacts are of great importance in the process of interneuronal interactions in the embryonic period of development. This type of contact in adult mammals occurs in limited areas, for example, they can be seen in the mesencephalic nucleus of the trigeminal nerve.

Chemical synapses . Chemical synapses for the transfer of excitation from one nerve cell to another use special substances - mediators from which they got their name. In addition to mediators, they also use modulators. Modulators are special chemicals that do not cause excitation themselves, but can either increase or decrease sensitivity to mediators (that is, modulate the cell's threshold sensitivity to excitation).

chemical synapse provides unidirectional transmission of excitation. The structure of a chemical synapse:

1) presynaptic zone- presynaptic extension, most often an axon terminal, which contains synaptic vesicles, cytoskeletal elements (neurotubules and neurofilaments), mitochondria;

2) synaptic cleft, which receives mediators from the presynaptic zone;

3) postsynaptic area is an electron-dense substance with receptors for a mediator on the membrane of another neuron .

FILM SYNAPSE

Synapse classification :

1. Depending on which structures of two neurons interact in the synapse, we can distinguish:

Axo-dendritic (presynaptic axon structure, postsynaptic - dendrite);

Axo-axonal;

Axo-somatic.

2. By function, they distinguish:

- exciting synapses, which lead to depolarization of the postsynaptic membrane and activation of the nerve cell;

- inhibitory synapses, which lead to hyperpolarization of the membrane, which reduces the threshold sensitivity of the neuron to external influences.

3. According to the main mediator contained in synaptic vesicles, synapses are divided into groups:

1. Cholinergic (acetylcholinergic): excitatory and inhibitory;
2. Adrenergic (monoaminergic, noradrenergic, dopaminergic): mainly excitatory, but there are also inhibitory ones;
3. Serotonergic (sometimes attributed to the previous group): excitatory;
4. GABA-ergic (mediator gamma-aminobutyric acid): inhibitory;
5. Peptidergic (mediators - a large group of substances, mainly: vasointerstitial polypeptide, vasopressin, substance P (pain mediator), neuropeptide Y, oxytocin, beta-endorphin and enkephalins (painkillers), dynorphin, etc.).

synaptic vesicles separated from the hyaloplasm by a single membrane. Choline-containing vesicles are electron-light, 40-60 µm in diameter. Adrenergic - with an electron-dense core, a light border, with a diameter of 50-80 microns. Glycine-containing and GABA-containing - have an oval shape. Peptide-containing - with an electron-dense core, a light border, with a diameter of 90-120 microns.

The mechanism of excitation transmission in a chemical synapse: an impulse arriving along an afferent fiber causes excitation in the presynaptic zone and leads to the release of a mediator through the presynaptic membrane. The mediator enters the synaptic cleft. On the postsynaptic membrane there are receptors for the neurotransmitter (cholinergic receptors for the mediator acetylcholine; adrenoreceptors for norepinephrine). Subsequently, the connection of mediators with receptors is broken. The mediator is either metabolized or reabsorbed by presynaptic membranes, or captured by astrocyte membranes with subsequent transfer of the mediator to nerve cells.

Regeneration of neurons. Neurons are characterized only by intracellular regeneration. They are a stable population of cells and do not divide under normal conditions. But there are exceptions. Thus, the ability to divide in nerve cells in the epithelium of the olfactory analyzer, in some ganglia (clusters of neurons of the autonomic nervous system) of animals has been proven.

neuroglia

neuroglia - a group of cells of the nervous tissue located between neurons, distinguish microglia and macroglia .

macroglia

Macroglia CNS subdivided into the following cells: astrocytes (fibrous and protoplasmic), oligodendrocytes and ependymocytes (including tanycytes).

Macroglia of the peripheral nervous system: satellite cells and lemmocytes (Schwann cells).

Functions of macroglia: protective, trophic, secretory.

Astrocytes - stellate cells, numerous processes of which branch out and surround other brain structures. Astrocytes are found only in the central nervous system and analyzers - derivatives of the neural tube.

Types of astrocytes: fibrous and protoplasmic astrocytes.

The process terminals of both cell types have button-like extensions (astrocyte pedicels), most of which terminate in the perivascular space, surrounding capillaries and forming perivascular glial membranes.

Fibrous astrocytes have numerous, long, thin, weakly or not at all branching processes. Mostly present in the white matter of the brain.

Protoplasmic astrocytes are characterized by short, thick and strongly branching processes. They are found predominantly in the gray matter of the brain. Astrocytes are located between the bodies of neurons, unmyelinated and myelinated parts of the nerve processes, synapses, blood vessels, subependymal spaces, isolating and at the same time structurally connecting them.

A specific marker of astrocytes is glial fibrillar acidic protein, from which intermediate filaments are formed.

Astrocytes have relatively large light nuclei, with a poorly developed nucleolar apparatus. The cytoplasm is weakly oxyphilic, it has poorly developed aER and rER, the Golgi complex. Mitochondria are few and small. The cytoskeleton is moderately developed in protoplasmic and well developed in fibrous astrocytes. There are a significant number of slit-like and desmosome-like contacts between cells.

In the postnatal period of a person's life, astrocytes are capable of migration, especially to areas of damage and are capable of proliferation (they form benign astrocytoma tumors).

Main functions of astrocytes: participation in blood-brain and liquor-hematic barriers(they cover capillaries, brain surfaces with their processes and participate in the transport of substances from blood vessels to neurons and vice versa), in this regard, they perform protective, trophic, regulatory functions; phagocytosis of dead neurons, secretion of biologically active substances: FGF, angiogenic factors, EGF, interleukin-I, prostaglandins.

Oligodendrocytes – cells with few processes , capable of forming myelin sheaths around the bodies and processes of neurons. Oligodendrocytes are located in the gray and white matter of the central nervous system, in the peripheral nervous system there are varieties of oligodendrocytes - lemmocytes (Schwann cells). Oligodendrocytes and their varieties are characterized by the ability to form a membrane duplication - mesaxon, which surrounds the process of the neuron, forming a myelin or non-myelin sheath.

The nuclei of oligodendrocytes are small, rounded, dark-colored, processes are thin, do not branch or branch slightly. At the electron-optical level, organelles, especially the synthetic apparatus, are well developed in the cytoplasm, and the cytoskeleton is poorly developed.

Some oligodendrocytes are concentrated in close proximity to the bodies of nerve cells ( satellite or mantle oligodendrocytes). The terminal zone of each process is involved in the formation of a segment of the nerve fiber, that is, each oligodendrocyte provides an environment for several nerve fibers at once.

Lemmocytes (Schwann cells) ) of the peripheral nervous system are characterized by elongated, dark-colored nuclei, poorly developed mitochondria and a synthetic apparatus (granular, smooth ER, lamellar complex). Lemmocytes surround the processes of neurons in the peripheral nervous system, forming myelinated or unmyelinated sheaths. In the area of ​​formation of the roots of the spinal and cranial nerves, lemmocytes form clusters (glial plugs), preventing the penetration of the processes of associative CNS neurons beyond its limits.

In the peripheral nervous system, in addition to lemmocytes, There are other types of oligodendrocytes: satellite (mantle) gliocytes in peripheral ganglions around the bodies of neurons, gliocytes of nerve endings, specific morphological features of which are considered in the study of nerve endings and anatomy of nerve nodes.

The main functions of oligodendrocytes and their varieties: forming myelinated or non-myelinated sheaths around neurons, provide isolating, trophic, supporting, protective functions; participate in the conduction of a nerve impulse, in the regeneration of damaged nerve cells, phagocytosis of the remnants of axial cylinders and myelin in violation of the structure of the axon distal to the site of injury.

Ependymocytes , or ependymal glia - low-prismatic cells that form a continuous layer covering the brain cavities. Ependymocytes are closely adjacent to each other, forming dense, slit-like and desmosomal junctions. The apical surface contains cilia, which in most cells are then replaced by microvilli. The basal surface has basal invaginations, as well as long thin processes (from one to several), which penetrate to the perivascular spaces of the brain microvessels.

In the cytoplasm of ependymocytes, mitochondria, a moderately developed synthetic apparatus are found, the cytoskeleton is well represented, there is a significant amount of trophic and secretory inclusions.

Ependymal glial variant are tanycytes . They line the choroid plexuses of the ventricles of the brain, the subcommissural organ of the posterior commissure. Actively participate in the formation of liquor (cerebrospinal fluid). Characterized by the fact that the basal part contains thin long processes.

Main functions of ependymocytes: secretory (synthesis of cerebrospinal fluid), protective (ensuring hemato-liquor barrier), supporting, regulatory (precursors of tanycytes direct the migration of neuroblasts in the neural tube in the embryonic period of development).

microglia

Microgliocytes or neural macrophages – small cells of mesenchymal origin (derivatives of monocytes), diffusely distributed in the CNS, with numerous strongly branching processes, are capable of migration. Microgliocytes are specialized macrophages of the nervous system. Their nuclei are characterized by the predominance of heterochromatin. Many lysosomes, lipofuscin granules are found in the cytoplasm; the synthetic apparatus is moderately developed.

Functions of microglia: protective (including immune).

Nerve fibers

A nerve fiber consists of a process of a neuron axle cylinder(dendrite or axon) and membranes of an oligodendrocyte or its varieties.

Types of nerve fibers:

1) Depending on how the sheath was formed, nerve fibers are divided into myelinated And unmyelinated.

In the peripheral nervous system, nerve fibers surround lemmocytes. One lemmocyte is associated with one nerve fiber. In the central nervous system, neuronal processes surround oligodendrocytes. Each oligodendrocyte is involved in the formation of several nerve fibers.

myelination fibers is carried out by elongation and "winding" of the mesaxon around the process of the nerve cell (in the peripheral nervous system) or by elongation and rotation of the process of the oligodendrocyte around the axial cylinder in the CNS.

myelinated (pulp) fibers in the peripheral nervous system have one neuron process surrounded by an elongated lemmocyte duplication (mesaxon). In the myelin fiber, the mesaxon repeatedly wraps around the axial cylinder, forming multiple turns of the membrane - myelin. The zones of myelin loosening (penetration of the lemmocyte cytoplasm) are called notches(Schmidt-Lanterman). Each lemmocyte forms a fiber segment, the border areas of neighboring cells are unmyelinated and are called interceptions of Ranvier Thus, along the length of the fiber, the myelin sheath has an intermittent course. The myelin sheath is a biological insulator. The spread of depolarization in the myelin fiber is carried out in jumps from node to node.

unmyelinated (non-fleshy) fibers in the peripheral nervous system consist of one or more axial cylinders immersed in the cytolemma of the surrounding lemmocyte. Mesaxon (membrane duplication) is short. The transmission of excitation in unmyelinated fibers occurs along the surface of the nerve through a change in surface charge.

2) Depending on the speed of the nerve impulse, the following types of nerve fibers are distinguished:

1. Type A has subgroups:

- Aa- have the highest speed of conduction of excitation - 70-120 m / s (somatic motor nerve fibers);

- Ab- the speed of conducting is 40-70 m / s. These are somatic afferent nerves and some efferent somatic nerves;

- Ag- conduction speed is 15-40 m/s - afferent and efferent sympathetic and parasympathetic nerves;

- Ad(delta) - the speed of conducting 5-18 m / s. This group of afferent somatic nerves carries primary (rapid) pain.

1. Type B - conduction velocity from 3 to 14 m / s - preganglionic sympathetic fibers, some parasympathetic fibers, that is, these are autonomic nerves.
2. Type C - conduction speed 0.5-3 m/s: postganglionic vegetative fibers (non-myelinated). Spend pain impulses of slow secondary pain (from the receptors of the pulp of the tooth).

Neurogenesis. On the 15-17th day of intrauterine development of a person under the inducing influence of the chord from primary ectoderm the neural plate is formed (an accumulation of longitudinally lying cellular material). From the 17th to the 21st day, the plate invaginates and first turns into neural groove and then in handset. By the 25th day of embryogenesis, the neural tube splits off from the ectoderm and the anterior and posterior openings (neuropores) close. On the sides of the neural groove are located neural crest structures.

In the early stages of development, the neural tube is formed meduloblasts - stem cells of the nervous tissue of the CNS. It is formed from the neural crest ganglion plate consisting of ganglioblasts– stem cells of neurons and neuroglia of the peripheral nervous system. Meduloblasts and ganglioblasts immigrate intensively, divide and then differentiate.

In the early stages of intrauterine development, the neural tube is a layer of process cells lying in the form of a single layer, but in several rows. They are bounded internally and externally by boundary membranes. On the inner surface (adjacent to the cavity of the neural tube), meduloblasts divide.

Subsequently the neural tube forms several layers . Among them are:

- Inner limiting membrane: separates the cavity of the neural tube from the cells;

- ependymal layer(ventricular in the region of the cerebral vesicles) is represented by blast progenitor cells of macroglia;

- Subventricular zone(only in the anterior cerebral vesicles), where the proliferation of neuroblasts occurs;

- Mantle (cloak) layer containing migrating and differentiating neuroblasts and glioblasts;

- Marginal layer(marginal veil) is formed by processes of glioblasts and neuroblasts. In it you can see the bodies of individual cells.

- Outer boundary membrane.

Differentons of the nervous tissue of the central nervous system

1. Neuron differon: meduloblast - neuroblast - young neuron - mature neuron.
   1. Astrocyte differon: meduloblast - spongioblast - astroblast - protoplasmic or fibrous astrocyte.
   2. Oligodendrocyte diferron: meduloblast - spongioblast - oligodendroblast - oligodendrocyte.
   3. Differon of ependymal glia: medulobast - ependymoblast - ependymocyte or tanycyte.
   4. Differon microglia: blood stem cell - half-stem blood cell (CFU HEMM) - CFU GM - CFU M - monoblast - promonocyte - monocyte - resting microgliocyte - activated microgliocyte.

Nervous tissue differons in the peripheral nervous system

1. Neuron differon: ganglioblast - neuroblast - young neuron - mature neuron.

2. Differon of a lemmocyte: ganglioblast - glioblast - lemmocyte (Schwann cell).

Mechanisms of neurogenesis. In the process of intrauterine development, neuroblasts migrate to the anatomical anlages of the nerve centers. At the same time, they stop sharing. In the CNS, neuroblast migration is controlled by adhesive intercellular interactions (with the help of cadherins and integrins of radial glia), signaling molecules of the intercellular substance (including fibronectins and laminins). After neuroblasts reach their area of ​​permanent localization, they begin to differentiate and form processes. The direction of growth of the processes is also controlled by the mentioned adhesive molecules (cadherins, integrins, signaling molecules of the intercellular substance).

In fetal development and after birth, there is a competitive interaction between similar neurons of the nerve centers. In this case, nerve cells that did not have time to occupy the corresponding zone or form contacts undergo apoptosis. In early development, from a third to a half of nerve cells die.

In subsequent development, a glial environment is formed around nerve cells and myelination of nerve fibers occurs. Nerve cells continue to form processes and synaptic contacts until puberty. The maximum development of the nervous tissue reaches 25-30 years.

With age, there is a death of some nerve cells and compensatory hypertrophy of others. Lipofuscin can accumulate in neurons. Areas with dead nerve cell bodies are replaced by glial scars formed by accumulations of hypertrophied astrocytes.

The dendrites branch heavily, forming a dendritic tree, and are usually shorter than the axon. From the dendrites, the excitation is directed to the body of the nerve cell. They form postsynaptic structures that perceive excitation. There are many dendrites, but there may be one. An axon is always present, one for each nerve cell. It does not branch or branches weakly in the terminal areas and ends with a synaptic bud that transmits excitation to other cells (presynaptic zone). Neurons transmit excitation using specialized contacts (synapses). The substance that provides the transfer of excitation is called mediator. In each neuron, one main mediator is usually found.

Regeneration of nerve fibers in the peripheral nervous system

After transection of the nerve fiber, the proximal part of the axon undergoes ascending degeneration, the myelin sheath in the area of ​​damage disintegrates, the perikaryon of the neuron swells, the nucleus shifts to the periphery, and the chromatophilic substance disintegrates. The distal part associated with the innervated organ undergoes a downward degeneration with complete destruction of the axon, disintegration of the myelin sheath, and phagocytosis of detritus by macrophages and glia. Lemmocytes persist and mitotically divide, forming strands - Büngner's bands. After 4-6 weeks, the structure and function of the neuron is restored, thin branches grow distally from the proximal part of the axon, growing along the Büngner bands. And as a result of the regeneration of the nerve fiber, the connection with the target organ is restored. If an obstacle arises on the path of the regenerating axon (for example, a connective tissue scar), innervation does not recover.

With additions from the teaching aid "General histology" (compilers: Shumikhina G.V., Vasiliev Yu.G., Solovyov A.A., Kuznetsova V.M., Sobolevsky S.A., Igonina S.V., Titova I. .V., Glushkova T.G.)

3.1.1. laying of the nervous system

The central and peripheral parts of the human nervous system develop from a single embryonic source of ectoderm. In the process of development of the embryo, it is laid in the form of the so-called neural plate of a group of high, rapidly multiplying cells along the midline of the embryo. On the. At the 3rd week of development, the neural plate plunges into the underlying tissue, takes the form of a groove, the edges of which rise slightly above the level of the ectoderm in the form of neural folds. As the embryo grows, the neural groove elongates and reaches the caudal end of the embryo. On the 19th day of development, the process of closing of the neural folds over the groove begins, resulting in the formation of a long hollow tube, the neural tube, located directly under the surface of the ectoderm, but separate from the latter.

When the neural groove closes into a tube and its edges grow together, the material of the neural folds is sandwiched between the neural tube and the skin ectoderm closing over it. In this case, the cells of the neural folds are redistributed into one layer, forming a ganglionic plate with a very wide potential for development. All nerve nodes of the somatic peripheral and autonomic nervous systems, including intraorganic nerve elements, are formed from this embryonic rudiment.

The process of neural tube closure begins at the level of the 5th segment, spreading both cephalically and caudally. By the 24th day of development, it ends in the head part, a day later in the caudal part. The caudal end of the neural tube temporarily closes with the hindgut, forming the neuroenteric canal.

The formed neural tube at the head end, at the site of the formation of the future brain, expands. Its thinner caudal part is transformed into the spinal cord.

In parallel with the formation of the neural tube, the formation of other structures (notochord, mesoderm) occurs, which, together with the neural tube, constitute the so-called complex of axial primordia. With the formation of a complex of axial rudiments, the human embryo, previously deprived of an axis of symmetry, acquires bilateral symmetry. Now the head and caudal sections, the right and left halves of the body are already quite clearly distinguishable in it.

The development of various parts of the central and peripheral nervous systems in the prenatal ontogeny of a person occurs unevenly. The central nervous system goes through a particularly difficult path of development.

The cells of the formed neural tube, which in their further development will give rise to both neurons and gliocytes, are called medulloblasts. Cellular elements of the ganglion plate, which apparently have the same histogenetic potency, are called ganglioblasts. It should be noted that at the initial stages of differentiation of the neural tube and ganglionic plate, their cellular composition is homogeneous.

In their further differentiation, medulloblasts are determined partly in a neutral direction, turning into neuroblasts, partly in a neuroglial direction, forming spongioblasts.

Neuroblasts differ from neurons in their significantly smaller size, the absence of dendrites and synaptic connections (hence, they are not included in reflex arcs), and the absence of Nissl substance in the cytoplasm. However, they already have a weakly expressed neurofibrillary apparatus, a developing axon, and are characterized by the lack of the ability to mitotic division.

In the spinal region, the primary neural tube divides early into three layers: inner ependymal, intermediate mantle (or mantle) and outer light marginal veil.

The ependymal layer gives rise to neurons and glial cells (ependymoglia) of the central nervous system. Neuroblasts are found in its composition, which subsequently migrate to the mantle layer. The cells remaining in the ependymal layer attach to the inner limiting membrane, send out processes, thereby participating in the formation of the outer limiting membrane. They are called spongioblasts, which, if they lose contact with the inner and outer boundary membranes, will turn into astrocytoblasts. Those cells that retain their connection with the inner and outer boundary membranes will turn into ependymal gliocytes that line the central canal of the spinal cord and the cavities of the ventricles of the brain in an adult. They acquire in the process of differentiation cilia, which contribute to the flow of cerebrospinal fluid.

The ependymal layer of the neural tube, both in the trunk and in its head, retains until relatively late stages of embryogenesis the potential to form very diverse tissue elements of the nervous system.

In the mantle layer of the developing neural tube, neuroblasts and spongioblasts are located, which, with further differentiation, give astroglia and oligodendroglia. This layer of the neural tube is the widest and richest in cellular elements.

Marginal veil The outer, lightest layer of the neural tube does not contain cells, being filled with their processes, blood vessels, and mesenchyme.

A feature of the cells of the ganglion plate is that their differentiation is preceded by a period of migration to areas of the body of the embryo more or less remote from their initial localization. The cells that make up the anlage of the spinal nodes undergo the shortest migration. They descend a short distance and are located on the sides of the neural tube, first in the form of loose, and then denser large formations. In a human embryo of 6-8 weeks of development, the spinal nodes are very large formations consisting of large process neurons surrounded by oligodendroglia. Over time, the neurons of the spinal ganglia transform from bipolar to pseudo-unipolar. Cell differentiation within the ganglia occurs asynchronously.

Those cells that migrate from the ganglion plate to the ganglia of the border sympathetic trunk, the ganglia of the prevertebral localization, and also to the adrenal medulla undergo a much more separated migration. Especially great is the length of the migration paths of neuroblasts invading the wall of the intestinal tube. From the ganglion plate, they migrate along the branches of the vagus nerve, reach the stomach, thin and most cranial parts of the large intestine, giving rise to intramural ganglia. It is precisely such a long and complex path of migration of structures that in situ control the process of digestion that explains the frequency of various kinds of lesions of this process that occur both in utero and after the slightest violation of the diet of a child, especially a newborn or a child of the first months of life.

The head end of the neural tube, after its closure, very quickly subdivides into three extensions - the primary cerebral vesicles. The timing of their formation, the rate of cell differentiation and further transformations in humans are very high. This allows us to consider cephalization, the advanced and predominant development of the head section of the neural tube, as a species trait of a person.

The cavities of the primary cerebral vesicles are preserved in the brain of a child and an adult in a modified form and form the cavities of the ventricles and the Sylvian aqueduct.

The most rostral section of the neural tube is the forebrain (prosencephalon); it is followed by the middle (mesencephalon) and rear (rhombencephalon). In subsequent development, the forebrain is divided into the final (telencephalon), including the cerebral hemispheres and some basal nuclei, and the intermediate (diencephalon). On each side of the diencephalon grows an eye bubble that forms the nerve elements of the eye. The midbrain is preserved as a whole, but in the process of development, significant changes occur in it associated with the formation of specialized reflex centers related to the work of the sense organs: vision, hearing, tactile, pain and temperature sensitivity.

The rhomboid brain is divided into the posterior (metencephalon), including the cerebellum and bridge, and the medulla oblongata (myelencephalon) brain.

One of the important neurohistological characteristics of the development of the nervous system of higher vertebrates is the asynchronous differentiation of its divisions. Neurons of different parts of the nervous system and even neurons within the same center differentiate asynchronously: a) differentiation of neurons in the autonomic nervous system lags far behind that in the main parts of the somatic system; b) the differentiation of sympathetic neurons somewhat lags behind the development of parasympathetic ones.

First of all, maturation of the medulla oblongata and spinal cord occurs, later the ganglia of the brain stem, subcortical nodes, the cerebellum and the cerebral cortex develop morphologically and functionally. Each of these formations goes through certain stages of functional and structural development. So, in the spinal cord, elements in the region of the cervical enlargement mature earlier, and then there is a gradual development of cellular structures in the caudal direction; spinal motoneurons differentiate first, then sensitive neurons, and lastly, intersegmental neurons and intersegmental pathways. The nuclei of the brain stem, diencephalon, subcortical ganglia, cerebellum and individual layers of the cerebral cortex also structurally develop in a certain sequence and in close connection with each other. Consider the development of individual areas of the nervous system.

In the early stages of development of the human embryo, the neural plate arises from the cells of the ectoderm, formed by a single-layer single-row prismatic epithelium (neuroepithelium), under which there is a chord that induces the formation of the neural plate (Fig. 224). The neural plate grows rapidly, thickens, becomes multi-layered, deepens, forming a groove, the edges of which rise and turn into neural folds. Neural crests are formed under the rollers - outgrowths in the form of strands of cells, which, after closing the groove into the neural tube, turn into ganglionic plates, located on the side of the neural tube and separated from it. The neural tube also separates from the ectoderm. After the formation of the tube, neuroepithelial cells differentiate into subventricular nerve cells - neuroblasts, the number of which rapidly increases due to active proliferation. These cells form the mantle layer. From the same cells, the primary supporting cells arise - glioblasts, which migrate into the mantle layer. Subsequently, the gray matter of the brain is formed from the mantle layer. Mitotic division of neuroblasts ends before the formation of processes. First, the growth of the axon begins, later - the dendrites. The processes of neuroblasts form a marginal (marginal) layer on the periphery of the neural tube, from which white matter is formed. Ventricular cells located on the inner surface of the neural tube differentiate into tanycytes and epithelioid ependymocytes. At the neural tube stage, the ganglionic laminae fragment to form rounded structures from which the spinal ganglia are formed.

So, three layers of the neural tube wall give rise to the ependyma lining the cavity of the central nervous system (internal), gray matter (middle, mantle) and white matter (outer) (Table 38). The lateral sections of the tube grow more intensively, from their ventral sections arise the anterior columns of gray matter (cell bodies and fibers) and the adjacent white matter (only nerve fibers). From the dorsal parts of the neural tube, the posterior columns of gray matter and the white matter of the spinal cord are formed. The head section of the neural tube grows unevenly. In some areas it is thicker, due to the increased longitudinal growth, it bends. Already at the 4th week of embryonic development, three primary cerebral vesicles are distinguished: anterior, middle and posterior. By the end of the 4th week, the anterior cerebral vesicle begins to divide into two: the telencephalon, from which the entire cerebral cortex subsequently develops, and the intermediate, from which the thalamus and hypothalamus develop. The lumen of the forebrain tube forms the lateral and III ventricles. The posterior (rhomboid bladder) also divides into two bladders during the 5th week, from which the cerebellum, medulla oblongata and pons are formed. From the middle bladder, which retains a tubular shape, the midbrain is formed, the lumen of the tube is the cerebral (Sylvian) aqueduct. As a result, the future brain consists of five bubbles (Fig. 225). In the area of ​​the middle brain bladder, the legs of the brain and the plate of the roof of the middle brain are formed. The lateral walls of the diencephalon grow, forming thalamus, outgrowths of the lateral walls give rise to eye vesicles. The lower wall of the diencephalon protrudes, forming a gray tubercle, funnel, hypothalamus (hypothalamus) and the posterior lobe of the pituitary gland. The origin of the various parts of the brain is presented in Table. 39.

Important transformations take place in the telencephalon. At stage I, olfactory structures and the limbic system (paleocortex) are formed, located around the edges of the developing telencephalon; at stage II, the walls of the forebrain thicken due to the intensive proliferation of neuroblasts, the beginnings of the basal ganglia appear; finally, at stage III, the cerebral cortex (neocortex) is formed. In connection with the active mitotic division of neocortical neuroblasts, when the rate of cell formation reaches 250,000 cells per minute, the formation of cerebral sulci and convolutions of the cerebral hemispheres begins. The mass of the brain of a newborn child is relatively large, it averages 390 g (340 - 430) in boys and 355 g (330 - 370) in girls (12 - 13% of body weight, in an adult - about 2.5%). The ratio of the mass of the brain of a newborn to the mass of his body is five times greater than that of an adult, respectively 1: 8 and 1:40. During the first year of life, the mass of the brain doubles, and by 3. At the age of 4, it triples, then it slowly increases and by the age of 20-29 reaches its maximum figures (1355 g for men and 1220 g for women). By the age of 20 - 25 and thereafter, up to 60 years for men and 55 years for women, the mass of the brain does not change significantly, after 55 - 60 years it decreases slightly. Up to 4 years of age, the child's brain grows evenly in height, length and width, and then the growth of the brain in height predominates. The frontal and parietal lobes grow most rapidly.

In a newborn child, phylogenetically older parts of the brain are better developed. The mass of the brain stem is 10 - 10.5 g (about 2.7% of body weight, in an adult - about 2%). By the time of the birth of the child, the medulla oblongata, the bridge and their nuclei are well developed, the mass of the first is about 4 - 5 g, the second - 3.5 - 4 g. The cerebellum, especially its hemispheres, is worse developed, the worm is better, the convolutions and furrows of the hemispheres are poorly expressed cerebellum. The mass of the cerebellum of a newborn child does not exceed 20 g (5.4% of body weight, in an adult - 10%). During the first 5 months of life, the mass of the cerebellum triples, at 9 months, when the child can stand and begins to walk,. four times. The hemispheres of the cerebellum develop most intensively. The diencephalon in a newborn is also relatively well developed. The formation of furrows and convolutions begins in the fetus from the 5th month of development. In a 7-month-old fetus, furrows and convolutions are already visible, by the time of birth they are fully developed (F.I. Valker, 1951), however, the branches of the main furrows and small convolutions are poorly expressed. The formation of the relief of the hemispheres continues during the first 6-7 years of life, the furrows become deeper, the gyrus between them is more embossed (VV Bunak, 1936). In a newborn child, the temporal lobes and the olfactory brain are most developed, the frontal ones are weaker. In a newborn child, the cerebral cortex is not fully differentiated. The ventricles of the brain of a newborn child are relatively larger than those of an adult. The hard shell of the brain of a newborn child is thin, tightly fused with the bones of the skull, its processes are poorly developed. The sinuses are thin-walled, relatively wide. After 10 years, the structure and topography of the sinuses are the same as in an adult. The arachnoid and soft membranes of the brain and spinal cord in a newborn are thin, delicate. The subarachnoid space is relatively wide.

private histology.

Private histology- the science of the microscopic structure and origin of organs. Each organ is made up of 4 tissues.

Organs of the nervous system.

On a functional basis

1. somatic nervous system- participates in the innervation of the human body and higher nervous activity.

a. Central department:

i. Spinal cord - nuclei of the posterior and anterior horns

ii. Brain - cerebellar cortex and cerebral hemispheres

b. Peripheral department:

i. spinal ganglia

ii. cranial ganglia

iii. nerve trunks

2. autonomic nervous system- provides the work of internal organs, innervates smooth myocytes and represents the secretory nerves.

1) sympathetic:

a. Central department:

i. Spinal cord - nuclei of the lateral horns of the thoraco-lumbar region

ii. brain - hypothalamus

b. Peripheral department:

i. Sympathetic ganglia

ii. nerve trunks

2) Parasympathetic:

a. Central department:

i. Spinal cord - nuclei of the lateral horns of the sacral region

ii. Brain - brainstem nuclei, hypothalamus

b. Peripheral department:

i. parasympathetic ganglia

ii. nerve trunks

iii. Spinal and cranial ganglia

Anatomically The organs of the nervous system are divided into:

1. Peripheral nervous system.

2. Central nervous system.

Embryonic sources of development:

1. neuroectoderm(gives rise to the parenchyma of organs).

2. mesenchyme(gives rise to the stroma of organs, a set of auxiliary structures that ensure the functioning of the parenchyma).

The organs of the nervous system function in relative isolation from the environment, separating from it. biological barriers. Types of biological barriers:

1. Hematoneural (delimits blood from neurons).

2. Liquoroneural (delimits cerebrospinal fluid from neurons).

3. Hematoliquor (delimits cerebrospinal fluid from blood).

Functions of the nervous system:

1. Regulation of the functions of individual internal organs.

2. Integration of internal organs into organ systems.

3. Ensuring the relationship of the organism with the external environment.

4. Ensuring higher nervous activity.

All functions are based on the principle reflex. The material basis is reflex arc, consisting of 3 links: afferent, associative And efferent. They are distributed to individual organs of the nervous system.

Organs of the peripheral nervous system:

1. Nerve trunks (nerves).

2. Nerve nodes (ganglia).

3. Nerve endings.

nerve trunks - these are bundles of nerve fibers, united by a system of connective tissue membranes. The nerve trunks are mixed, i.e. each has myelin and amyelin fibers, as a result of which the somatic and autonomic nervous systems are served.

The structure of the nerve trunk:

1. Parenchyma: unmyelinated and myelinated nerve fibers + microganglia.

2. Stroma: connective tissue membranes:

1) Perineurium(perineural sheaths: RVNST + blood vessels + ependymogliocytes + cerebrospinal fluid).

2) epineurium(PVNST + blood vessels).

3) Perineurium(cleavage from the epineurium into the trunk).

4) Endoneurium(RVNST + blood vessels).

There is a slit-like space in the perineurium - slit-like perineural sheath which is filled liquor(circulating biological fluid). Structural components of the walls of the perineural sheath:

1. Low-prismatic ependymogliocytes.

2. Basement membrane.

3. Subependymal plate.

4. Blood vessels.

Liquor in the perineural vagina may be absent. They are sometimes injected with anesthetics, antibiotics (because the disease spreads through them).

Functions of nerve trunks:

1. Conduction (conduct a nerve impulse).

2. Trophic (nutritional).

4. They are the initial link in the secretion and circulation of cerebrospinal fluid.

Regeneration of nerve trunks:

1. Physiological regeneration(very active restoration of membranes due to fibroblasts).

2. Reparative regeneration(that section of the nerve trunk is restored, the nerve fibers of which have not lost contact with the perikaryon - they are able to grow by 1 mm / day; peripheral segments of the nerve fibers are not restored).

Nerves (ganglia) - groups or cooperations of neurons, taken out of the brain. The nerve nodes are "dressed" in capsules.

Ganglia types:

1. Spinal.

2. cranial.

3. Vegetative.

spinal ganglia - thickenings on the initial sections of the posterior roots of the spinal cord; this is a cluster of afferent (sensitive) neurons (they are the first neurons in the reflex arc chain).

The structure of the spinal ganglion:

1. Stroma:

1) external connective tissue capsule, consisting of 2 sheets:

a. outer sheet (dense connective tissue - continuation of the epineurium of the spinal nerve)

b. inner sheet (multi-tissue: RVNST, gliocytes; analogue of the perineurium of the spinal nerve; there are splits passing to the intraorgan septa, filled with cerebrospinal fluid).

2) intraorgan partitions extending from the capsule into the node

b. blood and lymph vessels

c. nerve fibers

d. nerve endings

3) own connective tissue capsules of pseudo-unipolar neurons

a. fibrous connective tissue

b. single layered squamous ependymoglial epithelium

c. perineuronal space with cerebrospinal fluid

2. Parenchyma:

1) the central part (myelinated nerve fibers - processes of pseudo-unipolar neurons)

2) peripheral part (pseudounipolar neurons + mantle gliocytes (oligodendrogliocytes)).

Functions of the spinal ganglion:

1. Participation in reflex activity (the first neurons in the reflex arc circuit).

2. They are the initial link in the processing of afferent information.

3. Barrier function (hematoneural barrier).

4. They are a link in the circulation of cerebrospinal fluid.

Sources of embryonic development of the spinal ganglion:

1. Ganglionic plate (gives rise to the elements of the parenchyma of the organ).

2. Mesenchyme (gives rise to the elements of the stroma of the organ).

Ganglia of the autonomic nervous system - located after the spinal cord, participate in the creation of autonomic arches.

Types of ganglia of the autonomic nervous system:

1. Sympathetic:

1) Paravertebral;

2) Prevertebral;

2. Parasympathetic:

1) Intraorganic (intramural);

2) Perioorganic (paraorganic);

3) Vegetative nodes of the head (along the cranial nerves).

The structure of the ganglia of the autonomic nervous system:

1. Stroma: the structure is similar to the stroma of the spinal ganglion.

2.1. Parenchyma of sympathetic ganglia: neurons located randomly throughout the ganglion + satellite cells + connective tissue capsule.

1) large long-axon multipolar efferent adrenergic neurons

2) small equidistant multipolar associative adrenergic intensely fluorescing (MIF) - neurons

3) preganglionic myelin cholinergic fibers (axons of neurons of the lateral horns of the spinal cord)

4) postganglionic non-myelinated adrenergic nerve fibers (axons of large ganglion neurons)

5) intraganglionic non-myelinated associative nerve fibers (axons of the MIF - neurons).

2.2. Parenchyma of the parasympathetic ganglia:

1) long-axon multipolar efferent cholinergic neurons (Dogel type I).

2) long-dendritic multipolar afferent cholinergic neurons (Dogel type II): dendrite - to the receptor, axon - to types 1 and 3.

3) equidistant multipolar associative cholinergic neurons (Dogel type III).

4) preganglionic myelin cholinergic nerve fibers (axons of the lateral horns of the spinal cord).

5) postganglionic non-myelinated cholinergic nerve fibers (axons of Type I Dogel neurons).

Functions of the ganglia of the autonomic nervous system:

1. sympathetic:

1) Conducting impulses to the working bodies (2.1.1)

2) Impulse propagation within the ganglion (braking effect) (2.1.2)

2. Parasympathetic:

1) Conducting an impulse to the working bodies (2.2.1)

2) Conducting an impulse from interoreceptors within local reflex arcs (2.2.2)

3) impulse propagation within or between ganglia (2.2.3).

Sources of embryonic development of the ganglia of the autonomic nervous system:

1. Ganglion plate (neurons and neuroglia).

2. Mesenchyme (connective tissue, blood vessels).

In the development of the nervous system is associated with both motor activity and the degree of activity of the GNI.

In humans, there are 4 stages of development of the nervous activity of the brain:

1. Primary local reflexes are a "critical" period in the functional development of the nervous system;
2. Primary generalization of reflexes in the form of fast reflex reactions of the head, trunk and limbs;
3. Secondary generalization of reflexes in the form of slow tonic movements of the entire muscles of the body;
4. Specialization of reflexes, expressed in coordinated movements of individual parts of the body.
5. Unconditioned reflex adaptation;
6. Primary conditioned reflex adaptation (formation of summation reflexes and dominant acquired reactions);
7. Secondary conditioned reflex adaptation (the formation of conditioned reflexes based on associations - a “critical” period), with a vivid manifestation of orienting-exploratory reflexes and game reactions that stimulate the formation of new conditioned reflex connections such as complex associations, which is the basis for intraspecific (intragroup) interactions of developing organisms;
8. Formation of individual and typological features of the nervous system.

Bookmark and development of the human nervous system:

I. Stage of the neural tube. The central and peripheral parts of the human nervous system develop from a single embryonic source - the ectoderm. During the development of the embryo, it is laid in the form of the so-called neural plate. The neural plate consists of a group of tall, rapidly proliferating cells. In the third week of development, the neural plate plunges into the underlying tissue and takes the form of a groove, the edges of which rise above the ectoderm in the form of neural folds. As the embryo grows, the neural groove elongates and reaches the caudal end of the embryo. On the 19th day, the process of closing the ridges over the groove begins, resulting in the formation of a long tube - the neural tube. It is located under the surface of the ectoderm separately from it. The cells of the neural folds are redistributed into one layer, resulting in the formation of the ganglionic plate. All the nerve nodes of the somatic peripheral and autonomic nervous system are formed from it. By the 24th day of development, the tube closes in the head part, and a day later, in the caudal part. The cells of the neural tube are called medulloblasts. The cells of the ganglion plate are called ganglioblasts. Medulloblasts then give rise to neuroblasts and spongioblasts. Neuroblasts differ from neurons in their significantly smaller size, lack of dendrites, synaptic connections, and Nissl substance in the cytoplasm.

II. Brain bubble stage. At the head end of the neural tube, after its closure, three extensions are very quickly formed - the primary cerebral vesicles. The cavities of the primary cerebral vesicles are preserved in the brain of a child and an adult in a modified form, forming the ventricles of the brain and the Sylvian aqueduct. There are two stages of brain bubbles: the three bubble stage and the five bubble stage.

III. The stage of formation of brain regions. First, the anterior, middle and rhomboid brain are formed. Then, the hindbrain and medulla oblongata are formed from the rhomboid brain, and the telencephalon and diencephalon are formed from the anterior. The telencephalon includes two hemispheres and part of the basal ganglia.