Functional morphology of the nervous system

At the heart of the complex function of the nervous system is its special morphology.

In the prenatal period, the nervous system is formed and develops earlier and faster than other organs and systems. At the same time, the laying and development of other organs and systems goes synchronously with the development of certain structures of the nervous system. This process of systemogenesis, according to PK Anokhin, leads to functional maturation and interaction of dissimilar organs and structures, which ensures the respiratory, food, motor and other functions of life support of the organism in the postnatal period.

Morphogenesis of the nervous system can be conditionally divided into proper morphogenesis, ie, with. The consistent emergence of new structures of the nervous system at the appropriate gestational age, this process is only intrauterine, and functional morphogenesis. Actually, morphogenesis includes further growth, the development of the nervous system with an increase in the mass and volume of individual structures, which is due not to an increase in the number of nerve cells, but to the growth of their bodies and processes, myelination processes, the proliferation of glial and vascular elements. These processes partially continue throughout the childhood period.

The brain of a newborn is one of the largest organs and weighs 340-400. AF Tour pointed out that the brain of boys is heavier than that of girls by 10-20 g. By the age of one year, the weight of the brain is about 1000 grams to nine For years, the brain weighs 1300 g on average, and the last 100 is acquired in the period from nine to 20 years.

Functional morphogenesis begins and ends later than the proper morphogenesis, which leads to a longer period of childhood in humans compared to animals.

Concerning the development of the brain, it should be noted the work of BN Klossovsky, who considered this process in connection with the development of its feeding systems - liquor and blood. In addition, there is a clear correspondence between the development of the nervous system and the formation protecting it - shells, skull structures of the skull and spine, etc.

Morphogenesis

In ontogenesis, the elements of the human nervous system develop from embryonic ectoderm (neurons and neuroglia) and mesoderm (membranes, vessels, mesoglium). By the end of the third week of development, the human embryo has the form of an oval plate about 1.5 cm long. At this time, a nerve plate is formed from the ectoderm , which is located longitudinally along the dorsal side of the embryo. As a result of uneven reproduction and densification of the neuroepithelial cells, the middle part of the plate bends and a nerve groove appears that deepens into the embryo's body. Soon the edges of the nerve groove are closed, and it turns into a neural tube, separated from the skin ectoderm. On the sides of the nerve groove on each side a group of cells is allocated; it forms a continuous layer between the nerve beads and the ectoderm - the ganglion plate. It serves as the starting material for cells of sensitive nerve nodes (cranial, spinal) and nodes of the autonomic nervous system.

In the formed neural tube, three layers can be distinguished: the inner ependymic layer - its cells are actively dividing mitotically, the middle layer is mantle (cloak) - its cellular composition is replenished and due to the mitotic division of the cells of this layer, and as a result of moving them from the inner ependymic layer; the outer layer, called the marginal veil (formed by the shoots of the cells of the two previous layers).

Subsequently, the cells of the inner layer are transformed into cylindrical ependymal (glial) cells lining the central canal of the spinal cord. Cellular elements of the mantle layer differentiate in two ways. From them, neuroblasts arise , which gradually turn into mature nerve cells, and spongioblasts, which give rise to various types of neuroglia cells (astrocytes and oligodendrocytes).

Neuroblasts »spongioblastas are located in a special formation - germic matrix, which appears at the end of the second month of intrauterine life, and are in the region of the inner wall of the cerebral bladder.

By the 3rd month of intrauterine life, the migration of neuroblasts to the destination begins. And first the spongioblast migrates, and then the neuroblast moves along the appendage of the glial cell. Migration of neurons continues until the 32nd week of intrauterine life. During the migration, both neuroblasts grow, differentiate into neurons. The variety of the structure and functions of neurons is such that until the end it is not calculated how many types of neurons are present in the nervous system.

With the differentiation of the neuroblast, the submicroscopic structure of its nucleus and cytoplasm changes. In the core there are regions of different electron density in the form of tender grains and filaments. In the cytoplasm, large cisterns and narrower tubules of the endoplasmic reticulum are detected in large numbers, the number of ribosomes increases, and a plate complex develops well. The body of the neuroblast gradually acquires a pear-shaped form, the outgrowth, the neurite (axon), begins to develop from its pointed end . Later, other processes, dendrites, are differentiated . Neuroblasts are transformed into mature nerve cells - neurons (the term "neuron" for the totality of the body of a nerve cell with an axon and dendrites was proposed by W. Valdeir in 1891). Neuroblasts and neurons during the embryonic development of the nervous system are mitotically divided. Sometimes the picture of the mitotic and amytic division of neurons can also be observed in the postembryonic period. Neurons multiply in vitro, under conditions of nerve cell cultivation. At present, the possibility of dividing certain nerve cells can be considered established.

By the time of birth, the total number of neurons reaches 20 billion. Simultaneously with the growth and development of neuroblasts and neurons, the programmed death of nerve cells - apoptosis - begins . The most intensive apoptosis after 20 years, with cells that do not get involved in the work and do not have functional connections.

In violation of the genome, regulating the time of appearance and the rate of apoptosis, not isolated cells die, but synchronously separate systems of neurons, which manifests itself in a whole gamut of various degenerative diseases of the nervous system that are inherited.

From the nervous (medullary) tube, which extends parallel to the chord and dorsally from it to the right and to the left, a dissected ganglionic plate is formed protruding spinal nodes. Simultaneous migration of neuroblasts from the medullary tube entails the formation of sympathetic border trunks with paravertebral segmental nodes, as well as prevertebral, extra organ and intramural nervous ganglia. Spinal cord cells (motor neurons) approach the muscles, the outgrowths of the sympathetic knot cells spread to the internal organs, and the outgrowths of the spinal cord cells permeate all the tissues and organs of the developing embryo, ensuring their afferent innervation.

With the development of the brain end of the brain tube, the principle of metamerism is not observed. Expansion of the cavity of the cerebral tube and an increase in the mass of cells are accompanied by the formation of primary cerebral blisters, from which the brain is subsequently formed.

By the 4th week of embryonic development, 3 primary cerebral blisters form at the head end of the neural tube. For unification it is customary to use in the anatomy such notions as "sagittal", "frontal", "dorsal", "ventral", "rostral", etc. The rostral part of the neural tube is the forebrain (prosencephalon), followed by the middle brain mesencephalon) and the hindbrain (rhombencephalon). Later (at week 6), the forebrain is divided into 2 brain bubbles: the terminal brain (telencephalon) - the cerebral hemisphere and some basal nuclei, and the intermediate brain (diencephalon). On each side of the midbrain, an eyeball grows, from which the nerve elements of the eyeball form. The eye glass formed by this outgrowth causes changes in the ectoderm lying directly above it, which leads to the appearance of the lens.

In the process of development in the midbrain, significant changes occur, related to the formation of specialized reflexes; centers related to vision, hearing, and also to pain, temperature and tactile sensitivity.

The rhomboid brain is divided into the posterior brain (mefencephalon), which includes the cerebellum and bridge, and the medulla oblongata (medulla oblongata) of the medulla oblongata.

The growth rate of the individual parts of the neural tube is different, as a result of which several bends are formed along its course, which later disappear into the embryo. In the area of joining the middle and intermediate brain, the bend of the cerebral trunk is maintained at a 90-degree angle.

By the 7th week in the hemispheres of the brain, the striped body and the visual hillock, the pituitary funnel and the pocket (Ratke) are closed, a vascular plexus is indicated.

By the eighth week, typical nerve cells appear in the cerebral cortex, the olfactory lobes become visible, the hard, soft and spider veins of the brain are distinctly expressed.

By the 10th week (embryo length 40 mm), a definative internal structure of the spinal cord is formed.

By the 12th week (length of the embryo 56 mm), common features in the structure of the brain, characteristic of a person, are revealed. The differentiation of the cells of the neuroglia begins, the cervical and lumbar thickenings are visible in the spinal cord, the pony tail and the final thread of the spinal cord appear.

By the 16th week (the length of the zdroysha 1 mm, the parts of the brain become discernible, the hemispheres cover the greater part of the cerebral table, the tubercles of the quadruple form appear, the cerebellum becomes more pronounced.

By the 20th week (the length of the embryo is 160 mm, the formation of adhesions begins (commissure) and myelinization of the spinal cord begins.

Typical layers of the cerebral cortex are visible by the 25th week, furrows and gyrations of the brain are formed by the 28th-30th week; from the 36th week begins myelination of the brain.

By the 40th week of development, all the main convolutions of the brain already exist, the appearance of the furrows seems to remind them of their schematic sketch.

At the beginning of the second year of Georgia, such a schematic disappears and differences arise due to the formation of small nameless furrows that significantly change the overall picture of the distribution of the main furrows and gyri.

Myelination of nervous structures plays an important role in the development of the nervous system. This process proceeds ordering, in accordance with the anatomical and functional features of fiber systems. The myelination of neurons indicates the functional maturity of the system. The myelin sheath is a kind of insulator for bioelectric impulses that arise in neurons during excitation. It also ensures a more rapid excitation through nerve fibers. In the central nervous system, myelin is produced by oligodendrogliocytes located between the nerve fibers of the white matter. However, a certain amount of myelin is synthesized by oligodendrogliocytes in the gray matter. Myelinization begins in a gray matter near the bodies of neurons and advances along the axon to a white matter. Each oligodendrogliocyte is involved in the formation of the myelin sheath. It wraps a separate section of the nerve fiber with successive spiral layers. The myelin sheath is interrupted by the interceptions of the node (Ranvier intercepts). Myelination begins on the 4th month of intrauterine development and ends after birth. Some fibers are milled only during the first years of life. In the period of embryogenesis, structures such as pre- and postcentral convolutions, the spur groove and adjacent parts of the cerebral cortex, the hippocampus, the thalamostriopallidar complex, the vestibular nuclei, the lower olives, the cerebellar worm, the anterior and posterior horns of the spinal cord, the ascending afferent systems of the lateral and posterior cord, some downward efferent systems of the lateral cord, etc. Myelination of the fibers of the pyramidal system begins on the last month of intrauterine development and lasts for the first year Life. In the middle and lower frontal convolutions, the lower parietal lobe, the middle and lower temporal convolutions, myelination begins only after birth. They are formed the very first, associated with the perception of sensory information (sensorimotor, visual and auditory cortex) and communicate with subcortical structures. These are phylogenetically older parts of the brain. The areas in which myelination begins later are related to phylogenetically younger structures and are associated with the formation of intra-cortical connections.

Thus, the nervous system in the processes of phylo- and ontogeny passes a long path of development and is the most complex system created by evolution. According to MI Astvatsaturov (1939), the essence of the evolutionary laws reduces to the following. The nervous system arises and develops in the process of interaction of the organism with the external environment, it lacks rigid stability and changes and is continuously improved in the processes of filo and ontogenesis. As a result of the complex and mobile process of interaction between the organism and the external environment, new conditioned reflexes are being developed, refined and fixed, which underlie the formation of new functions. The development and consolidation of more perfect and adequate reactions and functions is the result of the action of the external environment on the organism, ie, its adaptation to the given conditions of existence (adaptation of the organism to the environment). Functional evolution (physiological, biochemical, biophysical) corresponds to the evolution of morphological, ie, newly acquired functions are gradually fixed. With the advent of new functions, the ancients do not disappear, a certain subordination of ancient and new functions is developed. With the fall of new functions of the nervous system, its ancient functions are manifested. Therefore, many clinical signs of the disease, observed in the disturbance of the evolutionarily younger parts of the nervous system, are manifested in the functioning of more ancient structures. When the disease occurs, it is as if a return to a lower stage of phylogenetic development. An example is the increase in deep reflexes or the appearance of pathological reflexes when removing the regulating influence of the cerebral cortex. The most vulnerable structures of the nervous system are phylogenetically younger divisions, in particular - the cortex of the hemispheres and the large brain, in which defensive mechanisms have not yet been developed, while in the phylogenetically ancient departments for thousands of years of interaction with the environment certain mechanisms of counteraction to its factors have been formed . Phylogenetically younger brain structures have a lesser capacity for recovery (regeneration).

[1], [2], [3], [4], [5], [6]