Functional morphology of the nervous system

The complex function of the nervous system is based on its special morphology.

In the intrauterine period, the nervous system is formed and develops earlier and faster than other organs and systems. At the same time, the formation and development of other organs and systems occurs synchronously with the development of certain structures of the nervous system. This process of systemogenesis, according to P.K. Anokhin, leads to the functional maturation and interaction of heterogeneous organs and structures, which ensures the performance of respiratory, nutritional, motor and other functions of life support of the body in the postnatal period.

Morphogenesis of the nervous system can be divided into morphogenesis proper, i.e. the sequential emergence of new structures of the nervous system at the appropriate gestation periods, this is only an intrauterine process, and functional morphogenesis. Morphogenesis proper includes further growth, development of the nervous system with an increase in the mass and volume of individual structures, which is caused not by an increase in the number of nerve cells, but by the growth of their bodies and processes, myelination processes, and proliferation of glial and vascular elements. These processes partially continue throughout the entire period of childhood.

The brain of a newborn human is one of the largest organs and weighs 340-400 g. A. F. Tur indicated that the brain of boys is heavier than that of girls by 10-20 g. By the age of one, the brain weighs about 1000 g. By the age of nine, the brain weighs an average of 1300 g, and it acquires the last 100 g between the ages of nine and 20.

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

Touching upon the issues of brain development, it is necessary to note the works of B. N. Klossovsky, who considered this process in connection with the development of the systems that feed it - the cerebrospinal fluid and the blood system. In addition, a clear correspondence can be traced between the development of the nervous system and the formations that protect it - the membranes, bone structures of the skull and spine, etc.

Morphogenesis

During ontogenesis, the elements of the human nervous system develop from the embryonic ectoderm (neurons and neuroglia) and mesoderm (membranes, vessels, mesoglia). By the end of the 3rd week of development, the human embryo has the appearance of an oval plate about 1.5 cm in length. At this time, the neural plate is formed from the ectoderm, which is located longitudinally along the dorsal side of the embryo. As a result of uneven reproduction and compaction of neuroepithelial cells, the middle part of the plate sags and a neural groove appears, which deepens into the body of the embryo. Soon the edges of the neural groove close, and it turns into a neural tube, isolated from the skin ectoderm. A group of cells stands out on each side of the neural groove; it forms a continuous layer between the neural folds and the ectoderm - the ganglion plate. It serves as the source material for cells of sensory nerve nodes (cranial, spinal) and nodes of the autonomic nervous system.

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

Subsequently, the cells of the inner layer transform into cylindrical ependymal (glial) cells lining the central canal of the spinal cord. The cellular elements of the mantle layer differentiate in two directions. From them arise neuroblasts, which gradually transform into mature nerve cells, and spongioblasts, which give rise to various types of neuroglial cells (astrocytes and oligodendrocytes).

Neuroblasts » spongioblasts are located in a special formation - the germinal matrix, which appears by the end of the 2nd month of intrauterine life, and are located in the area of the inner wall of the brain vesicle.

By the 3rd month of intrauterine life, the migration of neuroblasts to their destination begins. The spongioblast migrates first, and then the neuroblast moves along the process of the glial cell. The migration of neurons continues until the 32nd week of intrauterine life. During migration, neuroblasts also grow and differentiate into neurons. The diversity of the structure and functions of neurons is such that it has not yet been fully calculated how many types of neurons there are in the nervous system.

As the neuroblast differentiates, the submicroscopic structure of its nucleus and cytoplasm changes. In the nucleus, areas of different electron density appear in the form of delicate grains and threads. In the cytoplasm, wide cisterns and narrower canals of the endoplasmic reticulum are found in large quantities, the number of ribosomes increases, and the lamellar complex achieves good development. The body of the neuroblast gradually acquires a pear-shaped form, and a process, the neurite (axon), begins to develop from its pointed end. Later, other processes, the dendrites, differentiate. Neuroblasts turn into mature nerve cells, neurons (the term "neuron" to denote the totality of the nerve cell body with the axon and dendrites was proposed by W. Waldeir in 1891). Neuroblasts and neurons divide mitotically during the embryonic development of the nervous system. Sometimes the picture of mitotic and amitotic division of neurons can be observed in the post-embryonic period. Neurons multiply in vitro, under conditions of culturing the nerve cell. At present, the possibility of division of some nerve cells can be considered established.

By the time of birth, the total number of neurons reaches 20 billion. Along with the growth and development of neuroblasts and neurons, programmed death of nerve cells - apoptosis - begins. Apoptosis is most intense after 20 years, and the first to die are cells that are not included in the work and have no functional connections.

When the genome that regulates the time of occurrence and speed of apoptosis is disrupted, it is not isolated cells that die, but individual systems of neurons that die synchronously, which manifests itself in a whole range of different degenerative diseases of the nervous system that are inherited.

From the neural (medullary) tube, which extends parallel to the chord and dorsally from it to the right and left, a dissected ganglionic plate protrudes, forming the spinal ganglia. Simultaneous migration of neuroblasts from the medullary tube entails the formation of sympathetic border trunks with paravertebral segmental ganglia, as well as prevertebral, extraorgan and intramural nerve ganglia. The processes of the spinal cord cells (motor neurons) approach the muscles, the processes of the sympathetic ganglia cells spread into the internal organs, and the processes of the spinal ganglia cells penetrate all the tissues and organs of the developing embryo, providing their afferent innervation.

During the development of the head end of the neural tube, the principle of metamerism is not observed. The expansion of the cavity of the neural tube and the increase in the mass of cells are accompanied by the formation of primary brain vesicles, from which the brain is subsequently formed.

By the 4th week of embryonic development, 3 primary brain vesicles are formed at the head end of the neural tube. For unification, it is customary to use such designations in anatomy as "sagittal", "frontal", "dorsal", "ventral", "rostral", etc. The most rostral part of the neural tube is the forebrain (prosencephalon), followed by the midbrain (mesencephalon) and the hindbrain (rhombencephalon). Subsequently (on the 6th week), the forebrain is divided into 2 more brain vesicles: the telencephalon - the hemispheres of the cerebrum and some basal nuclei, and the diencephalon. On each side of the diencephalon, an optic vesicle grows, from which the neural elements of the eyeball are formed. The optic cup formed by this outgrowth causes changes in the ectoderm lying directly above it, which leads to the emergence of the lens.

During the development process, significant changes occur in the midbrain, associated with the formation of specialized reflex centers related to vision, hearing, as well as pain, temperature and tactile sensitivity.

The rhombencephalon is subdivided into the hindbrain (mefencephalon), which includes the cerebellum and pons, and the medulla oblongata (myelоncephalon or medulla oblongata).

The growth rate of individual parts of the neural tube varies, as a result of which several bends are formed along its course, which disappear during subsequent development of the embryo. In the area of the junction of the midbrain and diencephalon, the bend of the brain stem at an angle of 90" is preserved.

By the 7th week, the corpus striatum and thalamus are well defined in the cerebral hemispheres, the pituitary infundibulum and Rathke's recess close, and the vascular plexus begins to emerge.

By the 8th week, typical nerve cells appear in the cerebral cortex, the olfactory lobes become noticeable, and the dura mater, pia mater, and arachnoid mater are clearly visible.

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

By the 12th week (the embryo's length is 56 mm), common features in the structure of the brain characteristic of humans are revealed. Differentiation of neuroglial cells begins, cervical and lumbar thickenings are visible in the spinal cord, the equine tail and the terminal thread of the spinal cord appear.

By the 16th week (the length of the embryo is 1 mm), the lobes of the brain become distinguishable, the hemispheres cover most of the brain surface, the tubercles of the quadrigeminal body appear; the cerebellum becomes more pronounced.

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

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

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

At the beginning of the second year of life, this schematic nature disappears and differences appear due to the formation of small unnamed grooves, which noticeably change the overall picture of the distribution of the main grooves and convolutions.

Myelination of nerve structures plays an important role in the development of the nervous system. This process is ordered in accordance with the anatomical and functional features of the fiber systems. 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 faster conduction of excitation along nerve fibers. In the central nervous system, myelin is produced by oligodendrogliocytes located between the nerve fibers of the white matter. However, some myelin is synthesized by oligodendrogliocytes in the gray matter. Myelination begins in the gray matter near the bodies of neurons and moves along the axon into the white matter. Each oligodendrogliocyte participates 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 nodes of Ranvier. Myelination begins in the 4th month of intrauterine development and is completed after birth. Some fibers are myelinated only during the first years of life. During embryogenesis, such structures as the pre- and postcentral gyri, calcarine groove and adjacent parts of the cerebral cortex, hippocampus, thalamostriopallidal complex, vestibular nuclei, inferior olives, cerebellar vermis, anterior and posterior horns of the spinal cord, ascending afferent systems of the lateral and posterior funiculi, some descending efferent systems of the lateral funiculi, etc. are myelinated. Myelination of the fibers of the pyramidal system begins in the last month of intrauterine development and continues during the first year of life. In the middle and inferior frontal gyri, inferior parietal lobule, middle and inferior temporal gyri, myelination begins only after birth. They are the first to form, are 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 phylogenetically younger structures and are associated with the formation of intracortical connections.

Thus, the nervous system in the processes of phylo- and ontogenesis goes through a long path of development and is the most complex system created by evolution. According to M. I. Astvatsaturov (1939), the essence of evolutionary patterns is as follows. The nervous system arises and develops in the process of interaction of the organism with the external environment, it is deprived of rigid stability and changes and continuously improves in the processes of phylo- and ontogenesis. As a result of the complex and mobile process of interaction of the organism with the external environment, new conditioned reflexes are developed, improved and consolidated, underlying 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, i.e. its adaptation to the given conditions of existence (adaptation of the organism to the environment). Functional evolution (physiological, biochemical, biophysical) corresponds to morphological evolution, i.e. newly acquired functions are gradually consolidated. With the emergence of new functions, the ancient ones do not disappear; a certain subordination of the ancient and new functions is developed. When new functions of the nervous system disappear, its ancient functions manifest themselves. Therefore, many clinical signs of the disease, observed when evolutionarily younger parts of the nervous system are damaged, manifest themselves in the functioning of more ancient structures. When the disease occurs, there is a kind of return to a lower stage of phylogenetic development. An example is the increase in deep reflexes or the appearance of pathological reflexes when the regulatory influence of the cerebral cortex is removed. The most vulnerable structures of the nervous system are phylogenetically younger parts, in particular, the cortex of the hemispheres and the cerebral cortex, in which protective mechanisms have not yet been developed, while in phylogenetically ancient parts, over the course of thousands of years of interaction with the external environment, certain mechanisms for counteracting its factors have been formed. Phylogenetically younger structures of the brain have a lesser ability to restore (regenerate).

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