Development of the nervous system in homo sapiens

The human nervous system develops from the external embryonic leaf - ectoderm. In the dorsal regions of the embryo, the differentiating ectodermal cells form a medullary (nerve) plate. The latter initially consists of one layer of cells, which later differentiate into spongioblast (from which the supporting tissue develops - neuroglia) and neuroblasts (from which nerve cells develop). In connection with the fact that the intensity of cell multiplication in different parts of the medullary plate is not the same, the latter bends and constantly acquires the appearance of a groove or groove. The growth of the lateral parts of this nervous (medullary) groove leads to the fact that its edges approach, and then grow together. Thus, the nerve groove, closing in its dorsal parts, turns into a neural tube. The fusion initially occurs in the anterior part, somewhat receding from the anterior end of the neural tube. Then the rear, caudal, and its parts join together. At the anterior and posterior ends of the neural tube there remain small non-intact segments - neuropores. After the fusion of the dorsal parts, the neural tube is loosened from the ectoderm and immersed in the mesoderm.

During the period of formation, the neural tube consists of three layers. The ependymal lining of the cavities of the ventricles of the brain and the central canal of the spinal cord develops from the inner layer, and the gray matter of the brain from the middle layer ("cloak" layer). The outer layer, almost devoid of cells, turns into a white matter in the brain. Initially, all the walls of the neural tube have the same thickness. Subsequently lateral sections of the tube develop more intensively, which are increasingly thickened. The ventral and dorsal walls lag behind in growth and gradually sink between the intensively developing lateral divisions. As a result of this immersion, the dorsal and ventral longitudinal median grooves of the future spinal and medulla oblongata are formed.

On the inner surface of each of the lateral walls shallow shallow longitudinal boundary grooves are formed that separate the lateral sections of the tube into the main (ventral) and wing (dorsal) lamina.

The main plate serves as a rudiment, from which the front pillars of gray matter and the white matter adjacent to them are formed. The processes of the developing in the anterior columns of the neurons exit (sprout) from the spinal cord, form the anterior (motor) roots of the spinal and cranial nerves. From the wing plate develops the rear pillars of gray matter and the white substance adjacent to them. Even at the stage of the nerve groove in the lateral sections of it, cellular strands are distinguished, called medullary scallops. During the formation of the neural tube, two scallops, growing together, form a ganglionic plate, which is located dorsally of the neural tube, between the latter and the ectoderm. Subsequently, the ganglionic plate shifts to the lateral surface of the neural tube and turns into spinal cord nodes and sensitive nodes of the cranial nerves corresponding to each segment of the trunk . Cells evicted from ganglionic plates also serve as rudiments for the development of peripheral parts of the autonomic nervous system.

Following the isolation of the ganglionic plate, the neural tube at the head end is markedly thickened. This enlarged part serves as an embryo of the brain. The remaining parts of the neural tube subsequently become the spinal cord. Neuroblasts located in the emerging spinal nodules have the form of bipolar cells. In the process of further differentiation of the neuroblasts, the areas of two of its processes located in the immediate vicinity of the body of the cell merge into one T-shaped fission process. Thus, the cells of the spinal nodes become pseudo-unipolar in shape. The central processes of these cells go to the spinal cord and form a posterior (sensitive) spine. Other processes of pseudo-unipolar cells grow from nodes to the periphery, where they have different types of receptors.

In the early stages of embryo development, the neural tube extends along the entire length of the body. In connection with the reduction of the caudal sections of the neural tube, the lower end of the future spinal cord gradually narrows, forming a terminal (terminal) thread. Approximately within 3 months of intrauterine development, the length of the spinal cord is equal to the length of the spinal canal. Later, the spine grows more intensively. In connection with the fixation of the brain in the cranial cavity, the most noticeable lag in the growth of the neural tube is observed in its caudal divisions. The discrepancy in the growth of the spine and spinal cord leads, as it were, to the "ascent" of the lower end of the latter. Thus, in the newborn, the lower end of the spinal cord is located at the level of the third lumbar vertebra, and in the adult it is at the level of lumbar vertebrae I-II. Spines of spinal nerves and spinal nodes are formed early enough, so the "ascent" of the spinal cord leads to the fact that the roots are elongated and change their direction from horizontal to oblique and even vertical (longitudinal with respect to the spinal cord). The rootlets of the caudal (lower) segments of the spinal cord, which are vertical to the sacral orifices, form a bundle of rootlets around the terminal thread, the so-called ponytail.

The head of the neural tube is the rudiment from which the brain develops. In 4-week-old embryos, the brain consists of three cerebral blisters separated from each other by small constrictions of the walls of the neural tube. This prosencephalon is the forebrain, mesencephalon is the middle brain and rhombencephalon is the rhomboid (posterior) brain. By the end of the 4th week there are signs of differentiation of the anterior cerebral bladder to the future terminal brain (telencephalon) and the intermediate (diencephalon). Shortly thereafter, the rhomboid brain is divided into the posterior brain (metencephalon) and the medulla oblongata (myelencephalon, s. Medulla oblongata, s.bulbus).

Simultaneously with the formation of five cerebral blisters, the neural tube in the head region forms several bends in the sagittal plane. Earlier, others have a parietal bend, directed by convexity to the dorsal side and located in the region of the middle cerebral bladder. Then, on the border of the posterior cerebral bladder and the rudiment of the spinal cord, the occipital bend is prominent, directed also by the convexity to the dorsal side. The third curve - the pavement, facing ventrally, appears between the two previous ones in the region of the hindbrain. This last curve divides the rhomboid brain, as noted earlier, into two parts (a bladder): the medulla oblongata and the hindbrain, consisting of a bridge and a dorsally situated cerebellum. The common cavity of the rhomboid brain is transformed into the IV ventricle, which in its posterior parts communicates with the central canal of the spinal cord and with the intercellular space. Over a thin single-layered roof of the developing IV ventricle, blood vessels grow. Together with the upper wall of the IV ventricle, consisting of only one layer of ependymal cells, they form a vascular plexus of the IV ventricle (plexus choroideus ventriculi quarti). In the anterior parts of the cavity of the IV ventricle, the midbrain is opened , which is the midbrain cavity. The walls of the neural tube in the middle of the cerebral bladder thicken more evenly. The ventral sections of the neural tube here develop the legs of the brain, and from the dorsal parts - the plate of the roof of the midbrain. The most prevailing transformations in the development process undergoes anterior cerebral bladder.

In the midbrain (posterior part), the lateral walls reach the greatest development, which thickens considerably and form thalamuses (visual hillocks). From the side walls of the midbrain, protrusions to the lateral sides form the eye vesicles, each of which subsequently turns into the retina (mesh shell) of the eyeball and the optic nerve. The thin dorsal wall of the midbrain fuses with the choroid, forming the roof of the third ventricle containing the vascular plexus. In the dorsal wall there also appears a blind unpaired outgrowth, which subsequently turns into a pineal body, or an epiphysis. In the region of the thin lower wall another unpaired protrusion develops, turning into a gray mound, a funnel and a posterior lobe of the pituitary gland.

The cavity of the intermediate brain forms the third ventricle of the brain, which is communicated with the IV ventricle through the medium-water main.

The final brain, which consists in the early stages of development from an unpaired cerebral bladder, subsequently by the predominant development of the lateral divisions turns into two bubbles - the future hemispheres of the large brain. Unpaired at first the cavity of the terminal brain is also divided into two parts, each of which is communicated by means of an interventricular orifice with the cavity of the third ventricle. The cavities of the developing cerebral hemispheres are transformed into complex lateral ventricles of the brain.

The intensive growth of the cerebral hemispheres leads to the fact that they gradually cover from the top and sides not only the intermediate and middle brain, but also the cerebellum. On the inner surface of the walls of the forming right and left hemispheres, in the region of their base, a protrusion (thickening of the wall) is formed, in the thickness of which the nodes of the base of the brain develop - the basal (central) nuclei. The thin medial wall of each lateral bladder (each hemisphere) is screwed into the lateral ventricle together with the choroid and forms the vascular plexus of the lateral ventricle. In the region of the thin anterior wall, which represents the extension of the terminal (borderline) plate, a thickening develops, which subsequently turns into a corpus callosum and an anterior spike of the brain, connecting the two hemispheres to each other. The uneven and intensive growth of the walls of the hemispherical bubbles leads to the fact that in the beginning on their smooth outer surface there appear in some places grooves forming the furrows of the cerebral hemispheres. In the past, deep permanent furrows appear, and the first is the lateral (sylvia) furrow. With the help of such deep furrows, each hemisphere is divided into protrusions - gyrus - of the large brain.

The outer layers of the walls of the hemispheres are formed by the gray matter developing here, the cerebral cortex. Furrows and convolutions significantly increase the surface of the cerebral cortex. By the time the baby is born, the hemispheres of his large brain have all the main furrows and gyruses. After birth in the various parts of the hemispheres appear small non-permanent furrows, which have no names. Their number and place of appearance determine the variety of options and complexity of the relief of the cerebral hemispheres.

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