Development of the nervous system in homo sapiens

The human nervous system develops from the outer germ layer - the ectoderm. In the dorsal sections of the embryo's body, differentiating ectodermal cells form the medullary (nerve) plate. The latter initially consists of one layer of cells, which subsequently differentiate into spongioblasts (from which the supporting tissue - neuroglia - develops) and neuroblasts (from which nerve cells develop). Due to the fact that the intensity of cell proliferation in different parts of the medullary plate is not the same, the latter sags and constantly takes the form of a groove or a groove. The growth of the lateral sections of this neural (medullary) groove leads to its edges converging and then merging. Thus, the neural groove, closing in its dorsal sections, turns into a neural tube. Fusion initially occurs in the anterior section, slightly retreating from the anterior end of the neural tube. Then the posterior, caudal, sections of it grow together. At the anterior and posterior ends of the neural tube, small unfused areas remain - neuropores. After the fusion of the dorsal sections, the neural tube is pinched off from the ectoderm and immersed in the mesoderm.

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

On the inner surface of each of the lateral walls, shallow longitudinal border grooves are formed, which divide the lateral sections of the tube into the main (ventral) and alar (dorsal) plates.

The main plate serves as a rudiment from which the anterior columns of gray matter and the adjacent white matter are formed. The processes of neurons developing in the anterior columns emerge (grow) from the spinal cord, forming the anterior (motor) roots of the spinal and cranial nerves. The posterior columns of gray matter and the adjacent white matter develop from the alar plate. Even at the stage of the neural groove, cellular strands called medullary ridges stand out in its lateral sections. During the formation of the neural tube, two ridges, merging, form the ganglionic plate, located dorsal to the neural tube, between the latter and the ectoderm. Subsequently, the ganglionic plate shifts to the lateral surface of the neural tube and turns intothe spinal ganglia and sensory ganglia of the cranial nerves corresponding to each segment of the body . The cells that migrate from the ganglion plates also serve as rudiments for the development of the peripheral parts of the autonomic nervous system.

Following the separation of the ganglion plate, the neural tube noticeably thickens at the head end. This expanded part serves as the rudiment of the brain. The remaining sections of the neural tube later transform into the spinal cord. Neuroblasts located in the forming spinal ganglia have the form of bipolar cells. In the process of further differentiation of neuroblasts, the sections of its two processes located in close proximity to the cell body merge into one T-shaped process, which then divides. Thus, the cells of the spinal ganglia become pseudo-unipolar in shape. The central processes of these cells are directed to the spinal cord and form the posterior (sensory) rootlet. Other processes of pseudo-unipolar cells grow from the nodes to the periphery, where they have receptors of various types.

In the early stages of embryonic development, the neural tube extends along the entire length of the body. Due to the reduction of the caudal sections of the neural tube, the lower end of the future spinal cord gradually narrows, forming a terminal (end) thread. For approximately 3 months of intrauterine development, the length of the spinal cord is equal to the length of the spinal canal. Subsequently, the growth of the spinal column occurs more intensively. Due to 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 sections. The discrepancy in the growth of the spinal column and spinal cord leads to a kind of "ascent" of the lower end of the latter. Thus, in a newborn, the lower end of the spinal cord is located at the level of the III lumbar vertebra, and in an adult - at the level of the I-II lumbar vertebrae. The roots of the spinal nerves and spinal ganglia are formed quite early, so the "ascent" of the spinal cord leads to the roots lengthening and changing their direction from horizontal to oblique and even vertical (longitudinal in relation to the spinal cord). The roots of the caudal (lower) segments of the spinal cord, going vertically to the sacral openings, form a bundle of roots around the terminal thread - the so-called equine tail.

The head section of the neural tube is the rudiment from which the brain develops. In 4-week embryos, the brain consists of three cerebral vesicles separated from each other by small constrictions in the walls of the neural tube. These are the prosencephalon - forebrain, mesencephalon - midbrain and rhombencephalon - diamond-shaped (hindbrain). By the end of the 4th week, signs of differentiation of the forebrain vesicle into the future telencephalon and diencephalon appear. Soon after, the diamond-shaped brain is divided into the hindbrain (metencephalon) and medulla oblongata (myelencephalon, s. medulla oblongata, s. bulbus).

Simultaneously with the formation of the five cerebral vesicles, the neural tube in the head section forms several bends in the sagittal plane. The parietal bend appears earlier than the others, with its convexity directed to the dorsal side and located in the region of the middle cerebral vesicle. Then, on the border of the posterior cerebral vesicle and the rudiment of the spinal cord, the occipital bend stands out, with its convexity also directed to the dorsal side. The third bend, the pontine bend, facing ventrally, appears between the two previous ones in the region of the hindbrain. This last bend divides the rhombencephalon, as noted earlier, into two sections (vesicles): the medulla oblongata and the hindbrain, consisting of the pons and the dorsally located cerebellum. The common cavity of the rhombencephalon is transformed into the fourth ventricle, which in its posterior sections communicates with the central canal of the spinal cord and with the intermeningeal space. Blood vessels grow over the thin single-layer roof of the forming fourth ventricle. Together with the upper wall of the fourth ventricle, consisting of only one layer of ependymal cells, they form the choroid plexus of the fourth ventricle (plexus choroideus ventriculi quarti). In the anterior sections, the midbrain aqueduct opens into the cavity of the fourth ventricle, which is the cavity of the midbrain. The walls of the neural tube in the region of the midbrain vesicle thicken more uniformly. From the ventral sections of the neural tube, the cerebral peduncles develop here, and from the dorsal sections, the midbrain roof plate. The anterior cerebral vesicle undergoes the most complex transformations during development.

In the diencephalon (its posterior part), the lateral walls reach their greatest development, significantly thickening and forming the thalami (optic hillocks). From the lateral walls of the diencephalon, by protruding laterally, the eye vesicles are formed, each of which subsequently turns into the retina (reticular membrane) of the eyeball and the optic nerve. The thin dorsal wall of the diencephalon fuses with the choroid, forming the roof of the third ventricle, containing the choroid plexus. In the dorsal wall, a blind unpaired process also appears, which subsequently turns into the pineal body, or epiphysis. In the area of the thin lower wall, another unpaired protrusion is formed, turning into the gray tubercle, the funnel and the posterior lobe of the pituitary gland.

The cavity of the diencephalon forms the third ventricle of the brain, which communicates with the fourth ventricle via the midbrain aqueduct.

The end brain, consisting of an unpaired cerebral vesicle at the early stages of development, subsequently, due to the predominant development of the lateral sections, turns into two vesicles - the future hemispheres of the cerebrum. The initially unpaired cavity of the end brain is also divided into two parts, each of which communicates with the cavity of the third ventricle via the interventricular opening. The cavities of the developing hemispheres of the cerebrum are transformed into the lateral ventricles of the brain, which have a complex configuration.

The intensive growth of the cerebral hemispheres leads to the fact that they gradually cover from above and from the sides not only the diencephalon and midbrain, but also the cerebellum. On the inner surface of the walls of the forming right and left hemispheres, in the area 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 vesicle (of each hemisphere) is inverted into the lateral ventricle together with the vascular membrane and forms the vascular plexus of the lateral ventricle. In the area of the thin anterior wall, which is a continuation of the terminal (border) plate, a thickening develops, which subsequently turns into the corpus callosum and the anterior commissure of the brain, connecting both hemispheres with each other. Uneven and intensive growth of the walls of the vesicles of the hemispheres leads to the fact that at first on their smooth outer surface in certain places there appear depressions, forming the grooves of the cerebral hemispheres. Deep permanent grooves appear earlier than others, and the first to form among them is the lateral (Sylvian) groove. With the help of such deep grooves, each hemisphere is divided into protrusions - convolutions - of the cerebrum.

The outer layers of the walls of the hemisphere bubbles are formed by the gray matter developing here - the cerebral cortex. The grooves and convolutions significantly increase the surface of the cerebral cortex. By the time a child is born, the hemispheres of his cerebrum have all the main grooves and convolutions. After birth, small, inconstant grooves that have no names appear in different parts of the hemispheres. Their number and location determine the variety of options and complexity of the relief of the cerebral hemispheres.

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