Development of the nervous system

Any living organism in a certain environment constantly interacts with it. From the external environment, a living organism receives the food products necessary for life. Unnecessary substances for the organism are released into the external environment. The external environment has a favorable or unfavorable effect on the organism. The living organism reacts to these effects and changes in the external environment by changing its internal state. The reaction of a living organism can manifest itself in the form of growth, strengthening or weakening of processes, movements or secretion.

The simplest single-celled organisms do not have a nervous system. All the reactions noted in them are manifestations of the activity of one cell.

In multicellular organisms, the nervous system consists of cells that are connected to each other by processes capable of perceiving irritation from any part of the body surface and sending impulses to other cells, regulating their activity. Multicellular organisms perceive the effects of the external environment with external ectodermal cells. Such cells specialize in perceiving irritation, transforming it into bioelectric potentials and conducting excitation. From the ectodermal cells immersed in the depths of the body, a primitively structured nervous system of multicellular organisms arises. Such the most simply formed reticular, or diffuse, nervous system is found in coelenterates, for example, in hydra. These animals have two types of cells. One of them - receptor cells - is located between the cells of the skin (ectoderm). The other - effector cells are located deep in the body, connected to each other and to the cells that provide a response. Irritation of any part of the hydra's body surface leads to the excitation of deeper cells, as a result of which the living multicellular organism exhibits motor activity, captures food or escapes from the enemy.

In more highly organized animals, the nervous system is characterized by a concentration of nerve cells that form nerve centers, or nerve nodes (ganglia), with nerve trunks extending from them. At this stage of animal development, a nodular form of the nervous system arises. In representatives of segmented animals (for example, in annelids), the nerve nodes are located ventral to the digestive tube and are connected by transverse and longitudinal nerve trunks. Nerves extend from these nodes, the branches of which also end within the given segment. Segmentally located ganglia serve as reflex centers for the corresponding segments of the animal's body. Longitudinal nerve trunks connect nodes of different segments on one half of the body with each other and form two longitudinal abdominal chains. At the cephalic end of the body, dorsal to the pharynx, there is one pair of larger supraesophageal nodes, which are connected by a peripharyngeal ring of nerves to a pair of nodes of the abdominal chain. These nodes are more developed than others and are the prototype of the brain of vertebrates. This segmental structure of the nervous system allows, when irritating certain areas of the animal's body surface, not to involve all the body's nerve cells in the response, but to use only the cells of a given segment.

The next stage of development of the nervous system is that the nerve cells are no longer arranged in separate nodes, but form an elongated continuous nerve cord, inside of which there is a cavity. At this stage, the nervous system is called a tubular nervous system. The structure of the nervous system in the form of a neural tube is characteristic of all representatives of chordates - from the most simply structured skullless animals to mammals and humans.

In accordance with the metameric nature of the body of chordate animals, a single tubular nervous system consists of a number of similar repeating structures, or segments. The processes of neurons that make up a given nerve segment branch out, as a rule, in a specific area of the body and its musculature that corresponds to the given segment.

Thus, the improvement of animal movement patterns (from the peristaltic method in the simplest multicellular organisms to movement using limbs) led to the need to improve the structure of the nervous system. In chordates, the trunk section of the neural tube is the spinal cord. In the spinal cord and in the trunk section of the developing brain in chordates, in the ventral sections of the neural tube there are "motor" cells, the axons of which form the anterior ("motor") roots, and in the dorsal sections - nerve cells, with which the axons of "sensory" cells located in the spinal ganglia communicate.

At the head end of the neural tube, due to the sense organs developing in the anterior sections of the body and the presence of the gill apparatus, the initial sections of the digestive and respiratory systems, the segmental structure of the neural tube, although preserved, undergoes significant changes. These sections of the neural tube are the rudiment from which the brain develops. Thickening of the anterior sections of the neural tube and expansion of its cavity are the initial stages of differentiation of the brain. Such processes are already observed in cyclostomes. In the early stages of embryogenesis, in almost all cranial animals, the head end of the neural tube consists of three primary neural vesicles: the rhomboid (rhombencephalon), located closest to the spinal cord, the middle (mesencephalon) and the anterior (prosencephalon). The development of the brain occurs in parallel with the improvement of the spinal cord. The appearance of new centers in the brain places the existing centers of the spinal cord in a subordinate position. In those parts of the brain that belong to the hindbrain vesicle (rhombencephalon), the development of the nuclei of the gill nerves (the 10th pair - the vagus nerve) occurs, and centers arise that regulate the processes of respiration, digestion, and blood circulation. The development of the hindbrain is undoubtedly influenced by the static and acoustic receptors that appear already in lower fish (the 8th pair - the vestibulocochlear nerve). In this regard, at this stage of brain development, the hindbrain (the cerebellum and the pons) is predominant over other parts. The appearance and improvement of the receptors of vision and hearing determine the development of the midbrain, where the centers responsible for visual and auditory functions are laid. All these processes occur in connection with the adaptability of the animal organism to the aquatic environment.

In animals in a new habitat - in the air environment, further restructuring of both the organism as a whole and its nervous system occurs. The development of the olfactory analyzer causes further restructuring of the anterior end of the neural tube (the anterior cerebral vesicle, where the centers regulating the olfactory function are laid down), the so-called olfactory brain (rhinencephalon) appears.

From the three primary vesicles, due to further differentiation of the forebrain and rhombencephalon, the following 5 sections (cerebral vesicles) are distinguished: the forebrain, the diencephalon, the midbrain, the hindbrain, and the medulla oblongata. The central canal of the spinal cord at the head end of the neural tube turns into a system of communicating cavities, called the ventricles of the brain. Further development of the nervous system is associated with the progressive development of the forebrain and the emergence of new nerve centers. At each subsequent stage, these centers occupy a position that is increasingly closer to the head end and subordinate the previously existing centers to their influence.

Older nerve centers formed at early stages of development do not disappear, but are preserved, occupying a subordinate position in relation to newer ones: Thus, along with the auditory centers (nuclei) that first appeared in the hindbrain, at later stages auditory centers appear in the middle and then in the telencephalon. In amphibians, the rudiments of the future hemispheres are already formed in the forebrain, however, as in reptiles, almost all of their sections belong to the olfactory brain. In the forebrain (telencephalon) of amphibians, reptiles and birds, subcortical centers (nuclei of the striatum) and the cortex, which has a primitive structure, are distinguished. Subsequent development of the brain is associated with the emergence of new receptor and effector centers in the cortex, which subordinate the lower-order nerve centers (in the stem part of the brain and spinal cord). These new centers coordinate the activity of other parts of the brain, uniting the nervous system into a structural functional whole. This process is called corticolization of functions. The intensive development of the end brain in higher vertebrates (mammals) leads to this section dominating over all the others and covering all the sections in the form of a cloak, or cerebral cortex. The ancient cortex (paleocortex), and then the old cortex (archeocortex), occupying the dorsal and dorsolateral surfaces of the hemispheres in reptiles, are replaced by a new cortex (neocortex). The old sections are pushed to the lower (ventral) surface of the hemispheres and in depth, as if curled up, turning into the hippocampus (Ammon's horn) and the adjacent sections of the brain.

Simultaneously with these processes, differentiation and complication of all other parts of the brain occur: intermediate, middle and posterior, restructuring of both ascending (sensory, receptor) and descending (motor, effector) pathways. Thus, in higher mammals, the mass of fibers of the pyramidal pathways increases, connecting the centers of the cerebral cortex with the motor cells of the anterior horns of the spinal cord and the motor nuclei of the brainstem.

The cortex of the hemispheres reaches its greatest development in humans, which is explained by their work activity and the emergence of speech as a means of communication between people. I.P. Pavlov, who created the doctrine of the second signal system, considered the complexly structured cortex of the cerebral hemispheres - the new cortex - to be the material substrate of the latter.

The development of the cerebellum and spinal cord is closely related to the change in the animal's method of moving in space. Thus, in reptiles that do not have limbs and move by means of body movements, the spinal cord does not have thickenings and consists of approximately equal-sized segments. In animals that move by means of limbs, thickenings appear in the spinal cord, the degree of development of which corresponds to the functional significance of the limbs. If the forelimbs are more developed, for example in birds, then the cervical thickening of the spinal cord is more pronounced. In birds, the cerebellum has lateral protrusions - the flocculus - the most ancient part of the cerebellar hemispheres. The cerebellar hemispheres are formed, and the cerebellar vermis reaches a high degree of development. If the functions of the hind limbs are predominant, for example in kangaroos, then the lumbar thickening is more pronounced. In humans, the diameter of the cervical thickening of the spinal cord is larger than the lumbar one. This is explained by the fact that the hand, which is the organ of labor, is capable of producing more complex and varied movements than the lower limb.

In connection with the development of higher control centers for the activity of the whole organism in the brain, the spinal cord falls into a subordinate position. It retains the older segmental apparatus of the spinal cord's own connections and develops a suprasegmental apparatus of bilateral connections with the brain. The development of the brain manifested itself in the improvement of the receptor apparatus, the improvement of the mechanisms of adaptation of the organism to the environment by changing metabolism, corticolization of functions. In humans, due to upright posture and in connection with the improvement of the movements of the upper limbs in the process of labor activity, the cerebellar hemispheres are much more developed than in animals.

The cerebral cortex is a collection of cortical ends of all types of analyzers and is the material substrate of specifically visual thinking (according to I.P. Pavlov, the first signal system of reality). The further development of the brain in humans is determined by their conscious use of tools, which allowed humans not only to adapt to changing environmental conditions, as animals do, but also to influence the external environment themselves. In the process of social labor, speech emerged as a necessary means of communication between people. Thus, humans acquired the ability to think abstractly and a system for perceiving a word, or signal, was formed - the second signal system, according to I.P. Pavlov, the material substrate of which is the new cerebral cortex.

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