Development of the nervous system

Any living organism that is in a certain habitat constantly interacts with it. From the external environment, the living organism receives the necessary food for life. In the external environment is the allocation of substances that are unnecessary for the body. The external environment has a favorable or adverse effect on the body. The living organism reacts to these influences 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 unicellular organisms do not have a nervous system. All these reactions 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 parts of the body surface and sending impulses to other cells, regulating their activity. The effects of the environment multicellular organisms are perceived by external ectodermal cells. Such cells specialize in perception of stimulation, transformation of it into bioelectrical potentials and conducting excitation. From ectodermal cells immersing in the depth of the body, there is a primitive arranged nervous system of multicellular organisms. This most simply formed network-like, or diffuse, nervous system is found in coelenterates, for example, in hydra. In these animals, two types of cells are distinguished. One of them - the receptor cells - is located between the cells of the skin (ectoderm). Others - effector cells are in the depth of the body, are connected with each other and with cells that provide a response. The irritation of any part of the surface of the body hydra leads to the excitation of deeper-lying 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 the concentration of nerve cells that form nerve centers, or nerve nodes (ganglia), with the nerve trunks escaping from them. At this stage in the development of the animal world, a nodal form of the nervous system appears . In representatives of segmented animals (for example, in ringed worms), the nerve nodes are located ventrally of the digestive tube and are joined by transverse and longitudinal nerve trunks. From these nodes, nerves leave, the branches of which end also within this segment. Segmentally located ganglia serve as reflex centers of the corresponding segments of the body of animals. Longitudinal nerve trunks connect nodes of different segments to each other on one half of the body and form two longitudinal abdominal chains. At the cephalic end of the body, dorsal to the pharynx, there is one pair of larger nasopharyngeal nodes that connects to the pair of nodes of the abdominal chain with the peripheral nerve ring. These nodes are more developed than others and are the prototype of the brain of vertebrate animals. This segmental structure of the nervous system allows, when irritating certain areas of the animal's body surface, not to involve all the nerve cells of the body in the response, but to use only the cells of this segment.

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

In accordance with the metamerism of the body of chordates, a single tubular nervous system consists of a series of identical repeating structures, or segments. The processes of the neurons that make up this nerve segment branch, as a rule, in a certain segment of the body corresponding to this segment and its musculature.

Thus, the improvement of the forms of movement of animals (from the peristaltic method in the simplest multicellular to the movement with the help of the limbs) led to the need to improve the structure of the nervous system. In chordates, the trunk region of the neural tube is the spinal cord. In the spinal cord and in the trunk portion of the developing brain, the chordates in the ventral regions of the neural tube have "motor" cells, the axons of which form the front ("motor") roots, and in the dorsal nerve cells, with which the axons of the "sensitive" cells located in the spinal nodes.

At the head end of the neural tube, in connection with the sensory organs developing in the anterior parts of the trunk and the presence of the gill apparatus, the initial parts of the digestive and respiratory systems, the segmental structure of the neural tube is preserved, although it undergoes considerable changes. These parts of the neural tube are the germ, from which the brain develops. The thickening of the anterior sections of the neural tube and the expansion of its cavity are the initial stages of brain differentiation. Such processes are already observed in the cyclostomes. In the early stages of embryogenesis in almost all cranial animals, the head end of the neural tube consists of three primary nerve bubbles: rhombencephalon, located closest to the spinal cord, mesencephalon and anterior (prosencephalon). The development of the brain occurs in parallel with the improvement of the spinal cord. The emergence of new centers in the brain puts already existing centers of the spinal cord in a subordinate position. In those parts of the brain that belong to the posterior cerebral vesicle (rhomboid brain), the development of the nuclei of the gill nerve (X parabolic vagus nerve) occurs, centers that regulate the processes of respiration, digestion, and circulation occur. Undoubted influence on the development of the hindbrain is caused by the receptors of static and acoustics that are already occurring in lower fish (VIII pair - pre-door and cochlear nerve). In this regard, at this stage of the development of the brain, the hindbrain (the cerebellum and the bridge of the brain) is predominant over the other divisions. 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 habitat.

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

Of the three primary blisters due to further differentiation of the anterior and rhomboid brain, the following 5 divisions (cerebral bubbles) are distinguished: the terminal brain, the intermediate brain, the middle brain, the hindbrain and the medulla oblongata. The central canal of the spinal cord at the head end of the neural tube becomes a system of interconnected cavities, called ventricles of the brain. Further development of the nervous system is associated with progressive development of the forebrain and the emergence of new nerve centers. These centers at each subsequent stage occupy a position closer to the head end, and subordinate to their influence the pre-existing centers.

The older nerve centers formed in the early stages of development do not disappear but remain in a subordinate position with respect to the newer ones: Thus, along with the first centers of hearing (nuclei) that appeared in the hindbrain for the earliest stages, the centers of hearing appear on average, and then in the final brain. Amphibia in the forebrain is already forming a rudiment of the future hemispheres, however, like reptiles, almost all of their departments belong to the olfactory brain. In the anterior (final) brain amphibians, reptiles and birds distinguish between the subcortical centers (the core of the striatum) and the cortex, which has a primitive structure. Subsequent development of the brain is associated with the emergence of new receptor and effector centers in the cortex, which subordinate nerve centers of lower order (in the brain stem and spinal cord). These new centers coordinate the activities of other parts of the brain, integrating the nervous system into a structural functional whole. This process is called the function corticolization. Enhanced development of the terminal brain in higher vertebrate animals (mammals) leads to the fact that this department prevails over all the others and covers all the departments in the form of a cloak, or cerebral cortex. The ancient cortex (paleocortex), and then the old cortex (archeocortex), which occupy the dorsal and dorsolateral hemispheres in reptiles, are replaced by a new bark (neocortex). The old divisions are pushed to the lower (ventral) surface of the hemispheres and, as it were, coalesced, turn into a hippocampus (ammon horn) and into the adjacent parts of the brain.

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

The greatest development of the cortex of the hemispheres is in man, which is explained by his labor activity and the emergence of speech as a means of communication between people. IPPavlov, who created the doctrine of the second signal system, the material substratum of the latter considered a complex cortex of the cerebral hemispheres - a new cortex.

The development of the cerebellum and spinal cord is closely related to the change in the way the animal moves in space. Thus, in reptiles that do not have extremities and move due to movements of the trunk, the spinal cord has no thickening and consists of roughly equal segments. In animals moving by the limbs, thickening appears 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, the cervical thickening of the spinal cord is more pronounced. In the cerebellum the birds have lateral protrusions - a patch is the oldest part of the cerebellar hemispheres. The hemispheres of the cerebellum are forming, the cerebellar worm 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 greater than that of the lumbar spine. This is because the hand, which is the organ of labor, is capable of producing more complex and diverse movements than the lower limb.

In connection with the development of higher centers of control of the activity of the whole organism in the brain, the spinal cord falls into a subordinate position. It retains the older segmented apparatus of its own connections of the spinal cord and develops a supra-segmental apparatus of bilateral relations with the brain. The development of the brain was manifested in the improvement of the receptor apparatus, the improvement of the mechanisms of adaptation of the organism to the environment by changing the metabolism, corticolization of functions. In humans, due to the uprightness and in connection with the improvement of movements of the upper extremities in the process of labor activity, the cerebellar hemispheres are much more developed than in animals.

The cortex of the cerebral hemispheres is a set of cortical ends of all kinds of analyzers and represents a material substrate of specifically visual thinking (according to IP Pavlov, the first signal system of reality). The further development of the brain in man is determined by its conscious use of tools, which allowed a person not only to adapt to changing environmental conditions, as animals do, but also to influence the environment on their own. In the process of social labor, speech arose as a necessary means of communication between people. Thus, a person has the ability to abstract thinking and formed a system of perception of the word, or signal, - the second signal system, according to IP Pavlov, the material substrate of which is the new cortex of the large brain.

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