The nervous system in the living organism is represented bya network of communications that ensure its connection with the surrounding world and its own processes. Its basic element is a neuron - cell with processes (axons and dendrites), transmitting information by electrical and chemical means.
The nervous system first appeared in the livingorganisms, if necessary, more effective interaction with the environment. The development of a simple network for impulse transmission helped not only to perceive signals from outside. Thanks to it, it became possible to organize its own vital processes for a more successful functioning.
During the evolution of the structure of the nervous systemcomplicated: its task was not only the formation of an adequate response to external influences, but also the organization of its own behavior. Pavlov called this method of functioning a higher nervous activity.
For the first time the nervous system appeared in organisms,consisting of more than one cell, since it transmits signals between neurons forming a network. But even in the simplest, one can observe the ability to respond to external stimuli provided by intracellular processes.
Nerve system of multicellular qualitydiffers from the analogous formation in the protozoa. The last system of bonds is located within the limits of the metabolism of a single cell. On various processes that occur outside or inside, the infusoria "recognizes" because of changes in the composition of protoplasm and the activity of some other structures. Multicellular living beings have a system built of functional units, each of which is endowed with its own metabolic processes.
Thus, for the first time the nervous systemappears, who has not one, but several cells, that is, multicellular organisms. The prototype is the conduct of impulses in protozoa. Their level of vital activity reveals the development of protoplasmic structures that have a conductivity of impulses. Similarly, in more complexly organized living beings, this function is performed by separate nerve cells.
Multicellular animals that live in colonies do notshare functions among themselves, and they do not yet have a neural network. It occurs at the stage when different functions are differentiated in the multicellular organism.
For the first time the nervous system appears in the hydra andother coelenterates. It is a network that conducts non-purposeful signals. The structure is not yet formed, it diffusely distributed throughout the body of the coelenterate. Ganglion cells and their Nislev substance are not fully formed. This is the simplest version of the nervous system.
The type of animal's motility is determined by diffusethe reticular nervous system. Hydra performs peristaltic movements, as it does not have special body parts for movement and other movements. For motor activity, it needs a continuous connection between the contracting elements, and it is required that the bulk of the conducting cells be located in the contractile part. Which of the animals for the first time the nervous system appears as a diffuse network? Those who are the founders of the system of human regulation. Proof of this is the fact that there is gastrulation in the development of the embryo of animals.
The subsequent improvement of the nervous regulation was associated with the development of bilateral symmetry in place of the radial and the formation of neuronal clusters in various parts of the body.
In the form of cords, for the first time the nervous system appears in1 flatworm. At this stage, it is represented by paired head nerve nodes and formed from them formed fibers. In comparison with coelenterates, such a system is much more complicated. In helminths, groups of nerve cells in the form of nodes and ganglia are found. The prototype of the brain is the ganglion in the anterior part of the body that performs regulatory functions. It is called the brain ganglion. From it along the entire body go two nerve trunks, connected by jumpers.
All components of the system are locatedoutside, and are immersed in the parenchyma and thus protected from injury. For the first time the nervous system appears in flat worms, along with the simplest senses: touch, vision and a sense of balance.
The next stage of development is the formation ofring formation near the pharynx and several long fibers extending from it. With such characteristics, the nervous system first appears in roundworms. The ocellar ring is a single circular ganglion and serves as the basic organ of perception. With it is connected the ventral cord and dorsal nerve.
Nerve trunks in nematodes are locatedintraepithelial, that is, in hypodermal ridges. In the role of sensory organs are Sensilla - setae, papillae, supplementary organs, amphids and phasmids. All of them are endowed with mixed sensitivity.
The most complex organs of perception of nematodes are amphids.They are paired, can be different in shape and are in front. Their main task is to recognize chemical agents located far from the body. Part of the roundworms also have receptors that perceive internal and external mechanical effects. They are called metanemes.
The formation of the ganglion in the nervous system infurther develops in ringed worms. In most of them ganglionization of the abdominal trunks occurs in such a way that each segment of the worm has a pair of nerve nodes that are connected by fibers to adjacent segments. Ringworms have an abdominal neural chain formed by a brain ganglion and a pair of cords coming from it. They stretch across the abdominal plane. Perceptive elements are located at the front and are represented by the simplest eyes, olfactory cells, ciliated pits and locators. With paired nodes, the nervous system first appeared in ringed worms, but later it develops in arthropods. They have an increase in ganglia in the head and a combination of nodes in the body.
The peak of the evolutionary development of the nervous systemis the appearance of the brain and spinal cord in humans. However, even in the presence of such complex structures, the original diffuse organization remains. This network entangles every cell of the body: the skin, blood vessels, etc. But with such characteristics, for the first time the nervous system appears, who did not even have the opportunity to differentially perceive the environment.
Thanks to these "residual" structural unitsa person has the opportunity to feel different effects even on microscopic sites. The body can react to the appearance of the smallest foreign agent by developing protective reactions. The presence of a diffuse network in the human nervous system is confirmed by laboratory research methods based on the introduction of a coloring substance.
The evolutionary processes of the nervous system proceeded in three stages:
The structure and functioning of the CNS are very differentfrom earlier types. In its sympathetic department, ganglionic and reticular elements are represented. In its phylogenetic development, the nervous system became more and more fragmented and differentiated. The ganglionic stage of development from the reticular was distinguished by the presence of neurons still located above the system of conduction.
Any living organism is essentially a monolith consistingfrom various bodies and their systems, which continuously and continuously interact with each other and with the external environment. For the first time the nervous system appeared in the coelenterates, it was a diffuse network, providing an elementary impulse.
