Since evolution gave life on Earth a diffuse-type nervous system, many more stages of development have passed, which have become turning points in the activity of living organisms. These stages differ from each other in the types and number of neuronal formations, in synapses, in terms of functional specialization, in groupings of neurons, and in the commonality of their functions. There are four main stages - this is how the nervous system of the diffuse type, stem, nodal and tubular was formed.
Characteristic
Of the most ancient - the diffuse type nervous system. It is present in such living organisms as hydra (coelenterates - jellyfish, for example). This type of nervous system can be characterized by a plurality of connections in neighboring elements, and this allows anyexcitation is quite free to spread in all directions along the nervous network. The diffuse-type nervous system also provides interchangeability, which gives much more reliable functions, but all these reactions are inaccurate, vague.
The nodular nervous system is typical for crustaceans, mollusks, and worms. This type is characterized by the fact that excitation can only take place in clearly and strictly defined ways, since they have differently organized connections of nerve cells. This is a much more vulnerable nervous system. If one node is damaged, the functions of the body are completely disrupted. However, the nodal type of the nervous system is more accurate and faster in its qualities. If the diffuse type of the nervous system is characteristic of the coelenterates, then the chordates have a tubular nervous system, where features of both the nodal and diffuse types are included. Higher animals took all the best from evolution - both reliability, and accuracy, and locality, and speed of reactions.
How it was
The diffuse type of the nervous system is typical for the initial stages of the development of our world, when the interaction of living beings - the simplest organisms - was carried out in the aquatic environment of the primitive ocean. The protozoa secreted certain chemicals that dissolved in water, and thus the first representatives of life on the planet received metabolic products along with the liquid.
The oldest form of such interaction occurred between individual cells of multicellular organisms through chemical reactions. These are metabolic products - metabolites, they appear whenproteins, carbonic acid and the like break down, and are a humoral transmission of influences, a humoral mechanism of correlation, that is, connections between different organs. The humoral connection can also partly serve as a characteristic of the diffuse type of the nervous system.
Features
The diffuse type of the nervous system is characteristic of organisms in which it is already known exactly where this or that chemical substance coming from the liquid is directed. Previously, it spread slowly, acted in small quantities, and was either rapidly destroyed or excreted from the body even faster. It should be noted here that the humoral connections were the same for both plants and animals. When multicellular organisms developed a diffuse-type nervous system (coelenterates, for example) at a certain stage in the development of the living world, it was already a new form of regulation and communication, qualitatively distinguishing the world of plants from the world of animals.
And further in time - the higher the development of the animal's organism became, the more the organs interacted (reflex interaction). First, living organisms have a nervous system of a diffuse type, and then, in the process of evolution, they already have a nervous system that regulates humoral connections. A nerve connection, unlike a humoral one, is always precisely directed not only to the desired organ, but also to a certain group of cells; connections occur many hundreds of times faster than the first living organisms distributed chemicals. The humoral connection with the transition to the nervous did not disappear, it obeyed, andtherefore, neurohumoral connections arose.
Next step
From the diffuse type of the nervous system (existing in intestinal cavities), living beings left, having received special glands, organs that produce hormones that are formed from nutrients entering the body. The main functions of the nervous system are the regulation of the activity of all organs with each other, and the interaction of the whole organism as a whole with the external environment.
The environment exerts any external influence primarily on the sense organs (receptors), through changes that occur both in the external environment and in the nervous system.
Time passed, the nervous system developed, and over time its higher department was formed - the brain, the large hemispheres. They began to manage and distribute all the activities of the body.
Flatworms
The nervous system is formed by nervous tissue, consisting of an incredible number of neurons. These are cells with processes that read both chemical and electrical information, that is, signals. For example, the nervous system of flatworms no longer belongs to the diffuse type, it is the type of the nervous system of the nodal and stem.
Accumulations of nerve cells in them are paired head nodes with trunks and numerous branches that stretch to all organs and systems. This means that the nervous system of a planaria is not of a diffuse type (this is a flatworm, a predator that eats small crustaceans, snails). In lower forms of flatworms,there is a reticular nervous system, but in general they no longer belong to the diffuse type.
Annelled worms
Annelids also have a non-diffuse nervous system, it is much better organized in them: they do not have a nerve plexus that can be observed in mollusks. They have a central nervous apparatus, which consists of a brain (supraglottic ganglion), peripharyngeal connectives and a pair of nerve trunks that are located under the intestine and connected by transverse commissures.
Most annelids have completely ganglionized nerve trunks, when each segment has a pair of ganglia that innervates its own segment of the body. Primitive annelids live with nerve trunks widely spaced in the underbelly, connected by long commissures. You can call this structure of the nervous system ladder. Highly organized representatives have a shortening of the commissures and convergence of trunks almost to the point of confluence. It is also called the ventral nerve circuit. Much simpler living organisms have a diffuse-type nervous system.
Cnidarians
The simplest diffuse nervous system in cnidarians is the plexus, in the form of a grid that consists of multipolar or bipolar neurons. Hydroids have it on top of the mesoglea, in the ectoderm, while coral polyps and scyphoid jellyfish have it in the endoderm.
A feature of such a system is that activity can spread in absolutely any direction and from absolutely anystimulated point. This type of nervous system is considered primitive, but it eats, swims, and otherwise such an organism does not work very simply. It is worth watching how sea anemones move on mollusk shells.
Jellyfish, sea anemones and others
In addition to the nervous network, jellyfish and sea anemones have a system of long bipolar neurons that form chains, therefore they have the ability to transmit impulses faster without attenuation over long distances. This is what allows them to carry out a good overall response to all sorts of stimuli. Other groups of invertebrates may have both nerve networks and nerve trunks, noted in various parts of the body: under the skin, in the intestines, in the pharynx, in molluscs - in the leg, in echinoderms - in the rays.
However, already in cnidarians, there is a tendency for neurons to concentrate at the oral disc or in the sole, like in polyps. Along the edge of the umbrella, jellyfish have nerve endings, and in some places - thickenings on the ring - nerve cells in large clusters (ganglia). The marginal ganglia on the umbrellas of jellyfish are the first step towards the emergence of a central nervous system.
Reflex
The main form of nervous activity is a reflex, the reaction of the body to a signal about a change in the external or internal environment, which is carried out with the participation of the nervous system, responding to irritation of the receptors. Any irritation with excitation of receptors runs along centripetal fibers to the central nervous system, then through the intercalary neuron -back to the periphery already along centrifugal fibers, exactly getting to one or another organ whose activity has been changed.
This path - through the center to the working body - is called a reflex arc, and it is formed by three neurons. First, the sensitive one works, then the intercalary one, and finally the motor one. A reflex is a rather complex act; it will not work without the participation of a large number of neurons. But as a result of such an interaction, a response can occur, the body will respond to irritation. Jellyfish, for example, will burn, sometimes treat with deadly poison.
The first stage of development of the nervous system
Protozoa do not have a nervous system, but even some ciliates have a fibrillar intracellular excitable apparatus. In the process of development, multicellular organisms formed a special tissue that was able to reproduce active reactions, that is, to be excited. The network-like system (diffuse) chose hydroid polyps as its first wards. It was they who armed themselves with processes of neurons, diffusely (netlike) placing them throughout the body.
Such a nervous system conducts an excitation signal very quickly from the point where the irritation is received, and this signal rushes in all directions. This gives the nervous system integrative qualities, although not a single fragment of the body, taken separately, has such a feature.
Centralization
Centralization to a small extentalready noted in the diffuse nervous system. Hydra acquire nerve thickenings in the areas of the oral pole and sole, for example. This complication occurred in parallel with the development of the organs of movement, and was expressed in the isolation of neurons, when they went from the diffuse network into the depths of the body and formed clusters there.
For example, in coelenterates, free-living (jellyfish), neurons accumulate in the ganglion, thus forming a diffuse-nodular nervous system. This type arose primarily due to the fact that special receptors developed right on the surface of the body, which were able to respond selectively to light, chemical or mechanical influences.
Neuroglia
Living organisms, together with the above, in the process of evolution increase both the number of neurons and their diversity. Thus, neuroglia were formed. Neurons also appeared bipolar, having axons and dendrites. Gradually, organisms get the opportunity to carry out excitation in a directed way. Nervous structures also differentiate, signals are transmitted to cells that control responses.
This is how the development of the nervous system proceeded purposefully: some cells specialized in reception, others in signal transmission, and still others in response contraction. This was followed by evolutionary complication, centralization, and the development of a nodal system. Annelids, arthropods, and mollusks appear. Now the neurons are concentrated in the ganglia (nerve nodes), which are tightly connected by nerve fibersbetween themselves with receptors and organs of execution (glands, muscles).
Differentiation
Next, the body's activity is divided into components: the digestive, reproductive, circulatory and other systems are isolated, but the interaction between them is necessary, and this function was taken over by the nervous system. The central nervous formations have become much more complicated, many new ones have arisen, now completely dependent on each other.
The circumshield nerves and ganglia, which control nutrition and movement, evolved into receptors in phylogenically higher forms, and they now began to perceive smell, sound, light, and sense organs appeared. Since the main receptors were located at the head end, the ganglia in this part of the body developed more strongly, finally subordinating the activity of all the others. It was then that the brain was formed. For example, in annelids and arthropods, the neural chain is already very well developed.
