Since evolution gave a diffuse type nervous system to life that appeared on Earth, there have been many stages of development that have become turning points in the activities of living organisms. These stages differ from each other in types and number of neuronal formations, in synapses, in terms of functional specialization, in groups of neurons, in the generality of their functions. There are four main stages - this is how the nervous system of the diffuse type, the stem, nodular and tubular, was formed.
Characteristic
Of the most ancient, the nervous system is of a diffuse type. It is present in such living organisms as hydra (intestinal - jellyfish, for example). This type of nervous system can be characterized by a plurality of connections in neighboring elements, and this allows any excitation to spread quite freely in all directions along the nervous network. The diffuse type nervous system also provides interchangeability, which gives significantly greater reliability to the functions, but all these reactions are inaccurate, vague in nature.
The nodal nervous system is typical of crustaceans, mollusks, and worms. This type is characteristic in that the excitation can take place only in clearly and rigidly defined ways, since they have otherwise 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 quality. If the diffuse type of the nervous system is characteristic of the intestinal cavity, then the chordate possess the tubular nervous system, where features of both the nodular and diffuse types are included. Higher animals took all the best from evolution — reliability, accuracy, locality, and speed of reactions.
How it was
The diffuse type of the nervous system is characteristic of the initial stages of the development of our world, when the interaction of living beings - simple organisms - was carried out in the aquatic environment of the primeval ocean. The protozoa isolated some 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 when proteins, carbon dioxide and the like break down, and are a humoral transmission of influences, a humoral correlation mechanism, that is, connections between different organs. A characteristic of the diffuse type of the nervous system can partly serve as a humoral connection.
Features
The diffuse type of the nervous system is characteristic of organisms in which it is already known where a particular chemical substance from the liquid is directed. Previously, it spread slowly, acted in small quantities, and either quickly destroyed, or was even more rapidly excreted from the body. It should be noted here that the humoral bonds were the same for plants and animals. When the multicellular nervous system appeared of a diffuse type (enteric, for example) at a certain stage of development of the living world, this was already a new form of regulation and connections, which qualitatively distinguishes the world of plants from the animal world.
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 diffuse type of nervous system, and then, in the process of evolution, they already have a nervous system that regulates humoral bonds. The neural connection, in contrast to the humoral one, is always precisely directed not only to the desired organ, but also to a certain group of cells, the bonds occur many hundreds of times faster than the first living organisms distributed chemicals. The humoral connection with the transition to the nervous system did not disappear, it obeyed, and therefore neural-humoral connections arose.
Next stage
Living things have left the diffuse type of the nervous system (exists in the intestinal cavity), having received special glands, organs that produce hormones that are formed from nutrients entering the body. The main functions of the nervous system are both 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 senses (receptors), carried out through changes that occur both in the external environment and in the nervous system.
As time went on, the nervous system developed, and over time, its higher part was formed - the brain, the cerebral hemispheres. They began to manage and distribute all the activities of the body.
Flatworms
The nervous system is formed by the nervous tissue, which consists 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 does not belong to the diffuse type anymore, it is the type of the nervous system nodular and stem.
The accumulations of nerve cells in them are paired head nodes with trunks and numerous branches that stretch to all organs and systems. So, the planaria is not a diffuse type of nervous system (this is a flat worm, a predator that eats small crustaceans, snails). The lower forms of flatworms still have a reticular nervous system, but in general they are no longer of the diffuse type.
Annelids
Ringworms also have a non-diffuse type of nervous system; they have a much better organized system: they have no nerve plexus, which can be observed in mollusks. They have a central nervous system, which includes the brain (supraglottis ganglion), periopharyngeal connections and a pair of nerve trunks that are located under the intestine and connected by transverse commissures.
In most ringworms, nerve trunks are completely ganglionized when each segment has a pair of ganglia innervating its own body segment. Primitive annelids live with nerve trunks widely spaced in the abdomen, connected by long commissures. We can call such a structure of the nervous system staircase. Highly organized representatives have shortened commissions and rapprochement of trunks almost before the merger. This is also called the abdominal nerve chain. The nervous system of the diffuse type have much simpler living organisms.
Cnidaria
The simplest diffuse nervous system in vagabonds (cnidarians) is plexus, in the form of a grid that consists of multipolar or bipolar neurons. Hydroids have it on top of the mesogley, in the ectoderm, and coral polyps and scyphoid jellyfish in the endoderm.
A feature of such a system is that activity can spread in absolutely any direction and from absolutely any stimulated point. This type of nervous system is considered primitive, but it eats, swims, and otherwise the organism is not very simple. It is worth seeing how sea anemones move to mollusk shells.
Jellyfish, sea anemones and others
In addition to the neural 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 reaction to various stimuli. Other groups of invertebrates can have both nerve networks and nerve trunks, marked on various parts of the body: under the skin, in the intestines, in the pharynx, in mollusks - in the leg, in echinoderms - in the rays.
However, there is already a tendency among strifera, in which neurons are concentrated in the oral disc or in the sole, as in polyps. Nerve endings are formed along the edge of the umbrella in jellyfish, and in some places - thickening on the ring - nerve cells in large clusters (ganglia). The marginal ganglia on jellyfish umbrellas are the first step to the appearance of the central part of the nervous system.
Reflex
The main form of nervous activity is a reflex, the body's reaction 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 receptor irritation. Any irritation with excitation of the receptors runs through the centripetal fibers to the central nervous system, then through the insertion neuron - back to the periphery already through the centrifugal fibers, just getting to one or another organ whose activity is changed.
Such a path - through the center to the working body - is called a reflex arc, and it is formed by three neurons. First, the sensitive is triggered, then the insertion, and finally the motor. Reflex is a rather complicated act; it will not work out without the participation of a large number of neurons. But as a result of such interaction, a response can take place, 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
In protozoa, the nervous system is absent, however, even some ciliates have a fibrillar intracellular excitable apparatus. In the process of development, multicellular formed a special tissue that was able to reproduce active reactions, that is, to be excited. The reticular system (diffuse) was the first to choose hydroid polyps. It was they who armed themselves with the processes of neurons, diffusely (network-like) placing them throughout the body.
Such a nervous system very quickly conducts an excitation signal from the point where irritation is received, and this signal is carried in all directions. This gives the nervous system integrative (characteristic of the whole body, uniting) qualities, although not a single fragment of the body, taken separately, has such a feature.
Centralization
Centralization to a small extent is already observed in the diffuse nervous system. Hydras acquire nerve compaction in the areas of the oral pole and sole, for example. This complication occurred simultaneously with the development of the organs of motion, and was expressed in the isolation of neurons when they left the diffuse network in the depths of the body and formed clusters there.
For example, in intestinal, freely living (jellyfish) neurons accumulate in the ganglion, thereby forming a diffuse-nodal type nervous system. This type arose primarily due to the fact that special receptors developed directly on the surface of the body, which were able to react selectively to light, chemical or mechanical influences.
Neuroglia
Living organisms, along with the above, in the process of evolution increase both the number of neurons and their diversity. Thus, neuroglia was formed. Neurons and bipolar appeared, having axons and dendrites. Gradually, organisms are able to carry out stimulation in a directed manner. Nerve structures also differentiate, signals are transmitted to the cells that control the responses.
Thus, the development of the nervous system was deliberate: some cells specialized in reception, others specialized in signal transmission, and others specialized in contracting. What followed was evolutionary complication, centralization, development of a nodal system. Ringworms, arthropods, mollusks appear. Now neurons are concentrated in the ganglia (nerve nodes), which nerve fibers are tightly connected to each other with receptors and organs of execution (glands, muscles).
Differentiation
Then there is a division of the body into components: the digestive, reproductive, circulatory and other systems are isolated, but the interaction between them is necessary, and the nervous system took over this function. The central nervous formations became much more complicated, many new ones appeared, now completely dependent on each other.
The parathyroid nerves and ganglia, which control nutrition and movement, developed into receptors in phylogenically higher forms, and now they began to perceive smell, sound, light, and sensory organs appeared. Since the main receptors are located at the head end, the ganglia in this part of the body have developed more, finally subordinating the activity of everyone else. It was then that the brain formed. For example, in annelids and arthropods, the nerve chain is already very well developed.