Nervous System
Development and functions

Brain Of Amphibians and Mammals

December 19th, 2009

When amphibians left the water, they had a great many problems to face. In particular, their sense of smell became less acute. Fishes can perceive the odours of substances dissolved in water. In order to differentiate between smells on land, the pioneering amphibian had first to dissolve the odorous substances in some fluid present in its nose. Its olfactory receptors could not immediately adapt to the new conditions and its forebrain, receiving no information to digest, remained idle, as it were. This is, apparently, the reason why the forebrain in the amphibians assumed another function. It began to help in analyzing visual, auditory and, perhaps, many other stimuli. For the first time a division of the brain appeared which dealt with all sorts of information.

In mammals the brain developed particularly rapidly. First, it developed individual zones, which were not as yet strictly dillerentiated. Each zone was responsible for the analysis of a certain kind of stimulation — visual, auditory, olfactory, or skin irritation. Higher mammals developed small areas of so-called association cortex, which lay between the analyzing zones. These zones continually grew and prog­ressed in the course of the further evolution of the brain. In apes and humans they occupy a large part of the surface of the cerebral hemispheres. It is not difficult to guess that it is these zones that perform the most involved, purely human mental functions.

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December 19th, 2009 13:41:14

Ganglionated Type Of Nervous System

December 17th, 2009

The nervous system of the ganglionated type proved to be very convenient. In Annelida, which must have descended from the flatworms, all the nerve cells are concentrated in the ganglia, while the nerve strands connecting them hold only the long processes of these cells. Practically every segment of the worm has a pair of ganglia connected to each other. Besides, each ganglion is linked through the nerve strands with the corresponding ganglia of the preceding and following segments. This nervous system bears a close resemblance to a ladder. The anterior pairs of ganglia are the largest. They carry out the most important functions and have command over the rest of the nervous system.

In higher worms the ganglia come closer together, making up a single, compact formation. Their nervous system has some features characteristic of that of contemporary vertebrates.

We do not know what the brain of the first vertebrates was like. The lancelet, one of the most primitive represen­tatives of the chordates, has only a nerve cord, but as yet no cerebrum. This part of the brain first appears in the cyclostomes (lampreys and hagfish) and in fishes.

In these primitive animals the brain is divided up into the same sections as the brain in human beings. These sections are the same, but their structures and, what is more important, their functions essentially difler. The forebram is the main organ controlling the mental processes in a human being. All it does in lampreys and fishes is to analyze olfactory stimulations. In amphibians the functions of the forebrain are somewhat more complicated.

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December 17th, 2009 13:35:34

Nerve Cells And Nerve Strands

December 15th, 2009

The nerve cells in primitive Coelenterata are joined together by their processes to form a nerve network, the most primitive type of nervous system. The next improvement was the emergence of separate clusters of nerve cells, with their subsequent development into more organized and more compact nerve strands. These came into existence wherever the co-ordinated action of many contractile elements was required. Such clusters form the nerve rings encircling the umbrella of a jelly-fish, and cause the whole umbrella to tighten up or come loose, thus enabling the creature to move actively in the water.

In flatworms, the descendants of the Coelenterata, ail the nerve cells are concentrated in the form of strands arranged like braiding around the body in intricate patterns. Numerous constrictions between the strands, as well as the sites where the nerves come into direct contact, ensure the co-ordinated functioning of the entire nervous system. A diffuse network of nerve strands was undoubtedly an improvement compared with the network of randomly scat­tered nerve cells. However, this barrel-like nervous system proved too cumbersome and intricate to control the functions of the animal’s separate parts and organs, and a new organ was required to direct its operation.

Such a central organ first appeared in the higher represen­tatives of flatworms. It consists essentially of nerve strands with numerous nerve cells, aggregated into masses which are known as ganglia. These ganglia not only assumed the most difficult functions, but also influenced the work of other parts of the nervous system. Ganglia are primarily to be found near the sense organs, the eyes, the organ of equilibrium, and also near the gullet with which the flatworms catch their prey, hold it and push it into the intestine.

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December 15th, 2009 13:30:36

Long Evolution of Human Brain

December 14th, 2009

When reading the works of the ancient scholars, one cannot help being surprised at the number of scientific discoveries that were made merely as the result of observation and subsequent conjecture. More than two thousand years ago, scholars and physicians possessed quite a profound knowledge of how most of the human organs function. Nevertheless, they did not even suspect the real function performed by the brain. Strange as it may seem, Aristotle, a prominent Greek scholar who lived in the fourth century В. С, considered the brain to be merely a large gland for cooling the blood. Now we know that the brain is by no means a refrigerator. We also know what purpose this so-called “gland” serves, but the way it operates still remains largely a mystery.

The human brain developed as the result of long evolution of the nervous system which originated in the primeval oceans when individual biological molecules finally merged to produce little conglomerates of living matter. Those primary living particles, as well as the subsequent more complex unicellular organisms which settled in large colonies, already possessed two main properties, irritability and conductivity, i. e. the ability to transmit excitation to neighbouring cells.

Later, in multicellular animals there emerged a dineren-tiation between these functions. The Coelenterata were the first to develop special nerve cells with a high degree of irritability and conductivity. The function of these cells was to become ever more sensitive to external influences and to transmit the excitation to those cells or organs which could react in a way beneficial to the organism.

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December 14th, 2009 13:30:22