Nervous System
Development and functions

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