The nervous system
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The nervous system includes the nerves and the brain, and the nervous system carries information from various parts of the body to the brain. Not all living organisms have even a rudimentary nervous system--single-celled creatures, for instance, have no nervous system. The nervous system developed first in prehistoric chordates and then became more and more complex as evolution produced more and more complex creatures. We can see what the earliest nervous systems must have been like in more primitive chordata such as the Amphioxus. The nervous system can then be traced up through more complex organisms to its highest manifestation in the human being. Information about the outside world and about the inner workings of the body speeds to and from the brain through nerves, which are bundles of the long, tubelike extensions of nerve cells. Information is conveyed through them by impulses fired along their length. Nerve impulses travel only in one direction. As an example, if information from one part of the body is transmitted to the brain through one nerve, orders from the brain cannot travel back along the same nerve but must be carried by a different nerve. This shows how complex the system has become, but the system began in a much simpler form. The chordates are of the phylum Chordata and include the well known vertebrates (fishes, amphibians, reptiles, birds, mammals). Vertebrates and hagfishes together comprise the taxon Craniata, while the remaining chordates a
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known members of Vertebrata, the chordate subgroup that is most familiar to us. Fossils representing most major lineages of fish like vertebrates and the earliest tetrapods (Amphibia) can be found in deposits before the end of the Devonian Period. Reptile like tetrapods originated during the Carboniferous periods, while mammals differentiated before the end of the Triassic and birds before the end of the Jurassic (Lundberg, 1995, www.phylogeny.arizona.edu).
Studies of the origin of the nervous system have been given special attention since the latter portion of the last century based on a desire to reduce the extreme complexity of the vertebrate nervous system to its basic or elementary properties. comparative physiology and anatomy provide most of the evidence for determining the nature of the primitive nervous system and its origin. the nervous systems of lower invertebrates are identified as primitive because they lack all the specializations present in higher invertebrates and vertebrates. the most primitive metazoan groups today include the Porifera, Coelenterata, and Platyhelminthes. Most theories of the origin of the nervous system are based on the neural organization of the coelenterates because this is the lowe
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Approximate Word count = 2519
Approximate Pages = 10 (250 words per page)
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