Nervous System Evolution

The German philosopher Nietzsche wrote:

"Then you must be a scientist whose field is the leech" said Zarathustra, "and you must pursue the leech to its last rock bottom, you conscientious man!" "Oh Zarathustra!" answered the man, "that would be an enormity, how could I take up such a huge task? What I am the master and connoisseur of is the brain of the leech: that is my field and it is a whole universe."

The human brain appears to have evolved from predecessors of earthworms and leeches whose development was a milestone in eukaryotic evolution (Somjen, 1983). These organisms' nervous systems probably consisted of a chain of organized clumps of nerve cells called ganglia, or perhaps two chains of symmetrically paired ganglia with an enlarged head ganglion at the front end. The polarity and preferred axis of orientation which defined these basic nervous systems are related to polarity and asymmetry within their component nerve cells, or neurons, each a "universe" of its own. As will be described in the next chapters, neuronal orientation and asymmetry are determined by the cytoskeleton which, in many ways, is the nervous system within all higher plant and animal cells.

Over the course of evolution the primitive leech's head ganglion began to dominate other members of its chain, performing "decisions" which required cooperation of the entire assembly. Each segmental ganglion still retained some autonomy of action and, when cut into pieces, such a creature may have been able to regenerate complete new individual organisms like its current descendants. Pairs of leech ganglion chains resemble sympathetic ganglion chains of vertebrates which retain a measure of autonomy. For example, man's autonomic nervous system can efficiently regulate heart, intestine, blood vessels and other organ systems even when disconnected from the brain and spinal cord.

Transition from a segmented organism to a nervous system like our own probably occurred due to fusion of the paired chains of ganglia into a tubelike structure of nervous tissue. Paired nerve roots then emerged from the primitive central system similar to the spinal roots of today's vertebrates. These roots connected the central nervous system with the peripheral sensory organs, muscles and glands. Eventually, the head end of the neural tube increased in size and importance until it dominated most nervous system functions, a process termed "encephalization" by famed English neurologist Hughlings Jackson (Somjen, 1983). Encephalization, which occurred over eons and may be continuing presently within man, reflects development of a hierarchical organization in an otherwise parallel, distributed system. Brain components which are more highly organized and capable of more complex functions are generally newer on the evolutionary scale (i.e. "neocortex"). A collective hierarchy of parallel information processing systems based on functional organization which includes subcellular elements (cytoskeleton and cytoplasmic ground substance) is shown in Table 4.1.

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