nervous system
Created | Updated Jul 16, 2003
(To those concerned: A draft of this, also written by me, appears on another site. This has been revised in style for H2G2, which will be its final home if well received here.)
A nervous system is a set of specialized cells (neurons and glial cells) that enables most animals to sense their environment and the state of their own bodies and therefore to act quickly in response to changes in those conditions. The functions of the nervous system are thus to maintain the state of the body itself in a normal range (homeostasis) and to allow the animal to respond in beneficial ways to changes in its environment.
All animals of higher complexity than the sponges (Porifera) have a nervous system of some sort. A nervous system is the basis for the complex behavior of animals; the complexity of behavior is matched by the complexity of the nervous system. Nervous systems vary in complexity from the primitive, decentralized systems of the jellyfish and its cousins (Cnidaria) to the centralized nervous systems of the mammals (Mammalia), particularly the primates, with the human nervous system at the pinnacle.
Organisation
A nervous system has three types of neurons. Sensory neurons detect chemical reactions or changes in energy; they provide the input to the system. Motor neurons excite certain tissues in the animal’s body to cause a response to a change detected by the sensory neurons. Interneurons connect the sensory neurons to the motor neurons; they may form complex neural nets that process the input from sensory neurons. In the most advanced nervous systems, the interneurons form the central nervous system, which is the brain, brain stem and spinal cord.
The simplest nervous system could be formed by just two neurons, a sensory neuron and a motor neuron. The behavior produced by such a system would be a simple reflex, with no intermediate processing of information involved. This kind of neural configuration is called a reflex arc. It occurs even in complex nervous systems. The human knee-jerk reflex, for example, results when a stretch sensing neuron in the knee triggers a motor neuron of the spinal cord that causes the quadriceps muscle to contract, resulting in an involuntary kicking action.
Nervous systems become more interesting when a network of interneurons connects many sensory neurons to many motor neurons. This neural net configuration allows the input from the various sensory neurons to be integrated and processed before the motor neurons are triggered. This is the basis of complex behavior and the first step toward the development of brains and minds. Keep in mind though, that this is still on the development level of roundworms (Nematode).
Evolution
The flatworms (Platyhelminthes) represent an important advance in the development of the nervous system. The planarian flatworms (Planaria) were the first animal to achieve bilateral symmetry in body and a central nerve net that is complex enough to be thought of as a primative brain. The brain and two light-sensitive eyespots are located at one end of the worm, making Planaria the first animal to get a head, so to speak.1
Nervous system evolution has apparently been encouraged by the survival advantage of being able to move around to get food. That is an ability which requires just what the nervous system provides, sensation and motor control. It is interesting to note that development of the brain in particular seems to be associated strongly with the development of jaws and predation. Likewise, the development of other physiological mechanisms has placed further demands on the nervous system for sensing and control, and the nervous system has responded with corresponding increases in capacity and structural complexity.
It seems that humankind is currently at the pinnacle of nervous system development. Our marvelously complex nervous system provides us with astounding ability to understand our environment and ourselves, and to predict the outcome of our own complex behavior as individuals and as various levels of society. Thanks to our neocortex and related brain structures, we can not only sense and act, we are sentient. We can postulate and imagine. We can invent ways to extend our senses and our physical strength. We differ from any other living thing by our ability to control forces that are sufficient to completely destroy ourselves and the world we share with the rest of lifekind.
The future?
Still, reviewing the process of nervous system evolution, it’s extremely difficult to believe that we are the final end product of this process of increasing complexity. Religious considerations aside, we must wonder what new developments in the nervous system might eclipse us as we have eclipsed the lower primates? One must wonder even more whether we would ever allow ourselves to be eclipsed, as we are capable of recognizing and eliminating that which is both different and possibly better than us.
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