4 



The Conduction of Impulses 



by Nerves 



I. The Role of the Nervous System 



The nervous system is composed of units called neurons which transmit 

 information in the form of electrical pulses from one place within the 

 organism to another. This action is essential for the rapid responses of 

 animals to external stimuli. Animals respond more rapidly than plants 

 do to conditions outside themselves. For instance, certain plants have 

 flowers which are open only in bright sunlight, and deciduous trees shed 

 their leaves during the fall season. However, these are comparatively 

 slow responses involving time intervals from minutes to days. In con- 

 trast, the responses of a motorist to a red light or of a fly to an approach- 

 ing swatter are both very much quicker. These rapid responses of 

 animals timed in milliseconds or, at most, seconds are mediated by the 

 nervous system. 



The rapid coordinations and responses of animals strongly suggest 

 that the nervous system must transmit information in an electrical or 

 magnetic form. One might reach this conclusion without detailed 



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