70 The Conduction of Impulses by Nerves /4 : I 



knowledge of the structure and properties of the neurons. Studies of 

 nerves have shown that they consist of bundles of long processes called 

 axons or nerve fibers. The axons are each a part of an individual neuron. 

 Along the nerve fiber, the information is coded and transmitted in the 

 form of an "all-or-none" or "on-off" electrical pulse called an action 

 potential or spike potential. 



On a teleological basis, the problems of the nervous system are similar 

 to those of transmitting telephone messages over long distances. Either 

 there must be many parallel low frequency channels, or fewer high 

 frequency channels, each modulated by many separate signals. The 

 living organisms which respond rapidly to external stimuli (that is, 

 animals) have varying numbers of parallel low frequency electrical 

 channels. The number of channels increases with the complexity of 

 the animal. Along each of these channels (nerve fibers), information is 

 transmitted by electrical pulses, referred to as action (or spike) potentials. 

 The individual channel, with its energy supply and its connections, is 

 called a neuron. 1 Its distinguishing features involve the biological 

 generation and transmission of electrical potentials. 



The earliest experiments which could be called bioelectrical occurred 

 toward the end of the eighteenth century. Galvani put two dissimilar 

 metals into a frog's leg muscle and observed a twitch. He correctly 

 associated the response with electricity but assumed that the electricity 

 was generated within the muscle by a vital process. Volta proved that 

 Galvani's electricity was not of biological origin; the existence of true 

 biological potential generators was not discovered for almost another 

 century. Today, it is known that all nerve fibers, in fact, probably all 

 cell membranes, are charged electrically. The membrane charges, as 

 well as the spike potentials, are so small that they could not be observed 

 with the instrumentation of Galvani and Volta. The field of bio- 

 electricity is a fertile one for the application of electronic gadgeteering 

 and physical instrumentation; it has attracted many persons with a 

 background in physics who welcomed a challenging biological problem 

 to which they could apply their previous training. A major application 

 of bioelectricity is the study of the conduction of .impulses by 

 nerves. 



Animals possess other mechanisms, besides the bioelectrical properties 

 of the nervous system, for transmitting information from one part to 

 another. These other systems are called endocrine; they involve the 

 internal secretion of certain chemicals called hormones. The hormones 

 alter metabolic rates, dilation of blood vessels, and secretory rates at 



1 Some giant invertebrate fibers are fusion products of several embryonic 

 neurons. 



