286 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



body and axon, lis rate of movement varies in different animals and within 

 different nerves of the same animal; in warm-blooded animals it may travel 

 300 feet or more per second, about the speed of a pistol shot. 



Experiments and refined measurements have shown that the impulse is 

 not a purely electrical current as was formerly thought. It is an electro- 

 chemical reaction involving the consumption of oxygen, production of carbon 

 dioxide, the freeing of heat, and modification (depolarization) of electrical 

 charges on the surface membrane of the nerve fiber, followed quickly by their 

 restoration (Fig. 16.8). One such change starts another one just ahead 

 and thus the process travels along a fiber. It is something like a fuse burning 

 along a wire but the nerve fiber is in no way harmed by the passing of the 

 impulse. An impulse cannot be started unless the stimulus is of a certain 

 intensity, but beyond that the strength of the stimulation makes no difference 

 in the speed of the impulse. The stimulus is like a spark that may start 

 a small fire or a large one. The processes in the nerve impulse are in some 

 ways similar to those of muscular contraction. The nervous tissue, however, 

 expends an extraordinarily small amount of energy compared to muscle. 



Nerve cells are not easily fatigued. Impulses pass over a nerve cell sepa- 



FiG. 16.8. Diagram illustrating the membrane theory according to which the 

 nervous impulse is an electrochemical process that passes through a nerve cell. The 

 resting nerve fiber is polarized, that is, the outside is positively charged, and the 

 inside negatively charged. A, B, C, A stimulus passing along a fiber involves a 

 change in the membrane and a loss of polarization. In an interval of from one to 

 five one-thousandths of a second later the fiber becomes repolarized again {%) and 

 the fiber is ready for another impulse to pass over it. In any given nerve, stimulus 

 of a sense organ, perhaps a voice that is heard, results in hundreds of nerve im- 

 pulses each one on a nerve cell fiber that is insulated by its sheath from others 

 beside it. (By permission from Biology: Its Human Implications, 2nd. ed., by 

 Hardin. Copyright, 1952. W. H. Freeman and Company.) 



