xMULTICELLULAR ANIMALS 235 



But the nerve impulse that traverses a nerve fibre appears to be 

 more compHcated than a redistribution of electrical charges at the 

 boundary of the fibre. According to present views the propagation 

 of the impulse is conditioned upon both electrical and chemical 

 changes in the boundary of the fibre. The electrical changes are 

 regarded as essentially a sweep of depolarization of the membrane, 

 as indicated in the preceding paragraph. The metabolic changes are 

 thought to be oxidative chemical reactions initiated in the substances 

 of the fibre surface by the changes in its electrical condition. This 

 double nature of the impulse is revealed by its physiological char- 

 acteristics; for purposes of illustration we may list the more impor- 

 tant ones here: 



Characteristics of the Nerve Impulse. A stimulated nerve 

 ogives oflF more carbon dioxide and consumes more oxygen than a 

 resting nerve. Moreover, the impulse causes a slight rise in tempera- 

 ture, somewhat less than one ten-millionth of a degree. These facts 

 show that oxidative metabolism is in some way associated with the 

 impulse. Further evidence appears in the fact that in the absence 

 of oxygen the nerve soon loses its ability to transmit stimuli. 



The speed at which the impulse travels varies between different 

 types of animals and between different nerves in the same animal. 

 In human nerves the maximum rate of propagation reaches 125 

 meters per second. Obviously the nerve impulse is not an electrical 

 current in the ordinary sense, for the speed of electricity is approxi- 

 mately 186,000 miles per second. 



The speed of the impulse varies with the temperature of the nerve 

 fibre in much the same way that temperature influences chemical 

 reactions. Moreover, if the fibre is crushed, the impulse will not 

 pass the broken region, even though the crushing be done by tying 

 the nerve with a copper or other metallic wire. Here again is evi- 

 dence of the metabolic nature of the impulse and of its dependence 

 on the pecuhar structure of the nerve, not merely an electrical 

 conductor. 



