470 MUSCLE-NERVE PHYSIOLOGY 



mal nerve impulse arises in the nerve-cell body and passes out over the fiber 

 from its origin to its extremity. 



The nerve impulse travels over the nerve fiber with a velocity that was 

 first determined by Helmholtz. He found that in the sciatic of the frog the 

 nerve impulse travels at the rate of twenty-seven meters per second. The rate 

 has been measured in a number of animals and varies between wide limits. 

 In human nerves the rate is variously given, but thirty meters per second 

 may be taken as a fair average. 



The presence of the nerve impulse can be detected by the action current, 

 which exists in nerve as in muscle (see page 451 for methods of detecting 

 the action current). 



Rheoscopic Frog. The action current may be demonstrated by means of the follow- 

 ing experiment: 



The muscle current produced by stimulating the nerve of one muscle-nerve preparation 

 may be used to stimulate the nerve of a second muscle-nerve preparation. The hindleg 

 of a frog with the nerve going to the gastrocnemius cut long is placed upon a glass plate 

 and arranged in such way that its nerve touches in two places the gastrocnemius muscle, 

 exposed but preserved in situ in the opposite thigh of the frog. The electrodes from an 

 induction coil are placed behind the sciatic nerve of the second preparation, high up. 

 On stimulating it with a single induction shock, the muscles not only of the same leg are 

 found to undergo a twitch, but also those of the first preparation, although this is not near 

 the electrodes. The stimulation cannot be due to an escape of the stimulating current 

 into the first nerve, but is due to the action current of the second muscle. This experi- 

 ment is known under the name of the rheoscopic frog. 



When the nerve impulse is studied by means of the action current it is 

 found that a nerve impulse can be aroused by a weaker stimulus than is re- 

 quired to produce a minimal contraction of a muscle. The response of the 

 nerve to graduated strengths of the stimulus is increased very rapidly with 

 slight increase of strength of the stimulus, the augmentation extending through 

 a somewhat greater range than for muscle. If the stimulus is still further in- 

 creased there is only slight increase of the resulting nerve impulse. 



Fatigue of Nerve Fiber. Many efforts have been made to dis- 

 cover evidences of fatigue of nerve fiber, with practically complete negative 

 results. A difficulty has been to secure means of measuring change in intensity 

 of the nerve impulse. The muscle quickly fatigues so that the character of 

 the muscle response cannot be taken when measured in the ordinary way. 

 An effective method used by Howell, Budgett, and Leonard consists in cooling 

 a segment of nerve to suspend its conductivity, during stimulation of the free 

 end ; and periodically warming up the cooled segment of nerve to test the 

 strength of nerve impulse passing through it to the unfatigued muscle beyond. 

 By this and other methods it has been found that a motor nerve is not fatigued 

 by at least ten hours' continuous stimulation with induction currents. 



One must hesitate to draw the conclusion, however, that the nerve fiber 

 conducts the nerve impulse without loss of energy. The fiber can be anesthe- 

 tized, it responds to temperature changes, and gives other evidences of sus- 



