534 MUSCLE-NERVE PHYSIOLOGY 



normal 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 Helmholtz' figure 

 of thirty-four 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 494 for methods of detecting 

 the action current). 



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

 the following 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 hind leg 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, al- 

 though 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 experiment is known under the name of 

 the rheosco pic 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 increased there is only slight increase of the resulting nerve impulse. 



Fatigue of Nerve Fiber. Many efforts have been made to discover 

 evidences of fatigue of nerve fiber, with practically completely negative 

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

 tensity 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 un- 

 fatigued muscle beyond. By this and other methods it has been found that 



