Applications of Electronics in Medicine 35 



Nerve Action Potentials 



Nerve fiber is a tissue in which some of the properties of Hving 

 matter, especially conductivity and excitability, have become de- 

 veloped to an exceptional extent. The electrical stimulus is the 

 common one employed experimentally, but chemical or mechani- 

 cal stimuli are also effective. A nerve impulse travelling along 

 a nerve fiber is accompanied by a characteristic electrical change, 

 which is a diphasic potential wave. Once the impulse has been 

 initiated in a nerve, it is "all or none." If a nerve fiber is 

 stimulated electrically, the rate of travel and magnitude are 

 independent of the strength of the stimulus, and depend only 

 on the state of the nerve at the point under consideration. In 

 any particular fiber, stronger stimulation causes only an increase 

 in the frequency of the potential waves. A nerve trunk may 

 contain thousands of fibers of varying types and sizes, and 

 records may show a complex series of transients. In the human 

 body, the waves have a peak potential of about 1.0 millivolt 

 (which is only a fraction of that developed by the nerve owing 

 to the shunting effect of the inactive adjacent fibers in the nerve 

 trunk), and last about 1.0 milliseconds. 



The early work on nerve action potentials was handicapped 

 by the fact that the majority of recording instruments which 

 were sensitive enough for the purpose, for instance, the capillary 

 electrometer and the string galvanometer, required appreciable 

 power to work them, besides suffering from inertia. Pioneer 

 work was done by Adrian,^ using a capillary electrometer, and 

 Forbes and Thacher -^ with a string galvanometer. Gasser and 

 Erlanger -^ used the cathode-ray Oscillograph as the recording 

 device. Adrian - in his monograph on The Mechanism of Nerv- 

 ous Action, gives a review of the work done in this field to that 

 date. 



Wever and Bray ^^ had the courage to connect the auditory 

 nerve with an amplifier and telephone. They found that any 

 sound reaching the ear was reproduced in the telephone ; speech 

 could be understood, and the speaker identified by his voice. 



