Applications of Electronics in Medicine 37 



the activity of normally contracting motor units and of fibrillation 

 can be easily distinguished, and it is consequently possible to 

 decide whether a muscle is innervated normally, partially, or 

 not at all. For the exploration of the whole muscle, about six 

 punctures of the needle electrode may be required, but this is 

 rarely necessary and gives only trifling discomfort. Elliott ^'^ 

 made electromyographic studies of tender muscles in sciatica. 

 He demonstrated that the tender spots in the muscles are, as a 

 rule, the seat of a localized increase of irritability and a con- 

 tinuous discharge of action potentials, which lasts as long as 

 the needle remains in the muscle. 



A technique commonly employed in electromyography is to 

 insert a concentric electrode, made of fine wire running through 

 the center of a fine-gage hypodermic needle, into the belly of 

 the muscle. The needle's barrel acts as an earthed shield, and 

 the minute wire electrode picks up the electrical activity of units 

 within a radius of 1 millimeter. The electrical potentials are 

 amplified by a standard amplifier, and records can be observed 

 and photographed on a cathode-ray tube. Weddell, Feinstein, 

 and Pattle '^^ employ a special all-mains-operated amplifier. 

 Cathode-ray oscilloscope tracings are used for permanent records, 

 for practical purposes, however, only the sounds emitted from 

 an output loudspeaker are noted ; the detection of small differ- 

 ences in duration and frequency are more easily assessed by 

 auditory than by visual methods. 



Chronaxie Meters and Electronic Stimulators 



The effectiveness of a stimulus depends not only on its 

 strength, but also on the time duTing which it is allowed to 

 flow through the tissues. Chronaxie is defined as the time during 

 which a current, twice as great as the rheobase, must flow through 

 a tissue to set up activity. It is a measure of the excitability of 

 a tissue. 



Brian Denny ^ developed, from the original circuits of 

 Bauwens, an apparatus which aims at providing the means of 

 determining, accurately, the response to electrical stimulation of 



