36 Applied Biophysics 



These nerve action potentials can be demonstrated visually by 

 means of the cathode-ray oscillograph. 



A suitable amplifier for the demonstration of the electrical 

 changes in sensory nerves consists of a four-stage resistance- 

 capacity coupled amplifier employing MH4 thermionic triode 

 valves. The plate of the first valve is fed through a resistance 

 of 50,000 ohms, 20,000 ohms of which is used for decoupling 

 through a 4 mfd condenser. The second valve is fed through 

 a similar resistance, 10,000 ohms of which is used for decoupling 

 through a 4 mfd condenser. The third valve is fed through a 

 similar resistance, and the decoupling is the same as in the 

 preceding valve. The output valve is fed through all ,000 ohms 

 resistance, 1,000 ohms of which is employed for decoupling 

 through a 4 mfd condenser. The anode of this stage is fed via 

 a 2 mfd condenser, and the earth line to the Y plates of a 

 cathode-ray oscillograph. The intercoupling condenser of each 

 stage is 1 mfd, and the grid-bias resistor is 0.25 meg ohms, 

 giving for each stage a time constant of 0.25 seconds. 



Various specialized amplifiers and general purpose biological 

 amplifiers have been developed for this type of work. Other 

 recording devices besides the cathode-ray oscillograph, such as 

 the mirror oscillograph, have been used. 



Muscle Action Potentials 



The action potentials of muscle fibers are similar in shape 

 to those of nerve fibers, but are larger and slower. 



Wedensky ^- used the telephone as an indicator to study the 

 rate of electrical changes in voluntary muscular contraction. 

 Piper ^^ used the string galvanometer in recording the electro- 

 myogram. Adrian and Bronk ^ demonstrated that the action 

 potentials from voluntary muscle can be recorded by means of 

 a concentric needle electrode. Denny-Brown and Pennybacker ^^ 

 showed that the recording of action potentials from voluntary 

 muscle in certain pathological conditions gave useful information 

 concerning the nature and position of the underlying patho- 

 logical process. Weddell, Feinstein, and Pattle ^^ point out that 



