DIPHASIC ACTION POTENTIAL 



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high-potential output of a small induction coil. When the key in the 

 primary circuit of the induction coil is rapidly closed and opened, one or 

 more 0.2-milliampere high-potential induction discharges stimulate the 

 nerve fiber at this point and set up a nerve impulse i a which travels 

 away from G in both directions. 



This impulse, in a thick mammalian nerve fiber, having a diameter of 

 about 20 microns, may attain a speed of 100 meters per second; in 

 unmyelinated nerve it may be as low as 1 meter per second. 



The arrival of the pulse at B is shown by a deflection of the electric 

 impulse recorder E. The successive changes in the deflections indicate 

 that the external surface at the electrode B has been subjected to a rapid 

 drop in potential, followed by a slower recovery to its former value. A 

 record of the fall and rise of this action potential is inserted in the figure. 

 The action is designated as monophasic. The inference is that the 

 nerve impulse is an activity that travels with moderate speed, that it 

 has an electrical origin, and that it is a pulse of activity with a rather 

 steep wave front. It manifests itself as a local decrease and a slower 

 recovery in the surface potential of the nerve. 



Diphasic Action Potential 



Another method of approach is to investigate the propagation of the 

 nerve impulse uninfluenced by the complications introduced by the 

 demarcation current. The experiment may be modified as shown in 



Oh 



• \-\ 



In 1=1- 



V-/ 



To induction 

 coil 



Fig. VI-6. Diphasic action potential i\, ii, due to a wave pulse i a passing under 

 contact c and then under contact d. The potential is amplified and presented as 

 viewed on the fluorescent face of a cathode-ray oscillograph as two successive op- 

 positely directed pulses. 



Fig. VI-6. This diagram shows a normal nerve with the pick-up elec- 

 trodes placed on its surface at the points x\ — x 2 . Under these cir- 

 cumstances no current flows through E either from c to d or from d to c, 



