I02 



HANDBOOK OF PHYSIOLOGY -^^ NEUROPHYSIOLOGY 



FIG. 21. Recovery of the amplitude of the action potential 

 following abolition of a response of a single node. The ar- 

 rangement shown by the diagram in the upper part of fig. i6 

 was used. .4. Action potential of a single node (,top~) and a 

 truncated 60 cycle wave indicating 100 mv level in applied 

 stimulating and abolishing pulses (^bollom). B, C and D. Super- 

 posed recordings showing recovery after an abolished response. 

 Temperature, io°C. [From Tasaki (126).] 



pulse is needed to initiate a second action potential 

 and the amplitude of the second response decreases 

 continuously with decreasing interval between the 

 two responses. Record D shows that, following; the 

 action potential abolished at its peak, the node 

 exhibits no refractoriness to the following stimulating 

 pulse. In record C, the action potential has been 

 abolished after the potential has fallen slightly from 

 the peak; it is seen that the amplitude of the second 

 response is slightly subnormal at the beginning and 

 recovers gradually. 



These observations reveal how the process respon- 

 sible for the refractoriness progresses during the falling 

 phase of the action potential. As was pointed out by 

 Adrian (2) in 1921, the end of the action potential 

 coincides roughly with the beginning ot the relatively 

 refractory period [cf. Tasaki {119, 124)]. When the 

 action potential is abolished in the middle of its falling 

 phase, the recovery in the amplitude of the second 

 response starts in the middle of the normal recovery 

 curve (126). It has been suggested therefore that the 

 refractoriness is due to some chemical product which 

 accumulates during the falling phase of the action 

 potential. In the sodium theory (see p. 1 18) a different 

 explanation is given to the origin of the refractoriness. 



The rapid falling phase following the shoulder of a 

 normal action potential appears to be a transition of 



the membrane from the active state to the resting 

 state resulting from the gradually rising critical level 

 for abolition reaching the level of the continuously 

 falling potential level of the membrane. 



NERVOUS CONDUCTION .iiLONG UNIFORM .AXONS 



We are now ready to discuss nervous conduction as 

 a process that involves production of action poten- 

 tials in successive portions of the surface membrane of 

 the nerve fiber in an orderly fashion. In the squid 

 giant axon, the rise in the membrane potential** at the 

 peak of the action potential is 100 to 120 mv and the 

 critical depolarization necessary to initiate an action 

 potential is 12 to 15 mv. Furthermore, the resistance 

 of the membrane in the active area is far smaller 

 than that of the membrane at rest (see p. 89). 

 Therefore, when a portion of an axon membrane is 

 thrown into action by a pulse of stimulating current, 

 the adjacent portion of the membrane is automatically 

 brought to action Isy the restimulating effect of the 

 local circuit between the active and resting area of the 

 axon. By a repetition of this process of stimulation 

 by the local circuit, the activity spreads indefinitely on 

 both sides of the site of initial stimulation. 



The local circuit cannot be closed if there is no 

 conducting fluid medium outside the nerve filler. 

 Therefore, nervous conduction is expected to stop if 

 the saline solution outside the fiber is completely re- 

 moved. In practice, it is not possible to remove the 

 fluid outside the fiber completely, but it is easy to re- 

 duce it by immersing a cleaned single nerve fiber in 

 mineral oil. Hodgkin (52) has found that, when an 

 isolated nerve fiber of the cralo is immersed in mineral 

 oil, the velocity of the nerve impulse is markedly re- 

 duced. This is a clear-cut demonstration of the im- 

 portance of the local circuit in the process of propaga- 

 tion of a nerve impulse. 



In figure 22 a set of records from Hodgkin's paper 

 is reproduced. The velocity of the crab nerve fiber in 

 normal sea water was 4 to 5 m per sec. This was re- 

 duced by 20 to 40 per cent when the fiber was trans- 

 ferred into a bath of mineral oil. This reduction in the 



* The membrane potential is defined as the energy required 

 to transfer a unit charge across the membrane from the ex- 

 ternal medium to the axoplasm. If the potential difference 

 between the fluid in the intracellular micropipette and the 

 axoplasm (which is probably small but indeterminable) is 

 ignored, this coincides with the potential of an intracellular 

 electrode referred to the medium. Since the membrane potential 

 at rest is a negative quantity, a small rise in the membrane 

 potential represents a decrease in its absolute magnitude. 



