lOO HANDBOOK OF PHYSIOLOGY ^^ NEUROPHYSIOLOGY I 



subthreshold response. We shall first discuss the lime 

 factor. 



When a rectangular pulse of voltage is applied 

 across the air gap in the arrangement of figure ig, the 

 membrane potential at the nearest node (Ni and Nj) 

 rises (or falls) along a sigmoid curve. This sigmoid 

 time course arises from the situation that both the 

 myelin sheath and the nodal membrane have a 

 capacity which delays spread of the membrane po- 

 tential. The problem of spread of potential along a 

 uniform cable is discu.ssed in some detail on p. 86. 

 The situation in the myelinated nerve fiber is compli- 

 cated by the discontinuities at the nodes, and un- 

 fortunately no rigorous mathematical solution of the 

 problem is at present available. It is certain, however, 

 that both V (the membrane potential at the node) 

 and dV/dl are zero at / = o, and 1' rises first gradually, 

 then faster and finally approaches the plateau. 



The variation in the membrane potential caused by 

 a brief voltage pulse is given by the derivative, dl'/dt, 

 times a constant, because a brief pulse can be re- 

 garded as a diff"erence between two long rectangular 

 pulses of the same intensity but starting in succession 

 at a small time interval. From this it follows that the 

 maximum of the membrane potential change caused 

 at the node by the test shock is reached a certain 

 period of time, t,,, after the delivery of the shock [see 

 curve 1-/4 on p. 492 of Lorente de No (77)]. This time 

 (to) depends on the distance from the stimulating par- 

 tition to the node under study (122). Now, in the 

 range of voltage, v, where the relationship between S 

 and !> is expressed by straight line I in figure igfi, 

 action potentials are elicited when the algebraic sum 

 of the potentials caused by v and S reaches the 

 critical level. Therefore, the origin of time has to be 

 shifted to the left by to if the curves are to represent 

 the change in the state of the fiber caused by the sub- 

 threshold pulse, V. The argument along this line was 

 developed first by Erlanger & Blair (14, 38) and 

 later by Tasaki (118, 122). 



Next, we discuss the second factor that has to be 

 taken into consideration in the analysis of the curves 

 in A of figure 19. W'hcn the test shock, S, precedes the 

 start of the subthreshold pulse, v, the change in the 

 threshold of S is small. It has been mentioned that, 

 when a brief shock is close to its threshold, the fall in 

 the membrane potential at the node is far slower 

 than that expected from the physical constants of the 

 resting nerve fiber. If a weak (positive) rectangular 

 pulse (j/) follows such a barely subthreshold test pulse, 

 it is possible that the membrane potential is raised 

 to the critical level, thus initiating a full-sized response. 



This can account for a decrease in threshold in the 

 region where / is negative and v is positive. Katz (72) 

 developed this argument to explain the results of his 

 experiments in which the effect of a brief shock was 

 tested by another brief shock. His argument is not 

 entirely correct since he ignored the first (time) factor 

 mentioned above. Erlanger & Blair as well as 

 Tasaki neglected the second factor arising from the 

 subthreshold phenomenon; their argument, there- 

 fore, has to be partly modified. 



Finally, we shall discuss the significance of the 

 break in the v-S relation in the experiment of figure 

 195. Some physiologists believe that this break is a 

 sign of the development of subthreshold response to a 

 subrheobasic rectangular pulse alone [e.g. Nieder- 

 gerke (92)]. As we see, however, in the lower right 

 part of figure 16, this is not exactly the case. The 

 continuous transition from straight line I to II is evi- 

 dently due to the interplay of the two stimuli related 

 to the development of ' the slowly rising phase of the 

 membrane potential' which precedes a full-sized ac- 

 tion potential. 



."iBOLlTION OF THE .ACTION POTENTI.AL 



Initiation of an action potential can be regarded 

 as a transition of the membrane from its resting state 

 into the active state which is characterized by a low 

 membrane resistance and a high potential level. The 

 reverse process, i.e. a transition from the active state 

 of the membrane to the resting state, was first demon- 

 strated in the cardiac muscle of the kid (142), then 

 in the toad nodal membrane (126) and finally very 

 recently in the squid axon membrane. The action po- 

 tential of the cardiac muscle is associated with a 

 systolic contraction. The fact that this contraction can 

 be abolished by a strong (anodal) current pulse in an 

 all-or-none manner has been known since the time of 

 Biedermann (12, pp. 257-264). 



The regenerative process of initiating an action po- 

 tential is set off by a change (rise) in the membrane 

 potential up to a certain level. In an analogous man- 

 ner, the process of abolition of an action potential is 

 set off by a change (fall) in membrane potential 

 down to a critical level. This is shown in figure 20. 

 These records were obtained from a single node 

 preparation of the toad. The arrangement of the 

 stimulating and recording electrodes used is similar 

 to that for the experiment of figure 16. The first pulse 

 of outward membrane current raises the membrane 

 potential to the level slightly above the critical po- 



