CONDUCTION OF THE NERVE IMPULSE 



I'3 



the component of the action potential above the 

 notch dropped out completely. 



They did not consider this observation as indicating 

 the saltatory nature of nervous conduction in the 

 myelinated nerve fiber. However, they correctly ex- 

 plained this discontinuity as being related to the 

 existence of nodes along the myelinated nerve fiber. 

 Takeuchi & Tasaki (nS) repeated this observation 

 on isolated single nerve fibers and obtained substan- 

 tially the same result. 



The explanation of the discontinuous change in 

 the single fiber respon.se (fig. 32) is as follows. When 

 the threshold membrane current of the anodaliy 

 polarized node under the recording electrode rises 

 above the membrane current caused by the activity 

 of the adjacent node, the response of the node under 

 study drops out and a small potential variation arising 

 from the activity of the adjacent node is observed. 

 A further discussion on this subject may be found 

 elsewhere (124). 



Pfliign' s Law of Contraction 



The law of contraction formulated by Pfli'iger (loo) 

 in 1859 is at present of almost historical interest only. 

 To demonstrate this law one has to use a pair of non- 

 polarizable electrodes, e.g. long chlorided silver wires 

 imbedded in 2 per cent agar-Ringer's gel filled in 

 glass tubings or classical electrodes of the Zn-ZnS04 

 type. A sciatic-gastrocnemius preparation of the frog 

 or toad is the standard material used for this demon- 

 stration. When pulses of constant current (of about 

 10 sec. duration) are applied to the nerve trunk 

 through the nonpolarizable electrodes, one generally 

 finds that contractions of the muscle, if there are 

 any, occur only immediately following the onset or 

 following the end of the pulse but not during the 

 period of constant current flow. The presence or ab- 

 sence of contraction depends upon the intensity of 

 the current and also upon the arrangement of the 

 anode and the cathode of the stimulating electrodes 

 with respect to the muscle. In table 2 an example is 

 presented of the results of this type of observation. 

 The symbol -|- indicates the presence and — the 

 absence of a muscular contraction. The appearance 

 of a contraction is a sign of arrival of nerve impulses 

 in the muscle. 



If one takes nerve impulses carried to the muscle 

 by a single nerve fiber as an index, one obtains a 

 result somewhat different from that stipulated by 

 the classical law. The result obtained after cutting 

 all but one fiber near the mu.scle is also shown in 



T.\BLE 2. Demonstration of Pfliiger' s Law of Contraction 



This table indicates the presence, 4-, or the absence, — , 

 of a muscular contraction on make or break of long current 

 pulses applied to the nerve trunk of a sciatic-gastrocnemius 

 preparation of the toad. The orifice of the electrodes (Ag- 

 AgCl type) was about 6 mm in diameter and the space be- 

 tween the two electrodes was also about 6 mm. The resistance 

 of the nerve between electrodes was approximately 10 

 kilohms. The results obtained after cutting all the nerve fibers 

 near the muscle except one large motor fiber are presented 

 in parenthesis, and is mentioned only when it is different 

 from that for the whole nerve preparation. 



table 2. There are more negative signs in this case 

 than in the case for the whole nerve trunk. This 

 difference arises from the situation that there are in 

 the nerve trunk many fibers which are situated in 

 different parts of the potential field (produced by the 

 applied current). The existence of the small motor 

 nerve fibers which produce slow muscular contrac- 

 tions (134) in the nerve trunk makes also some dif- 

 ference between a single fiber and a nerve trunk ex- 

 periment. 



If one applies current pulses directly to the iso- 

 lated portion of a single motor nerve fiber in this 

 type of observation, one finds more negative signs 

 than in the two previous cases. In this type of direct 

 stimulation of a single nerve fiber, it is very difficult 

 to demonstrate excitation of the fiber on break of an 

 applied current. Break excitation which is readily 

 observable in the nerve trunk is evidently due mainly 

 to the capacities of the myelin sheath and of the con- 

 nective tissues. These elements in the nerve trunk 

 tend to generate outward membrane currents at the 

 nodes of the fibers on withdrawal of the applied cur- 

 rent. 



The mechanism of anodal block of nerve conduc- 

 tion has been discussed on previous pages. The ab- 



