CONDUCTION OF THE NERVE IMPULSE 



10 15 20 



SHOCK INTERVAL 



2 3 4 5 6 



SHOCK RESPONSE INTERVAL 



FIG. 4. Recovery curves of a toad nerse hber determined at two different temperatures. [From 

 Tasaki (119).] 



FIG. 5. Relation between the conduction distance and the shock response interval for two impulses 

 elicited at an interval of 2 msec. A motor nerve fiber of 1 1 /j in diameter inner\.ating the flexor 

 digitorum brevis of the toad. Temperature, 23°C. 



duction distance. Evidently, the first impulse travels 

 along the filler at a normal constant rate. 



If the second impulse had travelled at the normal 

 velocity, the shock response interval for the second 

 impulse should be represented by the dotted line in 

 the figure which has the same slope as the straight 

 line for the first impulse. Actually, it is seen that the 

 observed shock response interval increases with in- 

 creasing conduction distance more rapidly than that 

 for the first impulse. 



It is easy to figure out the space-time pattern of the 

 two impulses based on the experimental data present 

 in figure 5. Evidently, the tangent (slope) of the curve 

 in the figure represents the velocity of the second im- 

 pulse at that moment. At the point where the two 

 impulses were initiated, the velocity of the second 

 impulse is approximately 50 per cent of the velocity 

 of the first impulse. Because of this large difference in 

 velocity between the two impulses, the second im- 

 pulse lags, spatially and temporally, behind the first 

 as they travel along the fiber. As separation between 

 the two impulses increases, however, the second im- 

 pulse gains more speed because of increasing recovery 

 from the refractoriness left behind the first impulse. 

 Thus, as they travel along a nerve fiber, the interval 

 between the two impulses approaches asymptotically 

 a constant value which is independent of the initial 

 interval at which they started. 



c) Two-way conduction. It is simple to demon- 

 strate that a nerve fiber is capable of carrying im- 

 pulses in both directions, from its proximal end 

 toward the distal and also in the reverse direction. 

 An observation illustrated by figure 6 shows this. 

 Here a squid giant axon is used. An entirely analo- 



gous observation has been made on the vertebrate 

 myelinated ner\'e fiber. 



The axon is placed in a pool of fresh sea water on 

 a glass plate. Near each of the two ends of the axon 

 a pair of stimulating electrodes is placed. A recording 

 electrode, a glass pipette of about i ix at the tip filled 

 with isosmotic potassium chloride solution in this 

 case, is pushed into the axoplasm of the axon through 

 its .surface membrane. The grounded sea water is 

 taken as a reference point for measuring the action 

 potential. A stimulus applied at one end, A in the 

 figure, gives rise to a response of the all-or-none type, 

 indicating that the impulse starting at A trav"els 

 toward B. When another stimulating shock is applied 

 at the other end, B, sometime after the impulse from 

 A has swept o\-er the fiber, the impulse arising at B 

 can be recorded by the pipette in the middle of the 

 axon (see the top record in fig. 6 ). Since the recording 

 pipette can be placed anywhere between A and B 

 with essentially the same result, this observation proves 

 that the axon is capable of carrying impulses in both 

 directions. 



When the time intersal between the shocks at A 

 and B is reduced below a certain limit (see the record 

 in the middle), the second shock becomes ineffective. 

 The explanation of this fact is simple. Soon after 

 region B of the axon is traversed by the impulse 

 arising at A, this region becomes refractorv and does 

 not respond to the second shock. 



What happens if two shocks are applied simulta- 

 neously at the two ends A and B? There is no refrac- 

 toriness at the site of stimulation in this case since 

 these regions have not been traversed by any impulse. 

 Hence, an impulse should be initiated at A propa- 



