114 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



senceof a contraction on" make' of a strong' ascending' 

 current pulse in the table indicates that the nerve 

 impulse initiated under the cathode could not pass 

 through the anodally polarized region of the nerve 

 between the cathode and the muscle. 



Effect of Narcosis upon Nervous Conduction 



It has Ijeen pointed out that narcotics, such as 

 cocaine, urethane, ethanol and others, depress or 

 eliminate electric responses of the nerve fiber when 

 they are applied to the nodes of Ranvier of the fiber 

 (p. 109). The action of these chemicals, as well as 

 the effect of low sodium in the medium, progresses 

 with surprising rapidity; an equilibrium between the 

 single fiber and the surrounding fluid medium con- 

 taining these chemicals is established within one 

 second (68, 71, 124). 



It is well known that the action of these narcotics 

 upon the whole nerve trunk is extremely slow and 

 gradual, as emphasized by Winterstein (144). Evi- 

 dently the time required for dififusion of the chemical 

 into the nerve trunk accounts for the slow action of 

 these chemicals upon it. The diagram in figure 33 

 illustrates this great difference in the rapidity of ac- 

 tion of urethane between the intact sciatic nerve and 



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FIG. 33. Relation between the concentration of urethane 

 (abscissae) and the time required for conduction block (ordi- 

 nates') in a single fiber preparation (continuous line) and in the 

 whole sciatic nerve (circles'). [From Tsukagoshi, cited by Tasaki 

 in (124).] 



the exposed nerve fibers. The circles in the figure 

 represent the times required for conduction block at 

 various concentrations of urethane-Ringer's solution 

 in intact sciatic nerves. The narcotic was applied to 

 a 1 5 mm long uniform portion of the sciatic nerve 

 of the toad, and the disappearance of muscular con- 

 traction was taken as an index of block. The relation- 

 ship between the time required for conduction block 

 and the concentration of urethane is given by a 

 smooth curve. 



When a single nerve fiber preparation or a few 

 fiber preparations with a 15 mm long exposed region 

 are used in this type of e.xperiment, an entirely dif- 

 ferent result is obtained. For concentrations lower 

 than about i .8 per cent, conduction through the 

 narcotized region remains unblocked for an hour or 

 more; and for concentrations higher than about 2.2 

 per cent, conduction block sets in within one second. 

 At the critical concentration, which was appro.xi- 

 mately 2 per cent in this experiment, conduction 

 block occurs within about i minute. The relationship 

 between the concentration and the time required for 

 blocking is, therefore, given by the thick line (bending 

 at almost a right angle) in the figure. 



We have pointed out that a nerve impulse can 

 jump across one or two complete inexcitable nodes. 

 If one reduces the length of the narcotized region 

 down to about 5 mm or less, a longer time is required 

 to block conduction since time must be allowed for 

 diffusion of the narcotic along the nerve. This fact 

 was once taken as evidence for ' decremental conduc- 

 tion' in the narcotized region of the nerve (23, 69, 

 82). 



Narcotizing solutions of below the critical concen- 

 tration applied to a node raise the threshold and lower 

 the amplitude of the response. The magnitude of this 

 narcotizing effect depends on the concentration used. 

 Based on the experimental data on the effects of 

 narcosis on single nodes, it is possible to explain 

 nianv phenomena related to narcosis of the nerve. 

 The detail of the accounts along this line may be 

 found elsewhere (124). 



.\FTER-POTENTL^LS .^iND RHYTHMICAL .\CTIVITY 



We shall devote this section to two subjects which 

 are less clearly understood at present than tho.se 

 previously discussed, namely after-potentials and 

 rhythmical activity of the nerve fiber. The relation- 

 ship between after-potentials and rhythmical activity 



