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HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



In order to detect propagation of ner\e impulses 

 across an insulating air gap, it is necessary to have 

 an amplifier or the innervated muscle attached to 

 the single fiber preparation. Stimulating electrodes 

 and a muscle or recording electrodes connected to 

 the two sides of the insulating gap introduced an 

 electric capacity which, under ordinary experimental 

 conditions, is large enough to establish a local circuit 

 (by this capacitativc pathway). The resistance of a 

 single fiber preparation mounted across a wide air 

 gap is of the order of 50 MfJ. If there is a capacity of 

 about 2 MMf between the two portions across the gap, 

 the local circuit between the two portions of the 

 preparation will be very effectively closed by the 

 capacitative pathway for a period of about o. i msec. 

 In fresh single fiber preparations, it is actually im- 

 possible to demonstrate a conduction block at an 

 insulating air gap if muscular contractions are taken 

 as an index of such conduction'' (128, 145). 



The capacitative coupling between the two insu- 

 lated portions of a single fiber preparation can be 

 markedly reduced by the use of a positive feed-back 

 amplifier. In the diagram of figure 27 the small por- 

 tion of the preparation on one side of the insulating 

 air gap is connected to the input of a unity-gain 

 preamplifier and is completely enclosed in a metallic 

 shield driven by the output of the preamplifier. (Note 

 that, when the potential of the insulated portion 

 rises above the ground potential, the potential of the 

 shield around the fiber rises to the same extent and, 

 consequently, no electric charge is induced between 

 the insulated portion of the preparation and ground. 

 The input impedance of the preamplifier can l)e 

 made as high as 1000 MQ.) 



It is surprising to see that most single-fiber prepa- 

 rations mounted as shown in this figure are still 

 capable of carrying impuLses across the air gap (128). 

 Washing the surface of the internode in the gap with 

 a nonelectrolyte solution does not generally help to 

 bring about a block at the insulating air gap. Prob- 

 ably, the cell of Schwann on the surface of the nerve 

 fiber does not permit us to raise the external resist- 



' There are somewhat controversial viewpoints on this 

 subject in the literature. Huxley & Stampfli (66) reported 

 that conduction was blocked when the external resistance was 

 raised. Wolfgram & van Harreveld (145) failed to demonstrate 

 a block under similar experimental conditions and expressed 

 the view that their experimental results were inconsistent 

 with the concept of saltatory conduction. Frankenhauser & 

 Schneider (37) reported that they could demonstrate a block 

 with a 20 MSJ shunting resistance across the insulating air 

 gap. For a further discussion on this point, see Tasaki & Frank 

 (128). 



ance high enough to cause a conduction block in 

 fresh preparations. 



Record .-1 in figure 27 was obtained after circulat- 

 ing dry air around the portion of the fiber on the air 

 gap for a short period of time. This causes a rapid 

 e\aporation of water from the surface of the fiber 

 followed by a slow desiccation of the axis cylinder. 

 The monophasicity of the response indicates that the 

 block has actually taken place. Record B in the 

 figure was taken while the small insulated portion 

 of the preparation was grounded through the 20 M12 

 resistor in the figure. The response is now diphasic 

 (or rather binodal), indicating that conduction was 

 restored by the shunting resistance. A similar revers- 

 ible restoration of conduction can be obtained by re- 

 ducing the feed-i)ack voltage to the driven shield, 

 thereijy increasing the capacity of the insulated por- 

 tion of the preparation to ground. 



The obsersation just described indicates that the 

 abilit\- of the ner\e impulse to excite the adjacent 

 resting region is \cry large. As a consequence, a re- 

 versible conduction l:)lock by increasing the external 

 resistance has been demonstrated so far in prepara- 

 tions with a somewhat reduced safety margin. How- 

 ever, it seems safe to conclude from the observations 

 described above that ner\-ous conduction in the 

 myelinated nerve fiber does depend on the electric 

 pathway outside the myelin sheath. 



Safety Factor 



The safety factor in ner\ous conduction inay be 

 defined as the ratio of the action current of the nerve 

 fiber to the minimum current intensit\ necessary for 

 ner\ous condtiction. If an action current generated 

 at one point of the nerve fiber acts as an electric 

 stimulus to the adjacent point, it should be po.ssible 

 to measure the action current in terms of the normal 

 threshold. 



The first attempt to determine the safety factor was 

 made bv using a dilute narcotic solution to reduce 

 the action current from one portion of a nerve fiber 

 and by measuring the minimum intensity of the cur- 

 rent necessary to excite the adjacent portion of the 

 fiber (135). In the uppermost part of figure 28 is 

 shown the experimental setup used. A motor nerve 

 fiber of the toad is mounted across two pools of 

 Ringer's fluid separated by a narrow air gap. The 

 muscle innervated by the fiber is left uncut, and 

 twitches in the muscle resulting from stimulation of 

 the fiber near its proximal end are taken as an index 

 of nervous conduction. An ohmic resistor (of about 



