78 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the classical picture- is clearly defined as an IR drop 

 in the intercellular space. 



We shall now proceed to discuss the time course of 

 the action potential of a buUfroE; sciatic nerve re- 

 corded with this arrangement. When the distance 

 from the stimulating; electrode (S) to the recording 

 electrodes (Ei) is relati\ely large and the shock is 

 strong enough to stimulate most of the fibers in the 

 nerve, an action potential with several separate 

 peaks is observed (fig. i.-!, bottom). As the distance is 

 altered, the time intervals between the peaks are 

 found to alter, indicating that separate elevations 

 in the potential record represent processes travelling 

 along the nerve at different velocities. Gasser, Erlanger, 

 Bishop and others (13, 38) interpreted these findings 

 as resulting from differences in the conduction velocity 

 of the different fibers in the ner\'c trunk. 



In figure \B, a set of records is presented showing 

 the validity of the interpretation just mentioned. 

 Here, the connective tissue sheath around the sciatic 

 nerve is removed near its distal end and all except 

 three nerve fibers are cut (for the detail of this opera- 

 tion, cf. 113, 124). The two recording electrodes 

 are placed in two small pools of Ringer's solution 

 separated by a narrow air gap (o. i mm wide) across 

 which the exposed nerve fibers are mounted. Under 

 these circumstances, the electric currents which the 

 nerve fibers produce when the impulses arrive at the 

 site of recording inevitably flow through the resistor 

 QR in the figure) connected between the electrodes. 

 The IR drop thus produced is amplified and recorded 

 with an oscillograph. 



It is seen in the records that the number of peaks 

 observed is equal to the number of the fibers left 

 uncut. Three fibers are now generating three sepa- 

 rate potential variations. It is also clear that each 

 fiber carries an impulse at a rate which is approxi- 

 mately constant for the whole length of the sciatic 

 nerve. 



It is simple to demonstrate the statistical rule 

 formulated by Erlanger and Ga.sser that the con- 

 duction velocity increases with increasing fiber 

 diameter. If only one large fiber is left uncut, we 

 find a high conduction velocity; a weak electric 

 shock is sufficient to excite it. If one small fiber is 



- It is important to distinguish a negative potential from a 

 negative electric charge. The potential along a uniform electric 

 conductor is inevitably related by Ohm's law to a flow of 

 current in the conductor; it has to be expressed as a potential 

 difference between the two points on the conductor, for in- 

 stance, 'the potential of Ei is 10 mv below (or abose) that of 

 E;' but not E; is negative and Ei is positive.' 



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FIBER DIAMETER 

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FIG. 2. Conduction velocity of individual nerve fibers, V, 

 plotted against fiber diameter, D. Single fibers were isolated 

 from sciatic-gastrocnemius preparations of the bullfrog. The 

 outside diameter of the fiber was measured at the operated 

 region near the muscle. Temperature, 24°C. [From Tasaki 

 et al. (l3i)-] 



isolated in the region of recording, we find a small 

 response which arrives at the site of recording after a 

 long delay; a strong shock is needed to stimulate 

 such a fiber. 



In figure 2 the conduction velocities of about 50 

 different fibers in the bullfrog sciatic nerve are plotted 

 against their outside diameter. There is a rough 

 proportionality between the fiber diameter and the 

 conduction velocity, the correlation coefficient be- 

 tween the two being 0.92 in this measurement. The 

 relation between the minimum effective intensity or 

 threshold of shock and the fiber diameter determined 

 by this method can be found elsewhere (124). 



It is well-known that the internodal length (the 

 distance between the two neighboring nodes of 

 Ranvier) increases with the fiber diameter. For the 

 fibers in the bullfrog sciatic nerve, the relation be- 

 tween the diameter D and the internodal length L 

 was found to be expressed by the formula 



L = 0.146 X io'L», 



