CONDUCTION OF THE NERVE IMPULSE 



79 



the correlation coefficient between the two being 

 0.62. The relation between the conduction velocity 

 V (expressed in m per sec.) and the diameter (in fji) 

 presented in figure 2 can be expressed by 



V = 2.50/) 



(at 24°C)- From the two formulae abose, it follows 

 immediately that 



L 



- = 0.059 (fnsec). 

 I 



The ratio L'V represents the average conduction 

 time for one internodal length. The last expression 

 indicates that, statistically speaking, the internodal 

 conduction time is roughly independent of the fiber 

 diameter. 



In the experiments involving electric stimulation of 

 whole nerve trunks, it is customary to designate 

 groups of nerve fibers of different conduction velocities 

 as a, /3, 7, (6), B and C. Group a represents the 

 fastest myelinated nerve fibers in the nerve with 

 velocities of 20 to 30 m per .sec. in the frog, while B 

 fibers are the slowest group (5 m per sec. or less) 

 at room temperature. The first three (or four) groups 

 are often included in .1. Group C represents non- 

 myelinated fibers. This cla.ssification is somewhat 

 arbitrary. 



The distribution of the fiber sizes in a nerve trunk 

 generally shows several peaks of numerical pre- 

 dominance. Reflecting this situation, action potentials 

 recorded at some distance away from the site of 

 stimulation develop sev-eral peaks. However, be- 

 cause of the difference in size and duration of the 

 action potentials among diflferent fibers, it requires a 

 tedious calculation to predict the configuration of 

 the action potential of a whole nerve trunk on the 

 basis of its fiber size distribution. A detailed treat- 

 ment of this problem is found in a monograph by 

 Gasser & Erlanger (38). 



GENERAL CH.ARACTER OF THE NERVE IMPULSE 



In the preceding section we have seen an example 

 of simplicity and clarity of the experiments done 

 with isolated single nerve fibers. It was Adrian & 

 Bronk (5) in 1928 who made the first successful at- 

 tempt to reduce operatively the number of active 

 fibers in a nerve to record single fiber responses. 

 The operation of isolating single nerve fibers of the 

 frog and the toad was developed in Kato's laboratorv 

 (70). 



Another successful approach to single fiber experi- 

 ments was achieved by the use of nerve preparations 

 of invertebrates, such as crabs, lobsters, crayfish 

 or squid. The operative procedure of obtaining single 

 fibers in these invertebrate nerves is simpler than the 

 dissection of a single frog nerve fiber, since some of 

 the fibers in these lower animals are larger than 100 

 /i in diameter. So-called squid giant axons, which 

 Young (146) has introduced to electrophysiologists, 

 are as large as 400 to 900 n in diameter and are an 

 excellent material for investigating the potential 

 inside the axoplasm. 



Through the use of single fiber preparations, the 

 demonstration of some of the basic properties of the 

 propagated nerve impulse has become extremely 

 simple and direct. The following properties are 

 common to all the nerve fibers examined, vertebrate 

 and invertebrate. 



a) All-or-none law. The historical aspect of the 

 development of this law has been mentioned in the 

 introduction of this chapter. This law may be stated 

 as follows: with other factors constant, the size and 

 shape of any electrical sign of a propagated nerve 

 impulse is independent of the intensity of stimulus 

 employed to initiate the impulse. 



It has been mentioned that a definite threshold in- 

 tensity is needed to initiate an impulse in a nerve 

 fiber. As signs of an impulse, one may take the current 

 de\eloped by the fiber, the action current, or the 

 potential changes inside the axoplasm, or any other 

 electrical response of the fiber. When the stimulus 

 intensity is varied, these signs may appear slightly 

 earlier or later; but the whole time course remains 

 uninfluenced by how far above threshold the stimulus 

 intensity is. 



The records presented in figure 3 show the time 

 course of the action currents produced by a single 

 nerve fiber of a toad in response to electric shocks of 

 varying intensities. The shocks were applied to the 

 sciatic ner\e trunk and the current associated with 

 an impulse traveling along a single nerve fiber in the 

 nerve was recorded by the technique described in the 

 discussion of the experiment of figure iB. At threshold 

 (the lowest trace), the action current of the fiber 

 started after a long and variable delay. The time 

 course of this action current, however, was identical 

 with that of the other responses to stronger shocks. 



It is possible to modify the time course of the electric 

 response of a fiber by changing physical or chemical 

 environmental conditions, such as temperature or 

 composition of the fluid around the fiber. This fact 

 should not be regarded as a violation of the all-or- 



