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HANDBOOK OF PHVSIOLOGV ^ NEUROPHYSIOLOGY I 



developed from the observations on the model is that 

 nervous conduction may l^e mediated by a flow of 

 electric current between successive portions of the 

 nerve, i.e. by local circuits. Through very extensixe 

 investigations of bioelectricity by Matteucci (86), 

 Du Bois-Reymond (25), Biedermann (12) and others, 

 it became known that a transient potential variation 

 is generated by a stimulation of a nerve between the 

 portion of the nerve or the muscle carrying an impulse 

 and the killed or resting portion. The existence of a 

 local circuit is therefore a logical consequence of the 

 direct observations on the bioelectricity of the ner\e. 



A direct demonstration of the decisive role played 

 bv a local circuit in the propaeation of an impulse 

 was brought forward, a long time after Hermann's 

 prediction, first by Osterhout & Hill (95) who worked 

 not on the nerve but on a large plant cell, Xitella. 

 They found that propagation of an impulse along this 

 elongated cell can be reversibly blocked under certain 

 experimental conditions by removing or reconnecting 

 a salt bridge which constituted a part of the local 

 circuit. Later, similar obsersations were made both 

 on isolated invertebrate nerve fibers (52) [cf. also (50)] 

 and on single nerve fibers of the toad (i 17). 



The development of the concept of the all-or-none 

 relationship between the intensity of stimulus and 

 the 'size of the response' followed a long, confusing 

 course. In 1871, Bowditch (16) found that the 

 magnitude of contraction in an excised heart muscle 

 of the frog is independent of the intensity of the shock 

 used; a weak shock, if effective at all, causes a con- 

 traction which is as large as that caused by a strong 

 shock. A similar quantal relationship between the 

 twitch and stimulus intensity was demonstrated in 

 individual muscle fibers of the frog sartorius muscle 

 (loi) and also in a ner\-e-mu.scle preparation of the 

 frog containing a small number of nerve fibers (81). 

 In these cases the 'size of the response' represents 

 the magnitude of muscular contraction observed at 

 some distance away from the site of stimulation. 



Attempting to expand the concept of 'size of 

 response' to include the response in the nerve itself, 

 Lucas (82) and Adrian (i) introduced the idea of 

 measuring the nerve impulse by its ability to stimu- 

 late the adjacent portion of nerve, or by its capability- 

 to travel across a narcotized region of nerve — the 

 logic being analogous to measuring the power of a 

 man by his ability to cross a desert. Through a num- 

 ber of ingenious experiments, Lucas and Adrian 

 concluded that the size of the nerve impulse in in- 

 dividual nerve fibers was independent of the way 

 it was elicited. Kato (69) and his associates and also 



Da\is ('/ al. (23) pointed out that there was an er- 

 roneous assumption in this argument as to the 

 mechanism of narcotic action. However, the con- 

 clusion that a propagated ner\'c impulse obeyed the 

 all-or-none law turned out to be perfectly correct. 



Another .series of somewhat controversial argu- 

 ments was evoked among investigators when the 

 concept of 'local' or 'subthreshold' response was 

 introduced in physiology. In 1937 Rushton (105) 

 predicted the existence of a local response in nerve 

 by the following argument. If propagation of a nerve 

 impulse is due to successive stimulation of a resting 

 portion of ner\-e by the neighboring active (respond- 

 ing) area, a definite minimum area of a nerve has 

 to be excited by the stimulating current in order that 

 the response at the site of stimulation can generate a 

 propagated all-or-none response. In other words, 

 he stipulates that there should be a 'response' at 

 the site of stimulation that is too small to initiate a 

 full sized propagating response. 



.Soon after Rushton's prediction, Hodgkin (51) 

 obtained clear-cut records indicating the existence 

 of 'subthreshold responses' in the invertebrate nerve 

 fiber. However, it was found later that Hodgkin's 

 demonstration did not prove the legitimac\ of 

 Rushton's argument. Cole & Curtis (19) proved that 

 the resistance of the surface membrane of the squid 

 nerve fiber decreases at the peak of its response 

 to about '200 "' the resistance at rest; a responding 

 area of the squid axon behaves like a battery with no 

 appreciable internal resistance. Lender ordinary 

 experimental conditions, it is practically impossible 

 to elicit a full-sized response in an area too small to 

 initiate a propagated impulse. Furthermore, these 

 subthreshold respon.ses were demonstrated in sc)uid 

 axons of which a large area was subjected uniformly 

 to a stimulating current. Later we shall discuss 

 similar phenomena obser\-ed in the nixclinaied nerve 

 fiber (p. 98! 



We have discussed up to this poiiu the coin-se of 

 development of some of the basic concepts concerning 

 the nature of the nerve impulse. We shall describe 

 on the following pages the main experimental facts 

 known ai)Oul the nerve filler and its ai)ilit\' to carry 

 impulses. Emphasis will be placed on the data ob- 

 tained from in\ertebrate and vertebrate single nerve 

 fibers. There is good reason to belie\e that, at least 

 in this field of physiology, the iiehavior of an as- 

 sembh' of many nervous elements can be understood 

 if the beha\ior of indi\idual fibers under simple, 

 well-defined, experimental conditions is known. 

 It is generally extremely difiicult to infer the behavior 



