98 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



In the myelinated nerve fiber, the strength- 

 duration relation is determined primarily by the 

 complicated network formed by the nodal membrane, 

 the axis cylinder and the myelin sheath. Because of 

 the interaction between the applied current and the 

 start of a response, the rheobase is 20 to 30 per cent 

 smaller than that expected from the membrane 

 properties at rest (see fig. 16). So far, no one has 

 derived the equation describing the distribution of 

 the membrane potential caused ' by a rectangular 

 voltage applied at one point between two neighboring 

 nodes. In practice, however, the strength-duration 

 relation is expressed by a purely empirical formula: 



.= .(.+^). 



in which .S' is the threshold voltage, /; the rheobase 

 voltage and a a constant which has a dimension of 

 time and is known as'chronaxie'. It is known that the 

 chronaxie for a node varies markedly with the distance 

 between the node under study and the stimulating 

 partition (65). 



Subthreshold Response 



It has been shown in the explanations of figures 16 

 and 1 7 that the membrane potential raised by a 

 brief shock of barely subthreshold intensity decays 

 along a variable time course which is far slower than 

 that expected from the physical properties of the 

 resting nerve fiber. This delay in the fall of the mem- 

 brane potential is said to be due to a "subthreshold 

 response' or a 'local response'. Such delay occurs only 

 when the stimulus intensity is greater than 80 to 90 

 per cent of the threshold (in single node preparations). 

 This phenomenon is more marked in a preparation 

 with high threshold and a poor action potential than 

 in a fresh normal preparation. The phase of the 

 potential rise in these cases is determined bv the 

 physical properties of the resting membrane. The 

 subthreshold response is considered as a sign of the 

 beginning of the regenerative process which has 

 subsided without growing into a full-sized response. 

 The historical aspect of the concept of the sub- 

 threshold response has been discussed in the intro- 

 duction of this chapter. 



A subthreshold 'response' is different from an 

 ordinary full-sized response in that it does not leave 

 behind it a clear refractoriness. In the period during 

 which the membrane potential stays above the level 

 of the resting potential, the threshold for the second 

 shock (necessary to evoke a full-sized response) is 



lower than the threshold at rest. (In the squid axon, a 

 subthreshold response is followed by a small ' under- 

 shoot', during which the membrane potential is below 

 the resting level; the threshold is higher in this period 

 than at rest.) Like an ordinary response, a sub- 

 threshold response is associated with a reduction in 

 the membrane impedance; the reduction is, however, 

 far smaller than that associated with a full-sized 

 response (see fig. 12). 



The arriplitude of the full-sized action potential 

 depends slightly on whether or not it is preceded by a 

 marked subthreshold response. It is seen in the rec- 

 ords of figure 16 that the action potentials preceded 

 by a slow gradual potential rise are consistently 

 smaller than those preceded by an abrupt potential 

 rise. Because of this variation in the amplitude of the 

 response and of the subthreshold responses, the 

 response recorded at the site of stimulation is said to 

 be only approximately all-or-none. 



In the experiments of figures 16 and 17, the stimu- 

 lating current is applied uniformly through the ex- 

 citable inembrane. It is not possible, therefore, to 

 interpret the subthreshold response as an action po- 

 tential localized in a small area subjected to a strong 

 stimulating current (see p. 76). This area hypothesis 

 of the subthreshold response can be saved if one 

 assumes that the surface of the excitable membrane is 

 not uniform but that there are spots or patches where 

 the sensitivity to electric stimuli is higher than at the 

 remaining surface. In the sodium theory (59), a sub- 

 threshold response is attributed to a small increase in 

 the sodium conductance of the membrane. 



When a nerve fiber is excited by a stimulating cur- 

 rent distributed nonuniformly over the membrane, the 

 time course of the subthreshold response is compli- 

 cated by the spatial factor. Especially when the state 

 of the nerve filler has been altered locally by the 

 stimulating or recording electrode or when there are 

 large stimulation artifacts, pictures very different 

 from those in figures 16 and 17 can be obtained. Be- 

 cause of these complications, there have been a num- 

 ber of confusing reports on this topic. 



Measurement oj Excitability by L "sing Test Shocks 



In classical physiology writers used to .speak of 

 measuring the 'excitability' of the nerve by test 

 shocks. Insofar as we define the excitability as the re- 

 ciprocal of the threshold (p. 80), this procedure of 

 measuring the excitability is simple and straight- 

 forward. It seems, however, that to old phy.siologists 

 the term 'e.\cital3ilit\' or ' irritabilitv' had some 



