8o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



Fio. 3. Demonstration of the all-or-none behavior of the 

 electric response (binodal action current) of a single myeli- 

 nated fiber. The distance between the stimulating electrodes 

 and the site of recording was 20 mm. The strengths of the 

 stimulating shocks were, from the bottom upward, 100, 105, 

 150, 200, 250, 300 and 350 per cent of threshold, respectively. 

 Time marker, 5000 cycles per sec. Temperature, 20.5°C. 

 [From Tasaki (124).] 



none law. This law refers to the identity of the re- 

 sponses obtained by changing only the stimulus 

 intensity and nothing else. 



The all-or-none law is not applicable, at least not 

 in as strict a form as described above, to electrical 

 responses recorded at the site of stimulation 

 (cf. p. 98). 



/)) The refractory period. The time course of the 

 response of a nerve fiber is not influenced bv the rate 

 at which the stimulating shocks arc repeated as long 

 as the rate is less than about 10 per sec. When, how- 

 ever, the repetition rate is increased up to about 100 

 per .sec. at room temperature, it is found that the 

 responses are difl'crent in their size and shape from 

 the responses obtained at lower frequencies. During 

 a short period of time after an impulse has swept over 

 the fiber, the 'condition' of the fiber is different from 

 that of a normal resting filler. This period is called 

 the refractory period of the nerve fiber. 



It is customary to investigate the properties of a 

 nerve fiber in the refractory period by using a series 

 of paired stimuli, a brief conditioning shock followed 

 by a brief test shock at an adjustable interval. The 

 response of the fiber to the first conditioning shock 

 has the normal configuration, while the response to 

 the second test shock varies with the time interval 

 between the paired shocks. The threshold for the 

 second shock is known to undergo a pronounced 

 change during the early stage in the refractory period. 



The curve representing the time course of the 



gradual change in threshold with increasing shock 

 intervals is generally called a "recovery curve'. In the 

 first recovery curve published by Adrian & Lucas (6) 

 in 1912, the reciprocal of threshold, the 'excitability', 

 was plotted against the interval between the two 

 shocks. The thick continuous line in figure 4 shows a 

 recovery cur\e determined by using the propagated 

 impulses of a single nerve fiber as the index. The 

 threshold for the test shock alone (measured i sec. or 

 more after the conditioning shock) is taken as unity. 

 The oljserved data indicate that, as the interval be- 

 tween the conditioning and test shocks decreases, the 

 threshold for the test shock rises first gradually and 

 then more rapidly. There is a sharp break in the curve 

 at the moment when the threshold is about J. 5 to 

 3 times the normal \alue, namely, when the excita- 

 bility is about 30 to 40 per cent of the normal level. 



This break in the recovery cur\e indicates that, in 

 the period following initiation of a propagated nerve 

 impulse in a nerve fiber, there is a definite period 

 during which the fiber is incapable of carrying a 

 second impulse. This period was designated b\' pre- 

 vious workers as the 'aljsolutely refractory period', but 

 more recently the term the 'least (or critical) interval' 

 between two effective stimuli (124, 136) is preferred. 

 The reason for this recommendation is the fact that, 

 when one determines the recovery curve at the site of 

 stimulation, a continuous curve without a break is 

 obtained. The term 'functional' absolutely refractory 

 period has also been recommended to describe this 

 period (103). 



The period during which the excitability recovers 

 continuously is called the 'relatively refractory period'. 

 Following this period there is often a period of 

 heightened excitability which is called the supernor- 

 mal phase. During the 'supernormal phase', the size of 

 the action potential and the conduction velocity are 

 practically normal. 



The thin line in figure 4 shows the recovery curve 

 for the same fiber determined at low temperature. 

 The temperature-dependence of the recoxery curve 

 is pronounced, the Qio being about 3.5 (2, 1 19). The 

 effect of temperature change is reversible. 



The conduction velocity is known to be subnormal 

 during the relatively refractory period. This is shown 

 in figure 5, in which the shock response intervals for 

 two impulses were plotted against the distance be- 

 tween the site of stimulation and the site of recording. 

 The two impulses were set up at an interval slightly 

 longer than the least interval of the fiber. It is seen 

 in the figure that the shock response interval for the 

 first impulse increases proportionately with the con- 



