64 RAGNAR GRANIT 



goes on, recurrent inhibition becomes increasingly effective, in fact, inserted 

 at moment 52 sec it succeeded in blocking the cell altogether. The moments 

 1, 2 and 3, marked in the figure are reproduced in the inset from the original 

 records. We note that the time needed for recovery after a burst of antidromic 

 stimulation increased from 1 to 3 even though between tests the discharge 

 rose to the same level as before. 



It is concluded that, since for some reason a constant depolarizing pressure, 

 as gauged by a constant amount of recurrent inhibition, does not deliver a 

 constant inliibitory effect, some concealed factor must be present which is 

 not included in the simple formula (1) relating frequency of discharge to 

 depolarizing pressure. The very first question is whether or not this con- 

 cealed factor might be a temporal summation of the eff"ects of the individual 

 antidromic bursts so that for tliis reason the inhibitions actually increase in 

 strength from test to test though — physically speaking — identical as stimuli. 



It is not difficult to refute such arguments. To this end we make use of the 

 regularity with which a good tonic motor cell responds to stretches repeated 

 at regular intervals. Then it is possible to make each stretch an individual 

 experiment and throw in the test by recurrent inhibition at any chosen 

 moment in the reflex. Such experiments showed that it really is something 

 behind the discharge of the tonic cell that undergoes a gradual weakening 

 with time leading to a loss of resistance to inhibition. Thus, although the 

 frequency of discharge remains the same and outwardly everything is as 

 before, the stretch reflex in the end has lost the excitatory drive necessary to 

 enable it to withstand a suitably chosen dose of recurrent inhibition. 



From this result it is finally concluded that any given frequency of dis- 

 charge of the motoneuron (Fn), corresponding to a certain depolarizing 

 pressure (P^^p + -^poi)' ^^Y ^^ ^un on a greater or lesser amount of surplus 

 excitation. The concept of "surplus excitation" which now is introduced is. 

 as it were, another aspect of what was considered above under the term 

 "frequency limitation". As soon as there is frequency limitation, it is possible 

 to have a surplus of excitatory drive for which there is no equivalent increase 

 of output. Long ago it was shown by Denny-Brown (1929) with the stretch 

 reflex that the output frequency of discharge may be largely independent of 

 the degree of extension of the muscle and I have confirmed this result in a 

 recent study (Granit, 1958). We know very well that the muscle spindles 

 discharge in proportion to extension (Eldred et al., 1953; Granit, 1958) and 

 so, the more the muscle is extended, the greater the excitatory drive. Yet 

 the output frequency is hmited to a constant value. If, at different extensions 

 of the muscle, one tests with recurrent inhibition, it is easily shown that the 

 greater the extension and hence the excitatory drive, the better the reflex 

 resists recurrent inhibition. Throughout such experiments Fn can be kept 

 constant. Similarly antidromic stimulation is held constant. In this manner 

 then it is proved that the decisive factor is not the depolarizing pressure, as 



