REGULATION OF DISCHARGE RATE BY INHIBITION 65 



assayed by constant rate of discharge, but the amount of surplus excitation 

 or excitatory drive by which it is upheld. When the excitatory drive is low 

 the reflex is destroyed by recurrent inhibition. This is what we measure. 

 Early in a stretch reflex there is a good surplus behind the steady frequency 

 of discharge and therefore every loss of depolarizing current resulting from 

 recurrent repolarization is quickly replaced; later on the excitatory drive may 

 be barely sufficient to maintain a given depolarizing pressure (steady output 

 frequency) and so the cell falls an easy prey to repolarization by recurrent 

 inhibition. 



One broad generalization following from this work is that in problems of 

 regulation the important intracellular approach which has led to so much 

 conceptual clarification has definite limitations. This is when our concern is 

 with the rules of the game by which frequency of output is determined. It is 

 probably true that in the steady state condition depolarizing pressure deter- 

 mines impulse frequency in accordance with equation (1) and with the 

 results of Fuortes and Frank, mentioned above, with inside stimulation of 

 motoneurons. But up to a point the efficacy of an intercurrent inhibitory 

 force depends on how weU any particular depolarizing pressure is defended 

 by excitatory drive. Naturally, efficacy of recurrent inhibition — or any other 

 repolarizing variety of inhibition for that matter — must also depend upon 

 the slope constant by which impulse frequency is related to net depolarizing 

 current. Let us consider the extreme values of Fuortes and Frank, 4 and 

 13-6 for this slope constant and assume that we are studying two moto- 

 neurons adjusted to discharge at the same rate. Assume further that it 

 would be possible to test them by identical amounts of Pp^, of recurrent 

 inhibition. Merely because of the different slopes of the two curves (in a 

 diagram relating their impulse frequency to depolarizing current), the 

 effects of recurrent inhibition on the two cells would have to be in the ratio 

 of 4 to 13-6, other things equal. This is clearly because equal amounts of 

 repolarization will reduce frequency of firing in proportion to the constants 

 mentioned. 



One might think it unnecessary to introduce this distinction between 

 excitatory drive and depolarizing pressure and instead try to explain our 

 results by an uncertainty in the measurement of firing rate of the motoneurons. 

 In order to reply to this criticism it is possible to design an experiment in 

 which excitatory drive is maintained in spite of a reduction of depolarizing 

 pressure. Thus, with extensor motoneurons, the tonic reflex can be elicited 

 electrically by a maintained afferent tetanus. The depolarizing pressure can 

 at the same time be lowered by pulling on the antagonist flexor muscle 

 tibiahs anterior. Exceptionally, in this experiment, the nerve to the flexor 

 must be left intact. By these means it is easy to reduce the firing rate of the 

 extensor motoneuron, used as indicator, in excess of any variation in rate 

 during maintained stretch. When this is done, recurrent inhibition does not 



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