REGULATION OF DISCHARGH RATE BY INHIBITION 



63 



mum and later we shall consider the general problem of frequency limitation. 

 For the moment let us return to depolarizing pressure (7'J^,p I ^poi) ^id 

 use recurrent inhibition as our instrument of analysis. 



The simplest approach is to study situations in which depolarizing pressure 

 is constant which means that frequency of discharge Fn is held constant. 

 Assume that we pull out gastrocnemius-soleus to an extension of 10 mm 

 and leave it stretched. In a good, tonic decerebrate animal we shall then find 

 spikes which discharge for minutes at practically constant rates. By definition 

 this ineans that depolarizing pressure is constant. We then proceed to gauge 

 depolarizing pressure by a brief tetanic burst of antidromic stimulation 

 repeated at regular intervals. This means adding to equation (1) a term 

 Ppoi of recurrent inhibition by means of which we measure if the sum 

 ■^dep + -^poi' 3S reflected by a constant rate of discharge F«, really is 

 constant at different moments after onset of pull. 



Analysis of an experiment is shown in Fig. 1 from paper no. 3. The reflex 

 rate of discharge of the tonic cell was approximately constant between the 

 two horizontal lines, from 20 to 55 sec after onset of stretch. The tests by 

 constant brief tetani of antidromic stimulation vary in efficacy from moment 

 to moment but the general trend of the result is perfectly definite: as time 



Imp 



±d^ 



Fig. 1. A 15-mm steady pull on the knee extensors. Tetanic antidromic inhibition 

 at 114/sec inserted for 0-7 sec at regular intervals as marked by oblongs on 

 abscissa (running time). Frequency of discharge constant between the two 

 parallel horizontal lines. Black circles show number of impulses during the 

 periods of recurrent inhibition evaluated in imp/sec. (Inset) — Original records at 

 moments marked 1, 2 and 3 in the diagram. Note, that when delayed recovery 

 after recurrent inhibition begins, then discharge frequency fails to reach its 

 original level (at this rate of repetition of antidromic stimulation periods). Dis- 

 charge stopped for good with last period of stimulation having been five 

 times temporarily silenced (Granit and Rutledge, I960). 



