REGULATION OF DISCHARGE RATE BY INHIBITION 69 



Fig. 4. Plot as in Fig. 3. A, to show graph for Fn = Fi. B, initial control of spike 

 drawn to fit Fh = I -22 Ft — 2-8. Stimulation of lower frontal portion of anterior 

 lobe of cerebellum at Horsley-Clarke co-ordinates P6, H2 just contralateral to 

 midline at frequency 300/sec and 9 V, coaxial electrode and insulated tip 

 against its shield. Order of observations marked on graph; underlined numerals 

 are observations without stimulation of cerebellum. Curve C drawn to Fn = 

 1 • 39 fi - 2- 8, D to F„ = 1 • 58 Fi - 2- 8 (Granit et a!., 1960). 



As to other organizational features, we have the results of Brooks and 

 Wilson (1959), Wilson (1959) and of Wilson et al. (1959) which show that 

 there is recurrent excitation which is better developed from extensors to 

 flexors than the other way round, because inhibition is the dominant feature 

 between extensors and from flexors to extensors. This is a kind of asym- 

 metrical reciprocal innervation. Eccles et al. (1960) emphasize in the first 

 instance nuclear proximity in the spinal cord as the leading organizational 

 feature for recurrent inhibition. 



Thus this brief review shows that from many points of view recurrent 

 collaterals deserve to be studied. Recurrent inhibition seems to be dominating 

 in studies of extensors. We have only seen facilitation as rebound. 



I might end by saying a few words about limitation of discharge frequency. 

 We have no evidence that recurrent inhibition is decisive, except for tiie 

 tonic ventral horn cells where it can co-operate with after-hyperpolarization. 

 If we consider equation (1) it is clear that there are two fundamental possi- 

 bilities: (i) Fn may be cut, as in some of the Carcinus fibres of Hodgkin 

 (1948), or it may be cut by accommodation, (ii) Alternatively depolarizing 

 pressure P^ep + -^poi ^^Y ^^ the regulated quantity. It can be shown that 

 the latter generally is the case. Depolarizing pressure is Umited by many 

 factors such as limited number of aff"erent terminals, afferent inliibition, 

 natural recurrent inhibition, after-hyperpolarization. These factors are not 

 easily disentangled ; however, it is easily shown that in many cells depolarizing 

 pressure is hmited in response to muscular afferents when it still is capable 

 of rising in response to many other types of stimuli. The rule seems to be 



