62 RAGNAR GRANIT 



rate around 114/sec. In the former case truly tonic cells discharge, in the 

 latter case stimulation will be strong enough to activate tonic responses in 

 cells which normally — to muscular afferents — would respond phasically. 

 With electrical tetanization of muscular afferents the efferent roots have to 

 be cut. 



Now there are two ways of stimulating antidromically. In both cases the 

 stimulating electrodes are on the root delivering the filament whose spike 

 we analyze; in one case on the whole remaining root, in the other only on 

 some antidromically active filaments. With extensor reflexes it does not 

 seem to matter much if one selects the most strongly inhibiting adjacent 

 filaments or the whole remaining root. 



Let us first consider the tonic stretch reflex as exemplified by the single- 

 fibre preparation responding reflexly to pull on the gastrocnemius-soleus. 

 This situation requires some conceptual clarification. The muscle spindles 

 are responsible for what we call "excitatory drive" which may be regarded 

 as a barrage of impulses that activates a certain number of synaptic knobs 

 per unit time. The result emerges as net depolarizing current P^ across 

 the cell membrane. This process is opposed by repolarizing forces such as 

 orthodromic inhibition from afferents over internuncial cells and natural 

 recurrent inhibition initiated by the firing tonic ventral horn cells themselves. 

 Let the sum total of these opposing forces be Pp^\. As stated above, anti- 

 dromic stimulation was proved by Eccles et al. (1954) to repolarize the 

 ventral horn cell. The nonnal frequency of discharge F,, will be some function 

 of the net depolarizing current which is the algebraic sum of the two opposite 

 forces. 



^« =/(^dep + ^pol) (1) 



Long ago Barron and Matthews (1938) were interested in this function 

 whose right-hand term from now on I shall call depolarizing pressure, de- 

 fining thereby more precisely a term taken from a paper by Phillips (1959). 

 The experimental difficulty is, of course, how to eliminate P^^^x or to keep 

 it constant. Barron and Matthews tried to stimulate the spinal cord from the 

 outside with a depolarizing current and they published figures for one cell 

 in which Fn was proportional to strength of depolarizing current. The ideal 

 technique for elimination of Pp^i from equation (1) is to stimulate through 

 an inside microelectrode in the manner of Araki and Otani (1955). Systematic 

 measurements by this technique have been made by Fuortes and Frank who 

 kindly have allowed me to quote unpublished results. The firing frequency of 

 single motoneurons was found to be a linear function of depolarizing current. 

 Slope constants varied from cell to cell and their range of variation was as 

 wide as from 4 to 13-6 imp/sec per m^uA. With many cells linear curves 

 running up to 100 imp/sec were obtained. We recall that normal tonic firing 

 of motoneurons is at rates which are but a fraction of this theoretical maxi- 



