306 C. EYZAGUIRRE 



potential levels inhibition causes no electrical changes, or only small ones; if 

 the cell is displaced from this "inhibitory equiUbrium level" in either direction 

 the inhibitory action tends to restore this equihbrium by either polarizing or 

 depolarizing the structure. Since inhibitory potentials of both polarities may 

 be obtained with extracellular as well as with intracellular leads depolarizing 

 inhibitory potentials are not necessarily caused by recording artefacts or by 

 injury resulting from impalement of nerve cells. 



Usually, inhibitory potentials are not uniform and therefore a rigid des- 

 cription is difficult. It seems that during intracellular recording of these nerve 

 cells a number of variables appear which might change some characteristics 

 of these potentials, their ampHtude, polarity, or time course. However, in 

 general, one inhibitory impulse produces a large enough potential change 

 that a second impulse set up shortly afterwards does not add to the amplitude 

 of the first potential ; a ceihng effect occurs and this effect has been shown 

 to be present regardless of the polarity of the inliibitory potential. This ceiUng 

 effect is not present in all cells. In some preparations during repetitive 

 stimulation these potentials frequently sum building up to twice or more the 

 peak value of individual inhibitory potentials. 



Inhibitory After-effects 



In most preparations an inhibitory impulse train is capable of blocking the 

 sensory discharges abruptly, but, as soon as inhibition is stopped discharges 

 come back immediately to baseline levels. In a number of preparations, 

 however, two types of inhibitory after-effects have been observed : (a) a 

 post-inhibitory depression and (b) a post-inhibitory facilitation. 



(a) Post-inhibitory depression. A short burst of inhibitory impulses may be 

 able to clamp the membrane below the firing level. In this case inhibition 

 occurs and the only detectable membrane potential changes are those pro- 

 duced by the inhibitory train. However, in a number of cases, after applying 

 the inhibitory train, a hyperpolarization may be observed both with micro- 

 electrodes inserted into the cell soma and by means of extracellular electrodes 

 recording from the vicinity of the cell. The magnitude and the time course of 

 this post-inhibitory hyperpolarization is dependent upon the frequency of 

 the inhibitory impulses and on the duration of the train of these pulses (Fig. 

 16). Not all cells react in a similar manner ; for instance, this phenomenon may 

 not appear even after a few seconds of stimulation at 50/sec while on occasions 

 an indication of post-inhibitory depression appears even after a single 

 inhibitory stimulus suggested by a double phase during the inhibitory 

 potential decay. This post-inhibitory hyperpolarization prolongs inhibition 

 effectively and it may, perhaps, be an active process since it appears in 

 opposition to depolarization produced by stretch. 



(b) Post-inhibitory facilitation. An opposite phenomenon has been observed 



