VARIETIES OF INHIBITORY PROCESSES 



Harry Grundfest 



Department of Neurology, College of Physicians and Surgeons, Columbia University 



Inhibition is a negative effect, interference with ongoing or incipient activity. 

 Thus the character of inhibitory phenomena may vary depending upon the 

 activity under examination and upon the different processes which enter into 

 that activity. Tiie neurophysiologist usually tends to concentrate his attention 

 on conductile, electrically excitable activity which involves discharge of 

 spikes. However, inhibition, like facilitation, may be studied also in terms 

 of the non-propagated, electrically inexcitable activity of postsynaptic 

 potentials (p.s.p.'s), and in that case the quantitative and even quahtative 

 manifestations of inhibitory phenomena may be different. Among other 

 things, the underlying differences in mode of excitabihty create different laws 

 for electrically excitable and electrically inexcitable phenomena. By these 

 differences in properties, the interactions of transmissional and conductile 

 activity are comphcated. 



In his early work Sherrington fomiulated, though not explicitly, an 

 important distinction among various inhibitory phenomena. The inhibition 

 which he studied in spinal reflexes seemed to him due to active processes, 

 co-equal with excitatory events. He suggested that inhibition might involve 

 activity of synaptic membrane, rather than some "neutral" (or "neutraliz- 

 ing") event. Even when he later suggested that inhibition might perhaps be 

 due to "stabilization" of the excitable membrane, he apparently thought of 

 active interference with the excitatory processes. 



However, in principle, both "active" and neutralizing (or "passive") 

 phenomena can produce inhibition. In both types, either synaptic electrically 

 inexcitable membrane may be involved or the conductile electrically excitable 

 membrane may be affected. Thus, different inhibitory mechanisms, acting 

 upon two different electrogenic systems, can give rise to a number of specific- 

 ally different modes of inhibitory actions and effects. 



THE COMPONENTS OF THE TWO 

 ELECTROGENIC ACTIVITIES 



In both systems of electrogenic membrane the depolarizing component 

 may be regarded as a mechanism for valving Na+-conductance, while a re- 



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