VARIETIES OF INHIBITORY PROCESSES 11 



tion of the repolarizing factor, becoming relatively refractory for some time 

 after a response. 



NEUTRALIZING, OR PASSIVE EFFECTS 



Procedures wliich interfere with electrogenic transducer actions are also 

 inhibitory, but by preventing activity. Thus, these inhibitory phenomena are 

 "passive". Pharmacological agents wliich selectively block depolarizing or 

 hyperpolarizing p.s.p.'s, or both types non-selectively, provide the best known 

 examples of this type of action. However, the "inhibitory" effect of synaptic 

 drugs may be complicated by the functional relations of the whole system. 

 Thus, selective block of inhibitory synapses by strychnine or other drugs 

 results in convulsive activity of the central nervous system. Parallel effects 

 may also be observed in electrically excitable membranes. Cocaine and pro- 

 caine are known to depress activation of both Na+- and K+-conductances 

 of squid axons. This may be the mode of action of procaine as a depressant 

 of spikes in eel electroplaques and cat motoneurons. In other cells, however, 

 such as in crustacean muscle fibers, procaine apparently blocks selectively 

 the repolarizing component of the electrically excitable membrane. The result 

 is conversion of the normally gradedly responsive membrane to one that 

 produces large, overshooting spikes. 



In electrically excitable membrane two "inactivation" processes apparently 

 occur to counterbalance the activation of Na^- and K^-conductance. Block 

 of K+-inactivation would be an "inhibitory" process. Blockade of Na+- 

 inactivation has been found to occur and gives rise to one variety of pro- 

 longed spikes. This phenomenon therefore is "excitatory". What probably 

 may be related phenomena in electrically inexcitable membranes are "de- 

 sensitization" and "sensitization" effects. The effects probably depend upon 

 specific drug-membrane interactions. Thus, apphcation of acetylcholine 

 desensitizes Astroscopus electroplaques, the response to neural stimulation 

 becoming smaller. Carbamylcholine, however, causes little or no desensitiza- 

 tion. For the elucidation of these phenomena, however, more analytical 

 data are required than are currently available. 



ELECTRICAL INTERACTIONS 



Electrotonic effects, causing depolarization or hyperpolarization of electric- 

 ally excitable membrane by appropriate synaptic electrogenesis at distant 

 sites in the same cell may be classified as "active" excitatory and inhibitory 

 processes. At least for relatively long stretches the dendrites of cortico-spinal 

 neurons appear to lack a conductile, electrically excitable component. The 

 axodendritic p.s.p.'s of cortical neurons thus probably affect the soma of 

 the cells only by electrotonic spread. The effects are probably important to 



