VARIETIES OF INHIBITORY PROCESSES ^ 



polarizing or hyperpolarizing component is probably more varied in type. 

 The "excitatory" synaptic membranes, like those of primary or of final 

 common path receptor neurons, must produce depolarizing p.s.p.'s or 

 generator potentials, respectively, in order to excite the conductile com- 

 ponent. Thus, these electrically inexcitable membranes have both the de- 

 polarizing and repolarizing transducer actions. The earlier view that depolar- 

 izing synaptic membrane involves merely increased membrane permeabihty 

 to all ions (i.e. "Bernstein type" of electrogenic activity) apparently does not 

 hold for frog muscle endplates and Raia electroplaques, in which the re- 

 polarizing factor is probably only increased K+-conductance. Conductile, 

 electrically excitable membranes also have the two types of transducer 

 components. At present, theoretical emphasis is on the K+-conductance 

 valving system, but it is not unlikely that in some types of electrically excitable 

 membrane Ch-valving may also be involved, or may be the principal factor 

 of the repolarizing activity. 



ACTIVE SYNAPTIC INHIBITORY PROCESSES 



However, membranes also occur in which the Na+-conductance valving 

 action is absent. In the synaptic membranes some are known in which the 

 K+-conductance or CI -conductance, or both, may occur independently of 

 the absence of the Na+ component. Under the electrochemical conditions 

 that seem to prevail in excitable cells, the repolarizing transducer action 

 produces relatively little change in membrane potential. The membrane, 

 however, tends to remain at or near the resting potential during this activity. 

 Thus if electrically excitable membrane is present, any stimulus to depolarize 

 and excite the latter membrane is opposed by the "clamping" action of the 

 repolarizing excitable membrane. The action is equivalent to a decrease in 

 membrane resistance, or an increase in its conductance. The result may be a 

 diminution of depolarizing, excitatory p.s.p.'s, or of applied depolarizing 

 electrical stimuli, both effects therefore being inhibitory. 



In the absence of electrically excitable membrane, the depolarizing "excita- 

 tory" electrogenic activity and the repolarizing "inhibitory" activity produce 

 neither excitation nor inhibition. Thus, in the electrically inexcitable electro- 

 plaques of torpedine electric fishes or of the marine teleost Astroscopus, the 

 large depolarizing p.s.p.'s do not "excite" another component of the cells. 

 The electrical activity is itself an effector response. Likewise, a hyperpolarizing 

 electrogenesis is not "inhibitory" in the electrically inexcitable gland cells, 

 but signifies the processes associated with excitation of the secretory activity 

 of the gland. Furthermore, in cells that possess electrically excitable mem- 

 brane, activation of the "inhibitory" synapses may evoke spikes of the cells. 

 This has been observed in cat motoneurons and in crayfish stretch receptors 



