10 HARRY GRUNDFEST 



under electrochemical conditions in which activity of the "inhibitory" mem- 

 brane produces depolarization from the normal resting potential large 

 enough to excite the spike generating membrane. 



ELECTRICALLY EXCITABLE INHIBITORY PROCESSES 



Membranes in which electrically excitable repolarizing processes occur 

 without that of Na+-conductance valving are also known and properly should 

 also be termed electrically excitable. As in its electrically inexcitable counter- 

 part, the electrogenic manifestation of this activity is small and may be of 

 either sign, or absent, depending upon the electrochemical condition that 

 set the resting potential. As with repolarizing inhibitory synaptic membrane, 

 the active aspect of this effect is that produced on membrane conductance. 

 It is present in many spike-generating membranes and is well known as 

 rectification. This conductance change may increase and persist for a long 

 time during depolarizing test stimuli and is thus known as "delayed" 

 rectification. 



It also occurs in some cells whose active depolarizing electrogenesis is 

 only of the electrically inexcitable variety, such as the slow muscle fibers of 

 frog and certain fibers in crayfish muscle, as well as Raia electroplaques. 

 The phenomenon has been studied recently in some detail in the electro- 

 plaques and frog muscle fibers, in which absence of depolarizing spike 

 electrogenesis simplifies analysis. The electrically excitable repolarizing com- 

 ponent may be due only to K+-conductance changes or to CI", or to both 

 ions. The conductance change develops slowly, so that once initiated by a 

 depolarizing pulse it may grow after the stimulus for it is terminated. Thus, 

 the "delayed" feature of the conductance change is emphasized. It persists 

 for a considerable time after a brief initiating depolarizing pulse and in 

 Raia electroplaques it lasts for almost 1 sec. In the latter cells the electrogenic 

 concomitant of the conductance change may be a slight depolarization, 

 while in the frog muscle fiber it is a hyperpolarization, these aspects depending 

 upon the electrochemical conditions of the system. 



The electrically excitable repolarizing or rectifying component may be 

 blocked by applying hyperpolarizing currents or by treatment with pharmaco- 

 logical agents. The p.s.p.'s of frog slow muscle fibers or of Raia electro- 

 plaques are then correspondingly prolonged. In Raia electroplaques the 

 "inhibitory" action of the repolarizing component is only the decrease in 

 electrogenesis of the depolarizing p.s.p.'s. In the case of frog slow muscle 

 fibers, however, the delayed rectification probably diminishes the contraction 

 caused by the depolarizing p.s.p.'s. The repolarizing factor is of prime 

 importance in causing the graded potential of electrically excitable arthropod 

 muscle fibers and the resulting graded contraction of the fibers. Spike- 

 producing electrically excitable membrane is also "inliibited" by the opera- 



