EXCITATORY AND INHIBITORY PROCESSES 



315 



Fig. 23. Two series'(A and b) of records of inhibitory junctional potentials 

 obtained during the exchange of external NaCl by Na-glutamate. K^SOa-fiUed 

 internal electrodes were used. Arrows indicate the start of the exchange. Successive 

 records were obtained at 2 sec intervals. The upper trace shows the intracellular 

 potential change obtained with a K2S04-filled electrode while the lower trace 

 showed the potential just outside the cell recorded by a KCl-filled microelec- 

 trode. Voltage calibration, 10 mV. Time calibration 20 msec. (From Hagiwara, 

 Kusano and Saito,/. Neurophysiol. 23 : 505-515, 1960.) 



found that the inhibitory reversal potential is very near the resting membrane 

 potential level when K2SO4 filled electrodes are used. KCl electrodes seem to 

 displace this value. 



In summary, the available evidence obtained in the crustacean stretch 

 receptors indicates the following: (1) inhibitory stimulation may produce a 

 postsynaptic potential which accompanies postsynaptic conductance changes; 

 (2) these changes are determined by an increased permeabiUty to K+ and to 

 Ch ions. Whether a decrease of Na+ permeability also occurs is unknown, 

 although at present this possibihty seems unlikely since Kuffler and Eyza- 

 guirre (1955) have shown that inhibition is accompanied by increased con- 

 ductance of the stretch receptor cell (see Fig. 20). 



The Inhibitory Transmitter 



In recent years an effort has been made to find a transmitter responsible 

 for inhibitory processes. While in the heart acetylcholine release is responsible 

 for inhibition, in other structures the nature of the inhibitory transmitter is 

 unknown. Several amino acids have been explored in the crustacean nerve- 

 muscle junction and in sensory nerve cells; of these, y-aminobutyric acid 

 (GABA) has been found to be a very potent inhibitor. However, the available 

 evidence does not point in the direction of this compound as a possible 

 inhibitory transmitter in the crustacean stretch receptor (cf. Kuffler and 

 Edwards, 1958; Edwards and Kuffler, 1959). 



