THE CHANGE IN MEMBRANE PERMEABILITY 

 DURING THE INHIBITORY PROCESS 



Paul Fatt 



From the Biophysics Department, University College, London, England 



Evidence for a direct action of the inhibitory nerve impulse on the electrical 

 properties of crustacean muscle was already being sought — and in some 

 measure found — by Biedermann (1887). In 1953 Professor Katz and I 

 described an increase in membrane conductance of the muscle fibre pro- 

 duced by the inhibitory impulse (Fatt and Katz, 1953). It was manifest as a 

 reduction in the displacement from the resting level of potential, the dis- 

 placement being produced by an applied current. This electrical behaviour 

 was interpreted to indicate an increase in membrane pemieability toward 

 ions which, in the resting condition, were in electrochemical equilibrium 

 across the membrane. The ions thus involved were suggested to be either 

 K+ or Cb or both. At the time of this finding an increase in membrane 

 permeability specifically toward K+ ions was recognized to play a major 

 role in the electrical response of excitable tissue, while CI ions were seen 

 as providing merely a passive electrical leak (Hodgkin, 1951). Nevertheless 

 the possibility of an increase in permeability toward CI" as well as toward 

 K+ was seriously considered from the start as a result of comparing the 

 inhibitory response with the membrane alteration occurring during excitatory 

 junctional activity. At the excitatory junction the nerve impulse produces a 

 transient shift in membrane potential toward a level near zero membrane 

 potential (Fatt and Katz, 1951). This indicates an increase in permeability 

 toward more than a single species of ion. In the frog muscle where the 

 excitatory junctional process has been most fully analysed the ionic currents 

 added by junctional activity are found to reverse at a level of membrane 

 potential making the inside of the fibre between — 10 and 20 mV with 

 respect to the outside (Castillo and Katz, 1954). This is about the level to 

 be expected, if in the junctionally active areas the membrane lost all selectivity 

 toward the movement of the various ions known to be present inside and 

 outside the fibre. 



In order to examine the possible involvement of the two suggested species 

 of ions in the conductance increase produced during inhibitory activity, 

 experiments were carried out on the opener muscle of the claw of the cray- 

 fish, Astacus fluviatilis (Boistel and Fatt, 1958). The composition of the 



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