MEMBRANE PERMEABILITY DURING THE INHIBITORY PROCESS 89 



order of magnitude than the readily demonstrated increase in permeabiHty 

 toward CI . 



In order to inquire into the structural change in the membrane by which 

 the permeability increase just described is brought about, a more speculative 

 course will be followed, the inhibitory response being considered in relation 

 to other types of response of which the membrane is capable. Thus it is 

 reasonable to assume that there would be a basic similarity betv/een the 

 membrane permeability changes occurring in excitatory and inhibitory 

 junctional activity. One of the outstanding features of junctional activity is 

 the great magnitude of the conductance change over the area of the p3St- 

 junctional cell in which this activity originates. In the frog muscle fibre 

 junctional activity involves the addition of a conductance of about 5 ^ 10 -'ii^ 

 to the muscle fibre membrane in an area of 10"^ cm^ or less (Fall and Katz, 

 1951). From this one calculates a minimum conductance for unit area of 

 junctionally active membrane of 0-5 Q"^ cm -. Tliis exceeds by a factor of 

 10 the maximum conductance increase occurring for large depolarizations 

 in the squid axon, where the conductance changes, which are involved in 

 the production of the action potential, are due to increases in membrane 

 permeability specifically toward Na+ and toward K+. Quantities of similar 

 magnitude would probably hold for the excitatory junctional response and 

 for the action potential generating response, respectively, in various types 

 of cells. Moreover a similar high intensity of conductance change would 

 probably hold for inhibitory junctions as for excitatory junctions. The 

 underlying meaning of this is simply that within the small area of junctional 

 contact the total conductance increase must be sufficiently great to allow 

 currents to flow which on spreading into a much larger area of normal 

 membrane will be able to affect the membrane potential significantly and 

 thus control the excitability of the cell with respect to the initiation of an 

 action potential, or in the case of some muscle fibres, to the initiation of 

 contraction. A reasonable inference is that the very drastic increase in 

 penneabihty required in junctional activity would operate through a different 

 mechanism from that involved in the action potential and would possibly 

 involve a less complex structural organization of the membrane. Thus in 

 the case of excitatory junctional activity the behaviour of the membrane is 

 consistent with the creation of simple holes through which all ions diff'using 

 up to the membrane are able to penetrate. A similar alteration is envisaged 

 to occur in the inhibitory process, though in this case the holes would be of 

 smaller size and thereby capable of discriminating against the passage of 

 larger ions. Furthermore, if the walls of the holes contained fixed positive 

 charges, this would interfere with the passage of cations. 



An examination of the electrical behaviour of the crustacean muscle fibre 

 membrane may provide a clue as to how such holes are produced. When an 

 inward current is applied to a fibre in the normal bathing solution the resulting 



