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LIBRARY 



The ionic permeability of nerve membranes 



by 



R. D. KEYNES 



Physiological Laboratory, Cambridge 



One of the most striking characteristics of living cells is the existence of large ionic 

 concentration gradients across the membranes which bound them. In studying the 

 ionic permeability of cell membranes, we have to investigate not only the active 

 transport mechanisms by which the concentration gradients are built up in the first 

 place, and the properties of the membrane on which their maintenance depends, 

 but also the important question of the part played by ionic permeability in fulfilment 

 of the normal biological function of the cell. In many cases an active transport 

 system is necessary to maintain an osmotic balance between the intracellular and 

 extracellular fluids. In others a high internal concentration of certain ions may be 

 advantageous, or even essential, for the optimal working of enzyme systems. Some 

 cells form part of a secretory organ, and are capable of transferring, often against 

 considerable concentration gradients, large amounts of the particular ions for whose 

 transport they are adapted. In the example with which I am concerned, a rather 

 different adaptation has occurred, the ionic concentration gradients being utilized, 

 through special behaviour of the cell membrane, to form a system which can conduct 

 a transient reversal of membrane polarization rapidly from one end of the cell to the 

 other. I will consider first the role of ions in the passage of a nerve impulse, as it is 

 from this aspect of the permeability problem that we are likely to observe the greatest 

 specialization of the membrane. I will then turn to some evidence on the recovery 

 process in giant axons, where it would not be unreasonable for the mechanisms at 

 work to be less highly differentiated, and possibly similar to those in other types of 

 cell. 



The story begins with the discovery by Hodgkin and Huxley (1939, 1945) and 

 Curtis and Cole (1942) that the action potential in a nerve fibre does not consist 

 simply in a depolarization towards zero membrane potential, as Bernstein (191 2) 

 had supposed, but involves a temporary reversal of potential by some 40 mV. Since 

 these pioneer experiments on giant squid axons, the introduction of methods for 

 measuring membrane potentials by means of 0-5 /a glass microelectrodes thrust into 

 the interior of cells (Ling and Gerard, 1949; Nastuk and Hodgkin, 1950) has yielded 

 reasonably reliable values for the absolute sizes of the potentials in a wide variety of 

 excitable tissues. Recent additions to the list given by Hodgkin (1951) are the studies 

 of Brock, Coombs and Eccles (1952) on mammalian motoneurones, and of Keynes 

 and Martins-Ferreira (1953) on the electroplates of the electric eel. It is noteworthy 

 that although the duration of the action potential may vary from less than one milli- 

 second to several hundred milliseconds, the sizes of the membrane potentials cover a 



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