Ion Transport in Excitable Cells
All the available evidence indicates that there are two different and entirely distinct pathways by which sodium and potassium ions can cross the membranes of nerve and muscle fibres—the excitability mechanism and the pump (or recovery) mechanism. The two types of channel for the passage of cations exist side by side in the membrane, but as can be seen in Table 1 there are a number of ways in which one of them can be greatly affected while the other is quite unaffected, and their properties are so dissimilar that their separateness can no longer be doubted. It is possible that both mechanisms exploit the same fundamental physicochemical differences between hydrated Na+ and K+-ions in order to tell them apart, but even in this respect they behave differently towards Na+ and K+-ions, and may therefore lack a common basis for their discrimination. The discussion which follows is concerned only with the pump mechanism, and I must emphasize that my remarks are not relevant to the downhill ion transport involved in the propagation of impulses.
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