Ion channels: the concept emerges

The belief that the broad sweep of the Hodgkin–Huxley papers (Hodgkin & Huxley, 1952a,c,e,f Hodgkin et al. 1952) can be adequately summarised within a standard textbook chapter is fanciful at best and disingenuous at worst, because such summaries consist of scientific sound-bites that deprive students of the big picture; how decisive experiments create a linear narrative culminating in an internally consistent piece of work. In textbook chapters devoted to Hodgkin and Huxley’s work there are two topics that are inevitably omitted. The first is the separation of the trans-membrane current into INa and IK, the process usually described as a simple subtraction of currents recorded in Na+-free seawater from control currents, which is a gross simplification that neglects to recognise Hodgkin and Huxley’s elegant mathematical solution to the problem (Hodgkin & Huxley, 1952c). Fortunately there exists a superb account of this subject (Cronin, 1987). The second topic, and the subject of this editorial, relates to whether ion movements across a membrane conform to the independence principle. This principle, which was universally accepted at the time, derived from the equations of Teorell and Ussing, and described how the probability of the movement of ions across membranes was under the influence of electrical and chemical gradients but was independent of the presence of other ions (Teorell, 1949; Ussing, 1949). This topic appears deceptively simple upon initial inspection but consideration of the underlying mathematical foundations reveals unexpected complexity. Consider a cell bathed in saline where the influx of a particular ion can be expressed as