What was Hodgkin and Huxley's achievement?

The Hodgkin-Huxley (HH) model of the action potential is a theoretical pillar of modern neurobiology. In a number of recent publications, Carl Craver ([2006], [2007], [2008]) has argued that the model is explanatorily deficient because it does not reveal enough about underlying molecular mechanisms. I offer an alternative picture of the HH model, according to which it deliberately abstracts from molecular specifics. By doing so, the model explains whole-cell behaviour as the product of a mass of underlying low-level events. The issue goes beyond cellular neurobiology, for the strategy of abstraction exhibited in the HH case is found in a range of biological contexts. I discuss why it has been largely neglected by advocates of the mechanist approach to explanation. 1 Introduction 2 A Primer on the Hodgkin-Huxley Model 2.1 The basic qualitative picture 2.2 The quantitative model 3 Interlude: What Did Hodgkin and Huxley Think? 4 Craver's View 4.1 Mechanistic explanation 4.2 Sketches 4.3 Craver's view: The Hodgkin-Huxley model as a mechanism sketch 5 An Alternative View of the Hodgkin-Huxley Model 5.1 Another look at the equations 5.2 The discrete-gating picture 5.3 The road paved by Hodgkin and Huxley 5.4 Summary and comparison to Craver 6 Conclusion: The Hodgkin-Huxley Model and Mechanistic Explanation 6.1 Sketches and abstractions 6.2 Why has aggregative abstraction been overlooked?