This chapter shows how the theory developed in Chapters 1–4 illuminate important experiments in the development of Hodgkin and Huxley’s theory of the action potential. Although not an exhaustive review, the chapter discusses many of the critical experiments scientists used first to support Julius Bernstein’s “membrane theory” of the resting and action potentials, then Hodgkin and Huxley’s experimental work challenging that theory, then several further experiments from Hodgkin and Huxley (1946), Hodgkin and Katz (1949), Hodgkin and Huxley (1952a), and Hodgkin and Huxley (1952b) that were used to develop the Hodgkin–Huxley theory of the action potential.

Vertebrate skeletal muscle has a negative, -90 mV, resting potential arising from Na+-K+-ATPase-generated transmembrane ionic gradients and inwardly rectifying K+, and Cl- membrane conductances. Three-electrode and loose-patch voltage-clamp experiments demonstrated that, as in nerve, muscle action potentials involve voltage-dependent Na+ followed by K+ channel activation. An additional transverse tubular action potential contributes a discrete and separable delayed component to the recorded voltage change. It is triggered by low-frequency components of the surface-membrane action-potential leaving high-frequency components to ensure rapid propagation of the surface wave. Tubular Cl- conductances decrease in both fast and slow twitch muscle and ATP-dependent K+ channel conductances increase in fast twitch muscle, in early and prolonged exercise. Mathematical modelling demonstrates that these respectively enhance and reduce tubular excitability and its triggering of contractile activity. They potentially furnish enhancing and fatiguing mechanisms for muscle activation and for clinical myotonia congenita.

The cell surface membrane comprises an insulating lipid bilayer in which specialised proteins are embedded. This supports the membrane potential difference between intracellular and extracellular fluids brought about by their differing Na+, K+ and Cl- concentrations, their respective Nernst potentials for electrochemical equilibrium, and their relative membrane permeabilities. The concentration distributions arise from metabolically dependent active ion transport through membrane Na+-K+ ATPase activity, first demonstrated using radioactive tracers in cephalopod giant axons. In intact cells, these factors are combined with the Donnan distribution properties for intracellular and extracellular ions, reflecting the presence of impermeant intracellular charged protein, to determine the resting potential. The intact cell thereby forms an osmotically and electrically balanced system with relatively increased intracellular K+ and extracellular Na+ concentrations separated by a membrane across which there is a stable negative resting potential. The latter provides the electrophysiological background upon which cell excitation events are superimposed.