In a book by N.K. Adam entitled The Physics and Chemistry of Surfaces , Jens Christian Skou learnt of Langmuir's investigation of the monomolecular layers of lipids on a water surface, and of how Schulman had shown that capillary active substances dissolved in a liquid with a lipid layer on top would actually be drawn up into the lipid layer. As the cell membrane comprises a bimolecular layer of lipids, Skou reasoned that it must be possible to use a monolayer of nerve membrane lipids placed on a water surface as a simple model of a nerve membrane, and that this model could then be used to investigate the effects of local anaesthetics on the nerve membrane. His experiments showed that there was a connection between the ability of local anaesthetics to be drawn up from the liquid phase and into the fatty monolayer, and their ability to block neural conductance, i.e. to produce anaesthesia. As sodium carries a positive charge, this process leads to a reduction of the electric potential across the cell membrane. The next stage in the process involves the membrane opening up for potassium, which also carries a positive charge.
Potassium now flows out of the cell, with the result that the membrane potential is re-established.
What was not known was the nature of the structures in the membrane that mediated the transient increase in penetrability for sodium. Jens Christian Skou believed that it must be due to brief alterations in the structure of proteins in the membrane. In the light of his experiments with monolayers, Skou reasoned that local anaesthesia, by penetrating the lipids in the nerve membrane, brought about changes in the lipid part of the membrane, and in so doing indirectly obstructed the brief changes in the structure of proteins in the membrane which led to the increased permeability for sodium. To be able to prove this hypothesis, it was necessary to make a monolayer comprising a mixture of lipids and proteins, and then to prove that local anaesthesia could affect the structure of the proteins by their penetration of the lipid element of the monolayer. For this purpose it was necessary to use an enzyme which not only was so active that it would be possible to measure the level of activity in the limited quantities present in a monomolecular layer, but which also originated from a cell membrane.