Ill 



MEMBRANES OF NEURONS 



The Structure of Nerve Cell Membranes 



L. J. Mullins 



Biophysical Laboratory, Purdue University, Lafayette, Indiana 1 



Investigations or membrane structure must contend with the fact that 

 it is not technically possible to demonstrate a structure by optical methods, 

 and that the elements of the structure cannot be isolated in a pure form 

 for chemical analysis. An indirect approach, involving an examination of those 

 molecules that produce physiological effects by their interaction with the mem- 

 brane, appears to offer certain advantages in membrane structure analysis, and 

 forms the subject of this discussion. Interaction between the membrane and 

 certain molecules is presumably the result of a structural complementarity, 

 hence if enough different molecules acting on the same membrane are examined, 

 structural information can be obtained. The methods used in this study involve 

 first, the setting up of a model of the membrane; the presumed physical behavior 

 of the model may be compared with the results of experiment in order to decide 

 upon further improvements in the model. An early model of the membrane 

 (Davson and Danielli, 1943) suggested that the site of diffusion resistance was a 

 bimolecular lipid layer. Aside from the difficulty in seeing how such a structure, 

 between two aqueous phases, would remain stable, experiments of several sorts 

 have suggested a revision of the model. When measurements of the rate of pene- 

 tration of water, using a D 2 gradient, were compared with penetration rates 

 under an osmotic gradient, the latter were considerably higher (Hevesy et al, 

 1935; Prescott and. Zeuthen, 1953). This behavior is characteristic of pores 

 rather than a solution process in a non-aqueous medium, as first shown by 

 Kofoed-Johnsen and Ussing (1953). A second difficulty arises when one at- 

 tempts to alter the ion permeability of a lipid layer to conform with recent 

 studies on the action potential. The gross structural rearrangement (Danielli, 

 1941) required, appears incompatible with the constant membrane capacity 



1 Aided by a grant (B-139) from the National Institute for Neurological Diseases 

 and Blindness, Bethesda, Md. 



123 



