VOL. 4 (1950) PERMEABILITY AND NERVE FUNCTION, I 83 



the effect of inhibitors of acetylcholine-esterase on the ion permeability are also of 

 interest although still far from conclusive. 



It was mentioned above that the esterase in the red blood cell has the same charac- 

 teristic features as the esterase in conductive tissue. There, too, the enzyme is localized 

 exclusively in the surface membrane^^. It is therefore of interest that Greig and 

 Holland*" have described observations suggesting that inhibitors of choline ester 

 splitting enzymes may affect the permeability of red blood cells. If this hypothesis be 

 confirmed, it will be another support for the assumption of a similar function of acetyl- 

 choline in the neuronal surface membrane. Analogies as to the permeability of these 

 two types of cells have long been known to physiologists. 



Difference between conduction and synaptic transmission 



In view of the evidence that acetylcholine has an essential function in conduction 

 it appears necessary to reconsider the role of the ester in synaptic transmission. It is 

 the purpose of this article to analyse the question how the earlier observations, suggesting 

 the theory of chemical mediation, may be integrated into the picture resulting (I) from 

 the enzyme studies and (II) from the attempt to correlate the chemical and physical 

 events of nerve activity. 



The theory of chemical mediation was based essentially on two facts: i. the stimu- 

 lating effect of acetylcholine in relatively small amounts (a few //g) upon synaptic 

 junctions, and 2. the appearance of acetylcholine in the perfusion fluid of such foci 

 following nerve stimulation. The complete inertness of the fibre to acetylcholine even 

 if applied in high concentrations (up to 20 g per liter) was considered as definite proof 

 that the physiological function of the ester is limited to the synapse. 



a) Impermeability of the axonal surface membranes to acetylcholine. Studies on the 

 permeability of the axonal surface membranes have thrown new light on this problem 

 and have provided a satisfactory explanation for the discrepancy between the earlier 

 observations and the conclusions necessitated by the enzymatic studies. The investi- 

 gations were carried out on the giant axon of Squid. This material is unusually favourable 

 in view of the large diameter (0.5 to 0.7 mm) of the axon. It is possible to extrude the 

 axoplasm from the cell interior of this preparation without contamination by substances 

 attached to the outside surface. The axoplasm thus obtained may be analysed for com- 

 pounds to which the axon has been exposed for various periods of time. In this way the 

 inside concentration of these compounds and if desired the rate of penetration may 

 be determined. 



It was found that those inhibitors of acetylcholine-esterase which alter and abolish 

 conduction, like eserine and DFP, penetrate into the axoplasm, although the rates of 

 penetration of the different compounds may var^' considerably^". In striking contrast 

 to the compounds mentioned prostigmine, an extremely potent inhibitor of acetylcholine- 

 esterase, does not affect conduction even in h^'gh concentrations (lO"^ M)^*. This com- 

 pound was not found in the axoplasm, although the methods used were highly sensitive 

 and adequate to detect an extremely small fraction of the concentration of the com- 

 pound present on the outside. The experiments show that the axonal surface membranes 

 are impervious to prosfigTiine and, moreover, that the site of the acetylchoHne-esterase 

 associated with conduction must be inside a structural barrier which makes the enzyme 

 inaccessible to the inhibitor. Eserine is a tertiary amine and lipid soluble, prostigmine 

 is a quaternary ammonium salt and lipid insoluble. It appears likely that the difference 

 References p. 93/95. 



