STRUCTURE OF NERVE CELL MEMBRANES 



147 



& £ A 



Fig. 12. The fit of various substances into interspaces formed by membrane mole- 

 cules of radius 20 A. and 2 A. separation. Upper left, tetraethylammonium ion; upper 

 right tetramethylammonium ion (free space around the lower A site is more than 

 adequate to accommodate thiolacetic acid) ; lower left, ACh in a 'receptor' orientation; 

 lower right, ACh in an 'enzyme' orientation. 



interspaces. Fig. 13 shows these molecules fitted into interspaces of the appro- 

 priate size and indicates that the diethylphosphoryl enzyme derived from TEPP 

 has indeed a free anionic site on each side of the interspace while the DFP 

 enzyme has not. In addition, eserine and neostigmine bind differently in the 

 interspace. 



The Receptor for Acetylcholine 



The discussion of the ACh receptor will be confined to that at the n-m-j 

 of twitch-producing muscles because the nature and the speed of the processes 

 presumed to be taking place are more easily related to events accompanying 

 excitations in axons. The excitatory action of ACh at the junction is presumed 

 to produce a generalized permeability increase to all ions (Fatt and Katz, 1951) 

 in contrast with the data on axons that demands an increased Na + permeability. 

 Because ACh appears to be small compared with an interspace size that will 



