STRUCTURE OF NERVE CELL MEMBRANES 



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4.0 4.4 4.8 5 2 56 60 6 4 Radiu s. A 



Fig. 15. The interspace size distribution for the complex n-m-j membrane (solid 

 line) is compared with that for nerve (dotted line). The distribution is based on a 

 similarity of action of Mg" 1-1 " and curare (and their similar size), Mg ++ -Ca ++ antago- 

 nism, and on the need for assuming that the membrane becomes permeable to all 

 ions (doubly hydrated) during activity. 



spaces at the low end of the size distribution, and to produce, thereby, more 

 ACh sites, while Mg ++ at the high end of the distribution does precisely the 

 opposite, as shown in Fig. 15. 



There is difficulty in specifying the sequence of events concerned in junctional 

 transmission; this stems mainly from an uncertainty as to whether a particular 

 experimental treatment affects events leading to the release of ACh from the 

 pre-junctional terminals, or affects the reactivity of the post-junctional mem- 

 brane. The usual means of distinguishing between these processes, a comparison 

 between the response to nerve stimulation and topically applied ACh, appears 

 in several respects to be unsatisfactory. Fig. 15 has been drawn without any 

 attempt to separate actions on the pre- or post-junctional membranes. A recent 

 study (del Castillo and Katz, 1955) in which the rate of delivery of ACh to a 

 single junction has been controlled with great precision, illustrates part of the 

 difficulty. When ACh was applied by electrophoresis to junctions of the frog 

 sartorius, the maximal rate of rise of a transient depolarization was only a small 

 fraction of the rate of rise of a normal e.p.p. We lack the means for duplicating 

 the rapid changes in concentration of ACh with respect to time that are of 

 physiological interest. The slow depolarizations obtained with externally 

 applied ACh mean, according to the model, that the membrane is redistributing 

 its structure at a rate that is not greatly inferior to the rate at which ACh is 



