24 : 3/ The Molecular Basis of Nerve Conduction 443 



Na + ions or about acetylcholine, both of which clearly play an important 

 role in conduction of the spike potentials. In short, this model cannot 

 in itself lead to an explanation, on a molecular basis, of the excitation of 

 and conduction along axons. It is possible that these chemical and 

 mechanical changes, noticed from the analog, are all secondary effects. 

 Most of them are observations of the axoplasm as a whole, but squid 

 axons continue to conduct spikes even if more than half of their axoplasm 

 is replaced by an iron rod. 



The model has been included here for several reasons. First, it 

 emphasizes the approach of the physicist in trying simplified systems. 

 Second, unlike the next two systems discussed, it indicates the essential 

 role of Ca + + ions. And finally, it shows that structural rearrangements 

 of the protein membrane may really occur during conduction of the 

 spike potential. 



3. Biochemical Extractions 



An approach oriented more towards biochemistry is to first study extracts 

 of nerve axons, and then to investigate the effects of added amounts of 

 the characteristic compounds or their inhibitors. The compounds 

 which have been studied most are the ester, acetylcholine^ and the 

 enzyme which hydrolyzes it, called cholinesterase. This work is reviewed 

 by Nachmansohn in Reference 4. 



As discussed in Chapter 4, acetylcholine is an ester formed by removing 

 one molecule of water from acetic acid and the lipid molecule choline. 

 In terms of chemical symbolism, this is 



O ' OH 



H 3 C-0— GH 2 — GH 2 — N— (GHa) 3 



Acetylcholine 



These are long formulas ; it is easier to write AH for acetic acid, Ch for 

 choline, and ACh for acetylcholine. 



