404 ANNALS NEW YORK ACADEMY OF SCIENCES 



from the nerve ending to the second unit is considered. The observa- 

 tions of Eccles and his associates have shown that the electric current 

 set up by the pre-synaptic impulse initiates in the post-synaptic mem- 

 brane a special junctional potential (end-plate potential or, more gen- 

 erally, synaptic potential-^). 



These findings have recently found a morphological correlate by the 

 discovery of Couteaux that the sarcoplasm surrounding the presynap- 

 tic nerve ending has a very peculiar structure.^^ It is similar to that 

 described by several authors of the last century in the electroplasm 

 which surrounds the nerve endings in the electric plates of electric fish 

 and which shows a layer of "electric rods," the "palisades" of Remak, 

 at that particular point. 



The biochemical data support the assumption of a high rate of 

 ACh metabolism in the post-synaptic membrane of the neuro-muscular 

 junction. At the motor end-plate of guinea pig gastrocnemius, only 

 one third of the cholinesterase was found to disappear within three to 

 four weeks after section of the motor nerve.^^- ^^ The rest remained 

 there for many months, a long time after the end-plate had been trans- 

 formed into a sole plate. It appears, thus, probable that part of the 

 high concentration of cholinesterase observed at the motor end-plate 

 may be located at the post-synaptic membrane. The observations on 

 the electric organ support the assumption of such a localization. The 

 electric plates which form the electric organ are homologous to motor 

 end-plates. The discharge in these organs can be considered as com- 

 parable to the end-plate potential, that is, a response of the post-synap- 

 tic membrane. The direct proportionality found between the voltage of 

 the discharge and the cholinesterase activity is, therefore, another in- 

 dication for the importance of ACh in the post-synaptic membrane. 



Specificity of Cholinesterase 



In all the experiments on the activity of the enzyme, it was assumed 

 that cholinesterase is specific for ACh. In such a case, not only is the 

 conclusion justified that the substrate metabolized is ACh, but also, 

 the activity of a specific enzyme determined in vitro may well be used 

 as an indication for the potential rate of metabolism of the substrate oc- 

 curring in vivo. 



It appeared imperative, therefore, to demonstrate the specificity of 

 the enzyme for ACh in all those tissues which were used in the inves- 

 tigations leading to the new concept. The ester linkage in ACh shows 

 no peculiar properties. It has, therefore, to be expected that the ester 

 can be hydrolyzed by other esterases and, on the other hand, that 



