398 ANNALS NEW YORK ACADEMY OF SCIENCES 



The special case in which this problem of the time factor has been 

 studied and received a satisfactory answer, is the frog's sartorius mus- 

 cle.^° A small fraction of this muscle is free of nerve endings. By 

 determining the concentration of cholinesterase in this part of the 

 muscle, in the part containing nerve endings, and in the nerve fibers, 

 it is possible to calculate the concentration of cholinesterase at the 

 motor end-plates. Since the number of end-plates in a frog's sartorius 

 is known, the amount of ACh which may be split during one milli- 

 second at a single motor end-plate can be calculated. This turns out 

 to be 1.6 X 10" molecules of the ester. About one-third of the enzyme 

 at the motor end-plate appears to be localized inside the nerve ending. 

 On the assumption that one molecule of ACh covers about 20-50 

 square A, the amount which may be hydrolyzed during one millisecond 

 at one end-plate would cover a surface of 100-250 square microns. 



A high concentration of cholinesterase, of an order of magnitude 

 similar to that at motor end-plates, exists at all synapses, whether 

 central or peripheral, mammalian or fish, vertebrate or invertebrate." 

 In mammalian brain, for instance, 10^^ to 10^° molecules of ACh may 

 be activated per gram of tissue during one millisecond. This corre- 

 sponds to about 10-100 millions of square microns of neuronal surface. 



These experiments removed one of the chief difficulties from 

 the theory that ACh is involved in the transmission of nerve im- 

 pulses. They established that the ester may be metabolized at the 

 high speed required for a chemical reaction directly connected with 

 such a rapid event. 



The difference between synaptic region and fiber is, however, only 

 quantitative. The concentration of cholinesterase is high everywhere 

 in nerves, although it rises at the region of synapses. 



B. Localization of Cholinesterase at the Neuronal Surface 



The second essential feature is the localization of cholinesterase in 

 the neuronal surface. Direct evidence for this localization has been 

 offered with experiments on the giant axon of squid {Loligo pealii^-) . 

 This axon has a diameter ranging from 0.5 to 1.0 mm. The axoplasm 

 may be extruded and thus separated from the envelope. The envelope 

 is formed of connective tissue, lipoid and plasma membrane. The 

 axoplasm was found to be practically free of cholinesterase. The whole 

 enzyme activity is in the envelope. 



This exclusive localization of an enzyme in the neuronal surface has 

 been found only in the case of cholinesterase. Respiratory enzymes 

 are localized nearly completely in the axoplasm.^'' Bioelectric phe- 



