474 



ANNALS NEW YORK ACADEMY OF SCIENCES 





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SODIUM CITRATE 

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Figure 15. Responses recorded from the preganglionic and postganglionic nerves of a cat's 

 stellate sympathetic ganglion, during perfusion with acetylcholine and sodium citrate. Control 

 records in the absence of a chemical excitant in the left hand column. Time: 0.1 sec. 



immediately contiguous portions of their axons differ from axons in 

 general in some way that makes them sensitive to the action of this 

 agent. 



This is a striking example of a specificity of nerve structure involved 

 in the process of chemical excitation. It has been suggested that the 

 basis of this differential action is the presence or absence of a myelin 

 sheath that would prevent the rapid penetration of the acetylcholine." 

 That is not likely to be the explanation of the contrasting effects in the 

 experiments just reported, for the terminal portions of the pre-synaptic 

 fibers within the ganglion are considered to be non-myelinated/"* as 

 are the post-synaptic neurons. 



Certain chemical agents, such as acetylcholine, are highly specific, 

 with regard to the type of nerve structure they excite. Others, of which 

 citrate and calcium-deficient solutions are examples, are quite general 

 in their action.^^ Thus, a reduction of calcium ions in the perfusing 

 fluid, or the addition of sodium citrate, causes the discharge of recurring 

 impulses in both the pre-synaptic and the post-synaptic neurons, as in 

 axons generally (figure 15). 



The character of the response of ganglion cells to acetylcholine is, in 

 many respects, analogous to the response of peripheral axons to chem- 

 ical excitation. For each cell, there is a threshold concentration that 

 must be exceeded before impulses are developed. This threshold dif- 

 fers from cell to cell, but it is usually less than 25 micrograms of acetyl- 



