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HANDBOOK OF PHYSIOLOGY 



.NEUROPHYSIOLOGY I 



choline. When the acetylcholine concentration is as 

 high as I jumole per liter, muscle fibers are depolarized 

 sufficiently for spikes to be initiated. For low concen- 

 trations, not exceeding that required to elicit spikes, 

 the depolarization is nearly proportional to the acetyl- 

 choline concentration. With high concentrations the 

 depolarization elicited by acetylcholine can be meas- 

 ured in the wake of an initial burst of spikes, when 

 the muscle fiber in the region of the junction is re- 

 fractory to the initiation of further spikes. At the 

 lower concentrations the depolarization is maintained 

 for many minutes while the acetylcholine remains in 

 the surrounding fluid; at the higher concentrations a 

 perceptible decline is observed within a few minutes, 

 the rate of decline being greater the higher the con- 

 centration of acetylcholine. This effect is apparently 

 the result of a gradual desensitization of the receptor 

 by its forming a different and less readily reversible 

 combination with acetylcholine than that which 

 results in depolarization. 



More accurate information on the spatial distribu- 

 tion of the receptor and the time course of its reaction 

 may be obtained by applying brief pulses of acetyl- 

 choline with a micropipette (25, 70). It is found that 

 the high sensitivity to acetylcholine does not extend 

 beyond very limited regions in the neighborhood of 

 the nerve terminal branches, for in the frog, where the 

 terminals are spread over about a 200 /n length of fiber, 

 it is necessary to position the micropipette to within 

 10 or 20 M in order to obtain a high sensitivity. It is 

 further observed that acetylcholine exerts its power- 

 ful action only when applied externally; it has no 

 specific effect when released within the muscle fiber, 

 even though the pipette is situated immediately be- 

 neath a region of the fiber surface that is found to be 

 sensitive to external application. With the micro- 

 pipette critically placed over the junction so as to 

 obtain maximum sensitivity, the depolarization 

 evoked by a brief pulse of acetylcholine rises to a peak 

 in about 15 msec. This order of lime would no doubt 

 be required for the diffusion of acetylcholine from its 

 point of release to the receptor some microns away. 



Among agents that affect neuromuscular trans- 

 mission, the one that has received most attention is 

 curare. This term applies to a group of related sub- 

 stances which act by competing with acetylcholine 

 for the receptor. Combination of curare with the re- 

 ceptor does not itself aflfect the electrical properties of 

 the membrane, but it prevents acetylcholine combin- 

 ing and thereby exerting a depolarizing action, .\mong 

 the common inorganic ions, sodium appears to have 

 the most marked effect on the combination of acetvl- 



choline with the receptor (36, 42). After the complete 

 withdrawal of sodium ions from the bathing solu- 

 tion, the application of acetylcholine elicits a small 

 depolarization, which is augmented considerably by 

 the presence of only a small concentration of sodium. 

 This effect is not produced by the addition of calcium 

 or potassium ions. It is inferred to be due to a change 

 in the reaction between the receptor and acetylcholine, 

 rather than in a later stage of the process leading to 

 depolarization, from the fact that sodium ions also in- 

 crease the ability of acetylcholine to compete with 

 curare for the receptor. A facilitation of the reaction 

 between the receptor and acetylcholine in muscles of 

 the frog is also produced by the addition to the 

 bathing medium of very small concentrations of epi- 

 nephrine and norepinephrine, the substances released 

 by impulses at the terminals of sympathetic post- 

 ganglionic nerve fibers (49}. 



The anticholinesterases are a group of substances 

 that affect transmission by competitively inhibiting 

 the enzyme cholinesterase, which is concentrated in 

 the junctional region of the muscle fiber and normally 

 hydrolyzes acetylcholine soon after its liberation from 

 the nerve terminals. Unlike the reaction between the 

 receptor and acetylcholine or curare, which must be 

 very rapid in reaching an equilibrium, that between 

 the enzyme and a reversible anticholinesterase takes 

 many minutes. With the anticholinesterase exerting 

 its maximum effect and presumably completely in- 

 hibiting the enzyme, the time course of transmitter 

 action is in two stages (31, 40). The 2 msec, phase of 

 high intensity transmitter action is virtually un- 

 changed and accounts for the early rapid rise of the 

 endplate potential. This is succeeded by a prolonged 

 phase of low level transmitter action which heightens 

 and prolongs the endplate potential. 



Other organic compounds besides acetylcholine 

 exert a depolarizing action at the junction. Some of 

 the substances that have been examined combine in 

 various degrees the properties of acetylcholine, curare 

 and anticholinesterases (32, 74). In the case where 

 the first two actions are combined, the agent in a 

 concentration which produces a small depolariza- 

 tion prevents acetylcholine from adding to this to the 

 extent obtaining when the former is absent. Different 

 substances are found to follow various time courses 

 in their action, and where the same one exerts multi- 

 ple types of action, each may develop along a different 

 time course. Furthermore the relative effectiveness for 

 each type of action may vary between different 

 preparations. 



Transmission would be expected to be influenced at 



