SKELETAL NEUROMUSCULAR TRANSMISSION 



209 



conditions in which the number of units responding 

 to a nerve impulse is small. For this purpose the cal- 

 cium concentration is reduced (or magnesium added) 

 until the response to a single nerve impulse has a 

 mean amplitude of one or a few units. With two 

 nerve impulses at a short interval apart the response 

 to the second is found to be statistically larger, as in 

 the curarized preparation. Examination of the distri- 

 bution of amplitudes for the first and second re- 

 sponses in a number of trials reveals that the increase 

 in the second is accompanied by a reduction in its 

 fluctuation, indicating that the change is entirely 

 the result of an increase in the number of units re- 

 sponding (22). It is further found that the number of 

 units responding to the first nerve impulse in a par- 

 ticular trial has no effect on the number responding 

 to the second in that trial. This leads to the conclusion 

 that the potentiation of the second response depends 

 solely on the previous presence of an impulse in the 

 nerve and not on the number of units of acetylcholine 

 released by the impulse. 



Whereas in the amphibian the second of two 

 ner\e impulses elicits an endplate potential which is 

 larger than the first, in the curarized mammalian 

 preparation the response to the second is smaller up 

 to an interval of a few seconds (30, 65, 66). Evidence 

 of potentiation by previous activity of the nerve is 

 procured where the conditioning treatment is a large 

 number of ner\e impulses. When between a few 

 hundred and a few thousand impulses are set up in 

 the nerve within 5 to 20 sec, the later impulses in the 

 train elicit a considerably reduced response owing to 

 the depressant efifect of preceding volleys. The time 

 course of subsequent changes in the effectiveness of 

 transmission is revealed by testing with a single im- 

 pulse at a variable time after the termination of the 

 conditioning train of impulses. It is thus found that 

 the effectiveness of transmission gradually increases 

 from the depressed state to beyond that occurring in 

 the absence of previous activity (5, 48, 65). The mag- 

 nitude and time course of this potentiation depends 

 on the number of conditioning nerve impulses; it is 

 larger, arises later and is more prolonged, the greater 

 the number of impulses. Following a few thousand 

 impulses, the maximum is not reached until about 0.5 

 min. after conditioning, when the response as meas- 

 ured by the size of the endplate potential may be 50 

 per cent greater than the normal and the total dura- 

 tion of the potentiated state may be i o min. 



When the curare-free mammalian preparation is 

 subjected to calciuin depletion, a behavior is observed 

 which is similar to that in the frog. The second of two 



closely spaced nerve impulses now elicits a greater 

 response than the first (67). The effect of condition- 

 ing with a train of impulses is to cause a summation 

 of the potentiation left behind by individual nerve 

 impulses. It is apparent that the potentiation in the 

 wake of a nerve impulse has a very prolonged phase 

 of low level effectiveness, which, while hardly notice- 

 able after a single impulse, is able to sum over a large 

 number of impulses to produce an appreciable po- 

 tentiation of very great duration. When the calcium 

 concentration is normal, the earlier part of this po- 

 tentiation is outweighed by the depression which 

 follows each nerve impulse but does not sum over as 

 long a period of time. The fact that the depression 

 does not occur in the calcium depleted preparation 

 when the number of units of acetylcholine released by 

 each impulse is small makes it appear highly probable 

 that this effect, unlike the potentiation, depends on 

 the amount of acetylcholine released by previous 

 impulses. 



In the mammalian muscle under normal condi- 

 tions, the frequency of spontaneous discharges is 

 found to be increased immediately following the 

 response to a conditioning nerve impulse at which 

 time the response to a second impulse is diminished. 

 After conditioning with a large number of impulses, 

 the frequency is increased many times and returns 

 only very slowly to normal (6, 62). The final part of 

 its return parallels the time course of the subsidence 

 of the potentiation of transmission, as observed in the 

 curarized muscle. The effect of previous activity of the 

 nerve is apparently to increase the potentiality of the 

 terminals for releasing units of acetylcholine, both 

 spontaneously and in response to a nerve impulse. 



PROPERTIES OF THE JUNCTIONAL RECEPTOR 



The most direct method for investigating the 

 receptive properties of the muscle fiber is to add acetyl- 

 choline to the surrounding fluid without involving 

 the nerve terminals. Two techniques have been used: 

 the acetylcholine has been applied either uniformly 

 to the whole muscle fiber, or in a highly localized 

 manner to the region contacted by the nerve endings. 

 The effect is a depolarization of the muscle fiber in 

 the junctional region (12, 17, 36, 51). After pre- 

 liminary treatment with an anticholinesterase, which 

 prevents the enzymatic destruction of acetylcholine, 

 the technique of uniform application allows quantita- 

 tive information to be obtained on the reactivity of 

 the receptor with varying concentrations of acetyl- 



