204 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



be calculated. For an endplate potential of 20 mv 

 peak amplitude the maximum displacement of charge 

 is found to be about io~' coulombs. Given informa- 

 tion on the complete electrical characteristics of the 

 fiber, i.e. of the separate values of membrane capacity 

 and membrane and core conductances, it is possible 

 to analyze more completely the potential wave in 

 the fiber. An approximate treatment, in which the 

 observed response of the fiber is compared with a 

 theoretically derived potential wave for charge placed 

 instantaneously at a point along the fiber, confirms 

 the above interpretation of the generation of the end- 

 plate response by a brief transfer of charge in a small 

 area of membrane. 



In a normal uncurarized muscle, the rate of rise of 

 the endplate potential is about three times as fast as 

 in the above case, due evidently to a proportionately 

 more intense transfer of charge. On the endplate 

 potential reaching a level of depolarization of about 

 40 mv, an action potential is initiated, indicated by a 

 sudden increase in the rate of change of potential. 

 The threshold for the initiation of an action potential 

 has Ijeen examined by the direct application of a 

 current pulse, both at the junction and away from it, 

 and has been found to occur at all points at this same 

 level. The spike which follows the initial depolariza- 

 tion produced by the endplate potential is however 

 characteristically different at the junction where it 

 is evoked from elsewhere in the course of its propaga- 

 tion (40, 69; cf. fig. 4}. After rising from the level of 

 threshold to zero membrane potential at a rate which 



is not noticeably different in the two cases, the 

 junctional spike produces a smaller reversal of mem- 

 brane potential than the normal muscle spike away 

 from the junction. Thus, at the summit of the junc- 

 tional spike the membrane potential is reversed to the 

 extent of about 20 mv (total spike height of 1 1 o mv), 

 compared to a reversal of about 35 mv for the normal 

 spike (total height, 125 mv). The summit of the 

 junctional spike occurs earlier and is sharper than 

 that of the normal spike. After reaching the summit 

 the potential falls to the level of zero membrane 

 potential where it remains nearly steady for about 

 1.5 msec, before declining further. In contrast, the 

 normal spike declines rather slowly for about 2 

 msec, after its summit, but then falls more rapidly 

 past zero membrane potential. 



It can be shown that these features, which distin- 

 guish the junctional spike, do not depend on some 

 special characteristic of the action potential process in 

 the region where it occurs. When an action potential 

 is propagated into the junctional region without the 

 nerve having been active, the response is the normal 

 muscle action potential similar to that which is elicited 

 elsewhere along the fiber. Moreover, these features 

 cannot be attributed to the response having originated 

 in the region of observation rather than having 

 propagated into it, since the propagated action 

 potential and the one which is initiated in the region 

 of recording by a brief pulse of current show little 

 difference beyond the attainment of threshold. It is 

 concluded therefore that these features arise from a 



10 msec 



FIG. 3. Endplate potentials recorded intracellularly from a single curarized muscle fiber of a 

 frog. The series of five records were taken at intervals of i mm along the fiber. The top record shows 

 the response at the junction as inferred from the fact that the response was maximum at this posi- 

 tion. [From Fatt & Katz (38).] 



FIG. 4. Action potentials recorded in a muscle fiber in response to a nerve impulse. The upper 

 record was taken at the junction, the location of which had earlier been determined by the response 

 in the presence of curare. The lower record was taken 2.5 mm away in the same fiber. A trace of 

 the endplate potential can still be seen in the lower record, appearing as a gradual rise of potential 

 which precedes the foot of the spike. [From Fatt & Katz (38).] 



