SKELETAL NEUROMUSCULAR TRANSMISSION 



203 



tential, noivvithstanding that in the amphibian 

 muscle, where it has been studied most, the nerve 

 ending is not of the morphological form described as 

 an endplate. 



On increasing the concentration of curare in the 

 fluid bathing the muscle, the amplitude of the response 

 is reduced. When, on the other hand, the concentra- 

 tion is decreased from that required to block transmis- 

 sion, action potentials in individual muscle fibers 

 appear as more rapid and complex deflections super- 

 imposed on the endplate potential. With further 

 reduction of curare, the action potential component 

 increases and obscures the endplate potential. The 

 endplate potential was early inferred to be developed 

 across the surface membrane of the muscle fiber, 

 although confined to its junctional region, because of 

 the similarity of this potential with that which could 

 be evoked by a brief pulse of current applied any- 

 where along the muscle. More compelling evidence 

 came from the study of the interaction of the junc- 

 tional response and the muscle action potential, the 

 latter elicited by direct stimulation and propagated 

 into the junctional region. It was found by this 

 method that the action potential and the endplate 

 potential did not sum with each other, and that the 

 action potential was capable of aboli.shing the later 

 part of the endplate potential when timed to coincide 

 with its summit. 



The most accurate basis for an analysis of the 

 potential changes in the muscle fiber to determine the 

 manner of their generation is the results from intra- 

 cellular recording (40). This involves inserting a very 

 fine electrode through the surface membrane of 

 individual muscle fibers and recording potentials 

 between it and another electrode in the surrounding 

 fluid. Intracellular recording not only makes more 

 accurate measurements of the electrical response 

 possible but also greatly simplifies its interpretation. 

 After minor corrections for extracellular gradients of 

 potential when current is flowing, the potentials 

 observed by this method are those obtaining across the 

 surface membrane of the muscle fiber at the position 

 of insertion of the electrode. The frog muscle fiber is 

 found to have a resting membrane potential of 

 about 90 mv (inside negative with respect to outside), 

 which is the same in the junctional region as elsewhere 

 along the fiber. The addition of curare to the solution 

 bathing the muscle in a concentration sufiicient to 

 block transmission has no effect on this resting po- 

 tential. With the intracellular electrode situated in 

 the junctional region of the fiber, an endplate po- 

 tential is recorded in response to nerve stimulation. 



It appears as a transient positive deflection, i.e. as a 

 transient reduction of membrane potential from its 

 resting level. Its amplitude varies from fiber to fiber 

 and depends upon the concentration of curare. In a 

 frog sartorius muscle, critically curarized to abolish 

 contraction, different fibers have been found to dis- 

 play endplate potentials ranging from i mv to more 

 than 20 mv. The response would be expected under 

 these conditions to range in size up to the threshold 

 depolarization for initiating an action potential 

 which would be about 40 mv. Immediatelv at the 

 junction the endplate potential has a rising phase 

 lasting 1.5 msec. Following the attainment of the 

 summit, the potential declines to one half in another 

 2 msec. The rate of fractional decay decreases beyond 

 this, the time required to fall from one half to one 

 quarter being about 5 m.sec. A potential change can be 

 detected at points on the fiber up to a few millimeters 

 distant from the nerve ending, becoming progressively 

 more attenuated and slowed with increasing distance 



(fig- 3)- 



This potential wave has been analyzed to determine 

 the movement of charge underlying it. The amplitude 

 of the potential at various instants is plotted against 

 distance along the fiber. Assuming that the membrane 

 capacity remains constant during the response, the 

 curves thus formed indicate the spatial distribution of 

 charge displaced from the membrane capacity (rela- 

 tive to its initial condition of charge). The area 

 beneath each curve is a measure of the total charge 

 displaced at the given instant. The plot of these areas 

 against time shows that the charge is built up to a 

 maximum in about 2 msec, and after this it decays 

 exponentially with a time constant of about 25 msec. 

 A determination of the passive electrical characteris- 

 tics of the muscle fiber shows that this latter value 

 corresponds to the electric time constant of the mem- 

 brane. This result is consistent with the idea that 

 there is a brief phase of transmitter action, confined to 

 about the initial 2 msec, of the response, during which 

 charge is transferred inward across the membrane, 

 and that this is followed by a gradual dissipation of the 

 displaced charge at a rate determined by the electrical 

 characteristics of the inactive fiber membrane. It 

 agrees with the results of the interaction between the 

 endplate potential and action potential from which it 

 appears that the charge displacement built up by 

 junctional activity can be removed by the high con- 

 ductance of the spike at a time shortly following the 

 summit of the endplate potential. 



From a knowledge of the membrane capacity for a 

 imit length of fil)er, the displacement of charge may 



