HARRY ORUNDFEST I4I 



several types of action which are intermingled in electrogenic responses. Thus, 

 even the elementary nature of the excitable membrane, whether it is essentially 

 that of neuroplasm altered by its phase boundary position or whether it is a 

 special structural element, is unknown. Since electron microscopy has not 

 resolved highly dififerentiated structures comporting with the differentiation of 

 membrane function (156, 167, 168) it seems that the first mentioned [)ossibility 

 is the likelier. Also unknown are the molecular components of this membrane. 

 In consequence, 'models' of electrogenic mechanisms must resort to the vague 

 terms of 'pumps,' 'carriers' and 'receptors.' However, the grow'ing number of 

 analyses of specific chemical [)rocesses either in 'resting' brain (e.g. 82) or in 

 'active' (e.g. 88), and even in single cells (12, 117, 143), gives hope that neuro- 

 chemistry will in the near future provide needed restrictions on speculation 

 and useful specifications for processes. 



h) Electrogenic Action of Synaptic Transducers. The permeability changes 

 induced in the e.xcitatory and inhibitory transducers must be different to cause 

 oppositely directed potential changes. Under the normal conditions of ionic 

 distributions, increase only of potassium conductance would drive that ion out 

 of the cell and leave the interior more negative. Increase of chloride permeability 

 would tend to drive this ion inward and thereby also increase the internal nega- 

 tivity. Increase of sodium conductance alone, if it does not involve a regenera- 

 tive mechanism, could lead to depolarization and even to reversed polarization 

 of any level up to that of the sodium potential. However, there is evidence 

 (37, 72) that the end-plate potential does not exceed an equilibrium level 

 slightly negative to zero membrane potential. Therefore, it is likely that the 

 end-plate and other excitatory p.s.p.'s result from a general increase of mem- 

 brane conductance to all three ions.^ In addition to exciting the electrically 

 excitable component of the membrane, the p.s.p. would tend to poise the 

 potential of the latter at its own equilibrium potential, but this effect would 

 generally be small, since in most post-junctional cells the area of synaptic 

 transducer membrane is a small part of the total surface. However, at the 

 muscle end-plate a relatively large part of the fiber membrane is made up of 

 this transducer. The poising (or 'short circuiting,' cf. 72) action then is pro- 

 nounced. The peak of the spike is driven down while its falling phase is lifted 

 toward the equilibrium value of the p.s.p. (fig. 5). 



An interesting example of the electrochemical modification of a synaptically 

 activated system is disclosed by the electrogenic behavior of polarized Torpedo 

 electroplaques (100). Depolarizing currents diminish the amplitude of the 

 response, but the latter also reverses with still stronger applied currents (i). 

 This behavior does not comport with that of an electrically excitable membrane 



* At present the transducer effects on other, chietly the divalent Ca+"'" and Mg"*""*", ions are 

 neglected, although it appears likely that the membrane conductance for these is also changed 

 during electrogenic responses (83). 



