172 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



The drain on the energy reserves is greater in small than in large fibers of 

 similar structure and chemical composition. This follows from the dependence 

 of the fluxes during activity on surface area (20, 50) and of the potassium and 

 sodium content on volume. Thus, in the absence of recovery processes, a 500 n 

 axon will deliver 500 times as many impulses as a i /x fiber by virtue of its 500- 

 fold smaller surface to volume ratio. The much more active recovery mechanism 

 in small invertebrate fibers, reflected in the greater oxygen consumption (5) or 

 potassium absorption following activity (50), may therefore be accounted for 

 from an evolutionary standpoint in terms of the requirement to meet the 

 energy drain imposed by activity. This principle is applicable to vertebrate 

 nerve where heavy myelination replaces size in achieving a smaller drain on 

 the ion reserves. Thus, the potassium lost by i gm of frog nerve during an 

 impulse is about 30 ix^m, which is less than one-millionth of the reserve (44); 

 the corresponding potassium loss for spider crab nerve is approximately 5300 

 fifjM, which represents at least a 25-fold greater drain on its potassium store 

 (44, 46). The 18-fold greater recovery heat of crab nerve (11) therefore corre- 

 lates well with the factor by which crab and frog nerve differ in exhausting their 

 reserves. Small, weakly myelinated fibers from the vertebrates may be expected 

 to compare with crab nerve in having a more highly developed system for 

 active transport following nerve activity; the much greater activity respiration 

 of these fibers (32) probably reflects this. Such fibers may prove preferable for 

 studies of active transport, particularly following activity. 



CONCLUSION 



Theoretical and experimental developments, particularly within the past 

 five years, appear to offer prospects for a clarification of the more general 

 factors governing ion movement and distribution in nerve. Observations of 

 both influx and outflux as well as of the potential difference under conditions 

 of metabolic inhibition provide, on the one hand, a basis for distinguishing 

 between different energy requiring mechanisms which may contribute to the 

 ion balance and, on the other hand, provide a means of more critically testing 

 assumptions underlying hypotheses which have been developed to account for 

 the 'passive' electrochemical characteristics of biological systems. At present 

 there is no direct evidence that the permeability to different ions is actually 

 altered by the interruption of metabolism. The detailed submicroscopic 

 mechanisms of the movement of ions across the axon boundary, both active 

 and passive, remain unknown. The passive entry of sodium and the exit of 

 potassium during the impulse appear to involve separate pathways and chem- 

 ical interactions (25). This may be true of electrochemically governed transfer 

 in the resting state, but no detailed proof for this is at hand. Experimental 

 data on alterations in the kinetics of specific fluxes effected by drugs which 

 modify passive ion movement (44, 45) should help clarify this problem. 



