266 BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



TABLE 10-1. Natural Content of the Solution Within the Giant Axon of the Squid" 



Substance 



Concentration 

 (millimoles/lOOOg H 2 0) 



K 400 



Na 50 



CI 40 



Ca 0.4 



Mg 10 



isothionate 270 



aspartate 75 



glutamate 12 



succinate-fumarate 17 



orthophosphate 2.5 to 9 



ATP 0.7 to 1.7 



phosphagen 1 .8 to 5.7 



*Data collected by Hodgkin. 14 Compare with ionic content of erythrocytes (Table 2-1)'. 



Since 1947 experiments of essentially three kinds have added valuable 

 clues toward the explanation of just how the nerve carries information. They 

 have been: (l)radioactive tracer experiments on sodium and potassium 

 ions; (2) studies of the effects of changes in concentration of natural and 

 foreign ions and molecules; and (3) electrical studies such as fixing the po- 

 tential difference and following the current changes which result — the so- 

 called "voltage clamp' 1 technique. 



In short, these three techniques have established the facts that the im- 

 pulse is associated with: (a) a rapid increase in the membrane's permeabil- 

 ity to Na + , an increase which lasts only about one msec; and concurrently, 

 (b), a smaller and later increase in the K + permeability, which has a slower 

 recovery lasting over several msec. 



(1) Tracers: Hodgkin, Huxley, and Katz were the first to use effectively 

 the radioactive beta and gamma emitters, Na 24 and K 40 , to follow sodium- 

 and potassium-ion permeabilities across the axon wall. If the active Na + salt 

 is placed in the external solution, samples of the internal fluid can be with- 

 drawn via the micropipet and checked periodically for radioactivity. Al- 

 ternatively, small amounts of radioactive K + salt can be inserted into the 

 axon, and samples of the external fluid measured periodically for radio- 

 activity. The rate of permeation of these ions through the axon wall when 

 it is passive is much smaller than the rate of permeation when the axon is 

 repeatedly stimulated and is carrying impulses. This difference in rate of 

 penetration is greater the greater the number of pulses being passed along 

 the axon per second. 



