226 THE BIOPHYSICAL PROBLEM OF NERVE CONDUCTION 



rapidly increases with time and passes through a maximum, from which 

 it at first rapidly falls, and then decreases more slowly to attain a con- 

 stant value. In frog nerve this maximum is attained in 3 milliseconds; 

 in large mammalian nerve it is a fraction of a millisecond. 



On breaking of the current, the excitability of the nerve abruptly 

 decreases to a large value of inexcitability, from which it rapidly recovers 

 to attain its resting value exponentially with time (Erlanger and Blair 

 [1931]). 



At the anode the inverse of this phenomenon takes place. 



This change in excitability is called accommodation. 



Complete accommodation means that the excitability of the nerve 

 has returned completely to its resting value during the passage of a con- 

 stant current. This is probably due to the neutralization of the depolar- 

 izing effect of the current by the repolarization activity of the nerve so 

 that the two effects cancel, leaving the membrane at its resting value. 



Sequence of Events 



From the above experimental evidence we may roughly piece together 

 an explanation of the sequence of events which in the final analysis may 

 lead to a correct interpretation of the origin and method of propagation 

 of the impulsive wave of the nerve fiber. 



In general, the biological potentials are of two types : the static poten- 

 tials of the resting nerve indicating the relative difference of potential 

 between the two points under examination, and the transient potentials 

 accompanying the activity associated with pulse propagation. 



Structurally the nerve fiber has the form of a cylindrical condenser or 

 " submarine cable." The transmission of message pulses in an electrical 

 cable, however, is entirely different from the propagation of pulse mes- 

 sages in nerve fiber. 



The condenser-like properties of the nerve sheath and its potential 

 difference between the axis cylinder and the outer surface are probably 

 due to a difference in ionic concentration on the two sides of the bound- 

 ary. This difference in potential gradually disappears when the nerve 

 is deprived of oxygen. An explanation of this disappearance may be 

 found in the associated osmotic-pressure phenomenon, for part of the 

 potential is very probably due to the orientation of the organic molecules 

 forming a polar boundary at the surface of the axis cylinder. 



Suppose that these molecules are of the palmitic acid (C15H31 — COOH) 

 type, composed of a long (21 X 10 -8 cm) cylindrical carbon chain termi- 

 nating in an active polar group having a — COOH configuration. In a 

 closely packed array* they stand upright so that a maximum number of 



* Shaffer and Dingle [1938] find the monomolecular layer of crystalline egg albumin 

 to be 40 X 10 -8 cm thick. 



