NEURON PHYSIOLOGY INTRODUCTION 63 



Vnb 



• External fluid 



'\/\/\/\/\/\/\/\/\/\/\/\/\/\/ Inlenor of fibre 



llSmV 



12 mV 



FIG. 2. Theoretical action potential (F) and membrane conductance changes gNa and ^k obtained 

 by solving the equations derived by Hodgkin & Huxley (52) for the giant axon at i8.5°C. Inset 

 shows diagram of an element of the excitable membrane of a nerve fiber — a, constant capacity; 

 b, channel for K+; c, channel for Na+. [From Hodgkin & Huxley (52); Huxley (57)-] 



membrane potential. At any instant the nerve im- 

 pulse will be extended as a potential change along 

 the nerve fiber as shown in figure 35. According to 

 the ionic hypothesis, there will be a net inward move- 

 ment of Na ions during the rising phase of the impulse 

 (figs. 2, 3.4) because the Na conductance has been 

 greatly increased by the depolarization so that Na 

 ions move freely down their electrochemical gradient 

 carrying positive charges inwards, thus adding to the 

 depolarization and hence to the Na conductance. In 

 this self-regenerative manner, when the level of 

 depolarization of any element of the nerve membrane 

 increases above a critical value, it causes the mem- 

 brane potential to be carried almost up to the Na 

 equilibrium potential which is about +50 mv, i.e. 

 internally positive (fig. 2). The delayed development 

 of the other two ionic processes checks this potential 

 change and eventually restores the resting membrane 

 potential; the Na conductance is inactivated and the 

 K conductance increases so that, during the falling 

 phase of the impulse, the membrane potential is 

 dominated by the flux of K ions moving outwards 

 along their electrochemical gradient across the 



membrane (figs. 2, 3.-1}, which eventually is restored 

 to its original resting potential close to the potassium 

 equilibrium potential. Propagation occurs because 

 of the cable properties of the nerve fiber, current 

 flowing outwards across the membrane ahead of the 

 impulse in the circuits, as shown diagrammatically in 

 figure 3C. This current efTects a discharge of the mem- 

 brane capacitance so that in the zone ahead of the 

 impulse the membrane is depolarized sufficiently to 

 initiate the regenerative increase in Na conductance, 

 by which time the impulse may be said to have 

 arrived at this new zone, which will in turn go through 

 the conductance changes outlined above. It will be 

 appreciated that propagation will be a continuous 

 and uniform process along a stretch of nerve with 

 uniform properties. The propagation velocity calcu- 

 lated from the differential equation and the measured 

 cable properties of a nerve fiber is not only of the 

 correct order, but is in very close agreement with that 

 actually observed (52). Saltatory propagation along 

 the nodal structure of a medullated nerve also can be 

 satisfactorily explained by the occurrence of essen- 

 tially similar processes at each node. This propagation 



