PRINCIPLES OF PHYSIOLOGY 105 



side and negative ions inside is from 0.03 to 0.06 volt. When the nerve 

 is stimulated, its permeability is increased, the ions move through the 

 membrane, and the membrane is depolarized. Ions from the adjacent, 

 not-yet-activated region pass through the depolarized region and neu- 

 tralize each other. This depolarizes the adjacent region and makes it 

 permeable to the migration of ions from the next region, and so on. The 

 nerve impulse moves along the surface of the nerve fiber as a wave of 

 depolarization. It seems probable that certain chemical reactions must 

 occur in the depolarized membrane to make it permeable and other 

 reactions must occur during the refractory and recovery periods to re- 

 charge, repolarize the membrane and enable it to be depolarized by the 

 next impulse. This theory provides an explanation for the all-or-none 

 phenomenon of nerve transmission, for no matter what the strength of 

 the stimulus, the depolarization can go only to zero. 



+ + + + -f-i--(- + -t--»--f-h + + + 



A / RESTING NERVE FIBER 



+ ++-t--h+ + + + + -t--t--h-|--t--<- 



^+ + + + + + 4-H-f + +4-l- + 

 B / ACTIVE NERVE / 



f +-!- + -(■ + + +4- + +-(-+ + + 

 Depolarized region 



^+ +-t- + -t--(--(-+ + -t-+ + + 



C 



■M-.^.-l./tt'V 



+ + 



_^4 + + + + ++44 



4 4 4 + + + -^4 + + + + 



IMi 



. -■ V .-'i<:»>Ww<*^r;.l!:Lij-^«;ft>ii 



+ 4 + + + + + 



K^f£aaittV-jr;i^iiAft'»:t/;i>ii:-^/i*/>v;»*:«X 



Figure 5.11. Diagram illustrating the membrane theory of nerve transmission. A, 

 Resting nerve, showing the polarization of the membrane with positive charges on the 

 outside and negati\e ones inside. B, Nerve conducting an impulse, showing, from left to 

 right, the depolarized region where the impulse is, and the polarized region ahead of the 

 impulse. C, Stages in the passage of the impulse along the nerve. (Villee: Biology.) 



