208 The Vertebrate Organ Systems 



rest, that is, the membrane separates an outer layer of cations or posi- 

 tive charges from an inner layer of anions or negative charges. If this 

 nerve fiber is stimulated, the permeability of the membrane is increased 

 due to its depolarization. The section stimulated is then negative to 

 the following polarized section. A current which flows between these 

 two sections causes depolarization of the next region. Thus a wave 

 of negativity flows along: this is the nerve impulse. This change of 

 polarization is accompanied by chemical changes. The depolarized con- 

 dition lasts but a short time after the impulse has passed. During this 

 latter period, a new impulse cannot be conducted and the nerve fiber 

 is said to be refractory. 



+ -i- -t- + + + + 



Q -f- + -(--t--(--f- + 



-♦--+--*--»- 



-+--!-+-»- -*-+ ■*-■ 



Fig. 65. — Diagrams illustrating the membrane theory of nerve conduction. 

 A, Polarized nerve fiber not conducting an impulse; B, a nerve fiber conducting an 

 impulse from left to right; part of the fiber is depolarized; C, as the nerve impulse 

 moves along, the fiber becomes polarized again. 



If a nerve fiber reacts, it reacts completely. In this way it is very 

 similar to the individual muscle fiber in the manner in which it obeys 

 the "all-or-none" law. The difference in strength of nerve impulse can 

 be accounted for by the fact that a single stimulus does not necessarily 

 affect all the nerve fibers in an individual nerve. 



The manner in which the nerve impulse moves along a fiber has 

 often been compared to the way in which a gunpowder fuse burns ; that 



