CHAPTER XC 



THE CONDUCTION OP THE NERVOUS IMPULSE 



We have seen that the activity of the neuromuscular mechanism in- 

 volves the three processes of excitation, conduction, and response. These 

 processes may be regarded as fundamental properties of protoplasm 

 which may all be exhibited by a single cell. The evolution of the cen- 

 tral nervous system is the history of a gradual specialization of cells 

 each for the execution of one of these processes. Thus in a reflex arc 

 the receptor is a cell primarily fitted to react to slight changes in its 

 environment, the neurons are especially adapted to the conduction of im- 

 pulses, and the effector is constructed for the sole purpose of carrying 

 out some motor response or producing some specialized secretion. It 

 would appear as though a single cell could not develop the ability to 

 perform all of these functions with perfection and as a consequence a 

 high degree of division of labor has been established. 



It should be remembered, however, that this specialization for one 

 process has deprived the cells to only a limited degree of the ability 

 to be the seat of the other processes. Thus although excitability is pri- 

 marily a property of the receptors, nerve cells must also retain the abil- 

 ity to become excited by disturbances set up in the receptors, and muscles, 

 must be able to be excited by impulses reaching them from motor neurons 

 Similarly conduction must occur not only in nerve cells, but in receptors, 

 so that a disturbance set up at the distal end of the receptor cell may 

 reach the nerve fibers which terminate about its proximal end. Muscle 

 cells also must be able to conduct disturbances set up in the myoneural 

 junction to all parts of the muscle fiber, so that they may contribute to 

 the response. It has been seen that the conductivity of the cardiac mus- 

 cle is of great importance in the co-ordination of the action of the heart 

 (page 182). Even contractility which would appear at first sight to be a 

 function of the effector alone is displayed occasionally by the other parts 

 of the reflex arc. Thus embryonic nerve fibers have been observed to 

 perform ameboid movements and the rods and cones of the retina, which 

 are the receptors of light, become shorter or longer in response to changes 

 in illumination. 



While excitation, conduction, and response are seen to occur in all 

 parts of the reflex arc, when we come to study these processes it is con- 

 venient to examine each in that tissue in which it is most highly developed 

 and where the other processes will introduce a minimal complicating ele- 



836 



