234 ELEMENTS OF BIOLOGY 



stimulus upon a cell is electrochemical in nature. Let us imagine a 

 cylindrically shaped cell that is inactive (Fig. 162). Its boundary, as 

 we have seen, is a semi-permeable membrane that permits the 

 passage of some molecules and ions but holds back others. It is 

 known that the boundary of such a cell allows positive ions to 

 escape from the underlying cytoplasm but does not allow negative 

 ions to pass out. I>ue to the electrostatic attraction between opposite 

 charges, the positive ions cannot leave the outer surface of the mem- 

 brane, being held back by the attraction of the negatives that cannot 

 escape through the boundary. The cell boundary is therefore elec- 



FiG. 162. — Scheme to represent an interpretation of the nerve impulse. (Slightly 



modified, after Gerard.) 



trically polarized, bearing an outer positive charge and an inner 

 negative charge, so long as the cell is undisturbed. If a stimulating 

 agent is brought into contact with the cell boundary, its perme- 

 ability to negative ions is increased at the point stimulated and they 

 escape. The result of the stimulus is to increase the number of 

 negative ions at the point of contact and thus to set up a potential 

 difference between that point and adjacent regions of the mem- 

 brane, which are still positively charged (Fig. 162). Whenever two 

 regions with different electrical potentials are connected by an 

 element that will admit of electrical change, then a current flows 

 from one to the other. In the cell boundary, the electrical disturb- 

 ance at the point of stimulation causes a re-arrangement of the 

 electrical conditions elsewhere on the surface of the cell; thus the 

 excitation is responsible for some sort of transmission. 



