SECTION V 

 ELECTRICAL CHANGES IN LIVING TISSUES 



THE material composing living cells and tissues is permeated 

 throughout with water containing electrolvtes in solution. All salts, 



i/ 



as we have seen, undergo ionic dissociation in watery solution a dis- 

 sociation which, in the concentrations occurring in the animal body, 

 must be nearly complete. When an electric current passes through 

 the living tissues it is carried by the charged ions formed by the 



dissociation of the salts. Thus, n/10 solution of sodium chloride 



+ 

 contains almost entirely Na and Cl ions. In addition to these charged 



inorganic ions, the cell protoplasm contains in solution or suspension 

 various colloidal particles which in many cases are themselves charged. 

 By the presence of these colloidal particles marked differences may 

 be caused in the distribution of the inorganic ions owing to the power 

 of adsorption possessed by the colloids for many inorganic salts. It 

 is evident that any unequal distribution of the charged ions or colloidal 

 particles in a tissue or on the two sides of a membrane may give rise 

 to corresponding unequal distribution of electric charges, and therefore 

 differences of potential between different parts of the tissue, which 

 under suitable conditions may find their expression in an electric 

 current. It is therefore not surprising that practically every functional 

 change in a tissue has been shown to be associated with the production 

 of differences of electrical potential. Thus all parts of an uninjured 

 muscle are isopotential, and any two points may be led off to a 

 galvanometer without any current being observed. If, however, one 

 part of the muscle be strongly excited, as, for instance, by injury, so 

 that it is brought into a state of lasting excitation, it will be found 

 that, on leading off from this point and a point on the uninjured surface 

 to a galvanometer, a current flows through the latter from the 

 uninjured to the injured surface. Every beat of the heart, every 

 twitch of a muscle, every state of secretion of a gland, is associated 

 in the same way with electrical changes. In most cases the electrical 

 changes associated with activity have the same general character, the 

 excited part being found to be negative in reference to any other part 

 of the tissue which is at rest. The uniform character of the electric 

 response in different kinds of tissues suggests that an accurate knowledge 

 of the changes in the distribution of charged ions responsible for the 



189 



