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HOWARD J. CURTIS 



made between this inside electrode and one on the outside surface of 

 the cell directly opposite it (see 12). Here there can be no question 

 but that the true membrane potential is being measured, provided 

 elementary precautions are taken about insulation, input resistance 

 to the amplifier, etc., as discussed above. However, this method suf- 

 fers from the disadvantage that the manipulative procedure of getting 

 an electrode inside a cell is very difficult, and indeed has been ac- 

 complished for only two cells, the giant nerve fiber of the squid and 

 the large single plant cell Valonia. 



This measurement can be made indirectly in the case of certain 

 cells as the injury potential. Consider, for example, a nerve cell 

 that is a cylindrical cell in which the length is very much greater than 

 the diameter. It has been found that one end of a nerve fiber can 



Fig. 10. Equivalent circuit showing relation between membrane 

 potential E,,^ and the injury potential, Vo, m a single nerve fiber. 

 Shaded portion of the fiber represents injured region. Ri represents 

 the inside resistance and Rn the outside resistance. 



be destroyed without affecting the other end, at least for many hours 

 and even days. This destruction breaks down the membrane re- 

 sistance at one end and allows a current to flow freely between the 

 cytoplasm and the extracellular fluid. Contact can thus be made to 

 the inside of the cell by placing an electrode on the injured portion of 

 the cell. If now an electrode is placed on the iminjured portion of 

 the nerve fiber, a potential will exist between the two electrodes such 

 that the one on the uninjured portion is positive. This is known as 

 the injury potential. Whereas it is due to the membrane potential, 

 it certain!}'' cannot be taken as a measure of it with any degree of ac- 

 curacy. 



The relation between the injury potential and the membrane po- 

 tential is best understood in terms of the equivalent circuit, shown in 

 Figure 10. A simple calculation will show that the injury potential, 

 Fo, is given by the relation : 



