June 27, 1913] 



SCIENCE 



967 



stood. There is evidence, however, that it 

 is associated with a definite alteration in 

 the osmotic properties of the membrane. 

 The electrical properties of irritable tissues 

 like muscle indicate that during life there 

 exists permanently in the resting cell a dif- 

 ference of potential, equal roughly to 1/10 

 volt, between the outer and inner surfaces 

 of the plasma-membrane. When the un- 

 injured outer surface of a muscle or nerve 

 is connected through a galvanometer with 

 the exposed interior of the elements (cut 

 surface) a current, the so-called "demarca- 

 tion-current," flows from exterior to in- 

 terior, indicating that the outer surface of 

 the cells or nerve fibers has a higher poten- 

 tial than the interior. This potential-dif- 

 ference appears dependent on the semi- 

 permeability of the plasma-membrane ; it is 

 absent in dead cells whose plasma-mem- 

 branes have lost their semi-permeability, 

 and it is diminished by the application of 

 poisons which impair the normal semi-per- 

 meability. Briefly, the demarcation-cur- 

 rent potential appears in some way to be 

 inseparably connected with this semi-per- 

 meability of the membrane. Ostwald in 

 1890 suggested a possible explanation of 

 this condition when he pointed out that a 

 membrane might become the seat of a po- 

 tential difference by interfering unequally 

 with the diffusion of the anions and cations 

 of an electrolyte contained within the cell. 

 If the plasma-membrane allowed cations to 

 pass outward freely, but prevented the pas- 

 sage of anions, a state of things would be 

 produced comparable to what we observe 

 in living cells. But the actual conditions 

 are probably more complex than this, and 

 experimental substantiation of Ostwald 's 

 suggestion has not been satisfactory. The 

 subject is evidently one requiring further 

 investigation. In any case, however, the 

 existence of the demarcation-current poten- 

 tial appears dependent on the semi-perme- 



ability of the membrane, and whenever the 

 latter undergoes marked increase of perme- 

 ability, this potential is invariably de- 

 creased. The electrical variation or action- 

 current which normally accompanies 

 stimulation may be theoretically accounted 

 for by assuming that at this time the mem- 

 brane undergoes a decided but temporary 

 increase in permeability, i. e., loses the 

 semi-permeable properties which it pos- 

 sesses during rest, and there is independent 

 evidence that this change actually occurs 

 during stimulation. If this is the case the 

 plasma membrane is the seat of the most 

 constant and characteristic manifestation 

 of stimulation — the electrical variation or 

 action-current. The membrane responds 

 to the stimulating condition by suddenly 

 changing its permeability and hence its 

 electrical polarization. 



We are thus brought to the conclusion that 

 the plasma-membrane is characteristically 

 and intimately concerned in the stimulation 

 process. During stimulation it appears to 

 undergo a sudden and quickly reversible 

 increase of permeability. The electrical 

 variation is one expression of this change, 

 but there are others as well. Thus the 

 movements of sensitive plants, which occur 

 under the same conditions of stimulation 

 as those of irritable animal tissues, are due 

 to a collapse of turgid cells, consequent 

 upon a sudden loss of the semi-permeable 

 properties of the plasma-membranes en- 

 closing the osmotically active solution or 

 cell-sap. Here at least is one irritable tissue 

 where the connection between permeability- 

 increase and stimulation seems unmistak- 

 able. It might be held that the existence 

 of special osmotic motor mechanisms in cer- 

 tain plants affords no indication of the na- 

 ture of the conditions in irritable animal 

 cells ; but even in animals there are in some 

 cases very clear indications that stimulation 

 is constantly associated with an increase 



