MEMBRANES AND CELL-PROCESSES 



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can produce decided changes of condition only at the semi-permeable 

 surfaces, where the movement of ions is blocked; changes of electrical 

 polarization would be produced at such surfaces; ions of a given sign 

 would be carried against one face of the membrane by the current 

 and would concentrate there until the back-diffusion equalled the 

 current-transport ; the same effect, with the signs of the ions changed, 

 would result at the other face 

 (Fig. 3). He conceived that in 

 electrical stimulation something 

 of the kind occurs. The essential 

 or critical change occurs at the 

 semi-permeable membrane, and 

 consists in carrying to this mem- 

 brane sufficient ions to produce a 

 given ionic concentration-difference 

 corresponding to a given electrical 

 polarization. This is the deter- 

 mining condition of stimulation. 

 A certain time will be required 

 for the process, depending on the 

 strength of the current, and on 

 the specific diffusion-rate of the 

 ions. Nernst estimated that on 

 this hypothesis the stimulating action (S) of a given current ought to 

 vary directly with its strength {%), and with the square root of its 

 duration (t) (S = Kiyft, K being a constant characteristic of the 

 tissue). The experimental data show that a more intense current 

 requires for stimulation a shorter time than a weaker current, and in 

 approximately this proportion. The more recent work of Lapicque, 

 Lucas and Hill has confirmed and amplified Nernst's theory. There 

 is therefore strong evidence that a current stimulates by producing an 

 electrical polarization at the membranes. 



During life, however, the membranes are apparently already the seat 

 of a preexistent polarization, as we have seen. The polarization pro- 

 duced by the external current must, therefore, modify this. Now it 

 appears that in most, if not all irritable tissues, stimulation results 

 when the physiological polarization is diminished suddenly, but not 

 when it is increased. This is the simple inference from the law of 

 polar stimulation. When a current is passed through a tissue the 

 external positivity of the irritable elements is lowered on the side 

 directed toward the cathode and increased on the side directed toward 

 the anode, as may be seen by reference to Fig. 3. Now it has long 

 been known that the stimulus originates on the cathodal side of an 

 irritable tissue when the current is made, and on the anodal side when 



Fig. 3. Illustrating the polarization of 

 the current on a membrane difficultly 

 permeable to ions. The anions and cations 

 of the electrolyte, NaCl, move in the di- 

 rection indicated by the arrows. The cur- 

 rent, passing from left to right, carries 

 cations toward and anions away from the 

 left face of the membrane ; at its right 

 face the conditions are reversed. The 

 membrane thus becomes electrically polar- 

 ized, with its left face at the higher 

 potential. 



