302 



PHYSIOLOGY 



ELECTRICAL STIMULI AS APPLIED TO HUMAN NERVES 



When we attempt to apply the results gained on frog's nerves to 

 man, we are met at once by the difficulty that we cannot dissect out 

 the nerves and apply stimuli to them directly. So usually unipolar 

 excitation is used, one electrode, either anode or cathode, being 

 applied to the nerve to be stimulated, and the other to some indifferent 

 point, such as the back. It is evident under these circumstances that 



the current is concentrated as it leaves 

 the anode and reaches the cathode, 

 and diffuses widely in the body, seek- 

 ing the lines of least resistance. Thus 

 it is impossible to get pure anodic or 

 cathodic effects. If the anode be ap- 

 plied over the nerve, the current enters 

 by a series of points (the polar zone), 

 and leaves by a second series (the 

 peripolar zone). The polar zone will 

 thus be in the condition of anelectro- 

 tonus, and the peripolar zone in that 

 of catelectrotonus. The current, how- 

 ever, will be more concentrated at the 



polar than at the peripolar zone, and 

 FIG. 119. Electrodes applied to ,-, n - n , 



the skin over a nerve-trunk. In so tne former effect will predominate. 

 A the polar area is anelectrotonic These restrictions in the application of 



and the peripolar catelectro- ,-, , , . , 



tonic. The former condition tne current cause slight apparent irre- 

 therefore preponderates, since gularities in the law of contraction as 



the current here is more con- 

 tested on man. 



centrated. In B the conditions 

 are reversed, the polar zone 

 corresponding in this case to the 

 cathode. (WALLER.) 



In stimulating the nerves of man for the 

 purpose of determining the conditions of the 

 different muscles, we may use either induced 



currents (generally called faradic stimulation) or the make and break of a battery 

 current (galvanic stimulation). It is usual to employ the unipolar method, in 

 which one electrode is placed over the nerve at the point it is desired to stimulate, 

 while the other electrode, spoken of as the indifferent electrode, is applied to the 

 skin over a wide area, generally at the back of the neck. The current is then widely 

 diffused as it passes through the indifferent electrode, but is concentrated as it 

 passes between the skin and the stimulating electrode. The electrodes are covered 

 with chamois leather moistened with salt solution in order to diminish the 

 resistance of the skin. When it is desired to stimulate any given muscle, the 

 stimulating electrode is brought as nearly as possible over the spot where the 

 muscle receives its motor nerve. These ' motor points ' have been mapped 

 out, and reference is generally made to a diagram in determining the point 

 for any given muscle. By reversing the current the stimulating electrode 

 may be made either anode or cathode. It is found that stimulation occurs 

 most easily on closure of the current when the stimulating electrode is the 

 cathode; with the greatest difficulty when the current is broken and the 



