CHAPTER XI 



CONDITIONS OF EXCITATION OF NERVE AND MUSCLE BY THE 



GALVANIC CURRENT 



THE passage of a galvanic current through a nerve or muscle produces secondary 

 polarisation of these tissues, caused by the accumulation of positive and negative 

 ions at or near the poles of the constant current. This polarisation is accom- 

 panied by certain physiological changes, the tissue being more excitable in the 

 neighbourhood of the negative pole or kathode, and less excitable in the neigh- 

 bourhood of the positive pole or anode. These effects both physical and 

 physiological spread for some distance beyond the actual poles. And not only 

 is the tissue rendered more excitable by the kathode, but this itself sets up 

 excitation, which, in the case of a muscle, may cause its contraction not only 

 at the moment of closure, but during the whole time of passage of the current. 

 On breaking the circuit the part of the nerve which was more excitable 

 during the passage of the current becomes instantaneously less so than the rest 

 (physiological rebound). On the other hand, the presence of the anode of a 

 constant current not only renders the tissue less excitable whilst the current is 

 passing, but on breaking the circuit there is again a rebound ; the part which 



was the less excitable becom- 

 ing the more excitable ; this 

 passage from less to greater 

 excitability again acts as a 

 stimulus. Hence, when a con- 

 stant current is sent through 

 a nerve or muscle, there is 

 excitation at the kathode on 

 making and at the anode 

 on breaking the circuit. But 

 the latter furnishes a rather 

 weaker excitation than the 

 former. 



Polar excitation of muscle. 

 1. Engelmann's sartorius 

 experiment. A curarised sar- 

 torius is connected with a 

 pair of non-polarisable elec- 

 trodes which are joined up 

 through a mercury key with 

 a battery (fig. 44). It will 

 be observed that the twitch 

 begins at the kathode when 

 the current is closed ; indeed, 

 the muscle may remain more or less contracted at that end during the whole time 

 of the passage of a strong current. On the other hand, on opening the circuit 

 the twitch begins near the anode, and may again be followed by a prolonged 



50 



PIG. 44. POLAR EFFECTS OF CONSTANT CURRENT 

 UPON CURARISED SARTORIUS. 



