THE MUSCULAR SYSTEM 



407 



If an uninjured muscle, which is giving no currents, be stimu- 

 lated by means of an interrupted current into contracting activity, 

 it exhibits electrical phenomena, called the current of action. It 

 is the production of this current which accounts for the negative 

 variation of the injury current. The action current is of such a 

 nature as to give rise to what is known as a diphasic variation in 

 the current of a muscle, as shown by the needle of the recording 

 galvanometer swinging first one way and then in the opposite 

 direction. The double variation is due to the fact that the point 

 on the muscle to which the stimulus is applied becomes negative 

 to all points of the muscle at which' the wave of contraction, 

 resulting from the stimu- 

 lation, has not yet arrived. 

 This negativity occurs 

 during the ' latent period ' 

 (p. 396), and passes along 

 the muscles as a wave 

 which precedes, or in some 

 cases accompanies, the 

 wave of contraction. Thus 

 if, as in Fig. 122, a muscle 

 be stimulated at a, while 

 the points b and c are con- 

 nected through a very sen- 

 sitive galvanometer, at the 

 moment of stimulation a 

 becomes negative to the 

 rest of the muscle. As this 

 negativity sweeps along 

 the muscle it passes first 

 over the point b, which 

 thus becomes negative to 

 c, and the current in con- 

 sequence flows from c to b 

 through the galvanometer. 

 Midway between b and c is a neutral point, as shown by the 

 fact that no current is passing through the galvanometer. Im- 

 mediately afterwards the current passes over the point c, and c 

 then becomes negative to b, as shown by the current flowing 

 through the galvanometer in the opposite direction. Every 

 contraction of a muscle gives rise to a diphasic variation, 

 which passes along the muscle either in advance of or accom- 

 panying the contraction wave. These phenomena, while of the 

 greatest interest in the case of muscle, become still more im- 

 portant in the case of a nerve, since they provide the only 

 accurate means of following the passage of an impulse along a 



Fig. 122. — Sartorius Muscle arranged 

 to Demonstrate the Diphasic Varia- 

 tion of Action Current in Muscle (or 

 Nerve). 



S, Sartorius ; a, stimulating electrodes ; b, c, 

 non-polarisable electrodes as * leads ' to G, 

 the galvanometer. The electrode c is in- 

 tentionally not placed on the injured end 

 of the muscle as it would be for demon- 

 strating more ' negative variation,' since 

 the strong negativity of the injured end 

 would mask the desired phenomenon. A 

 similar arrangement suffices to demonstrate 

 the same phenomenon in a piece of nerve. 



