Manchester Memoirs, Vol. xlii. (1898), No. 1^. 19 



themselves, in the two cases of movement and of 

 inhibition. Whether an afferent impulse stirs up a motor 

 unit to activity or checks an activity already in play, is 

 determined by the relations of the afferent impulse to the 

 condition and circumstances of the motor unit, not by any 

 essential differences in the nature of the afferent impulse 

 in the two cases. 



This is true, at least, so long as we judge of the nature 

 of a nervous impulse by what we have seen to be the only 

 objective token in our possession, the electric current of 

 action. 



We must not, however, trust too much to these currents 

 of action. We certainly are not justified in assuming that 

 the only phases of a nerve unit are the two phases, the 

 one of rest, in which no currents of action are developed, 

 the one of activity, signalled by a current of action. We 

 have increasing evidence of cases in which a motor unit, 

 quite apart from its being thrown into an activity mani- 

 fested as movement, exerts on the peripheral, muscular or 

 other elements with which it is connected, a more gentle 

 continued influence, commonly spoken of as tonic action. 

 Now this tonic action, so far as we know, is not accom- 

 panied by any such electric change as that which marks 

 the grosser ordinary nervous impulse. It is the effect of 

 gentler, subtler influences passing along the axon, influences 

 more nearly allied, of which we spoke a little while back 

 as concerned in the nutrition of the unit. 



Such a tonic influence exerted by a unit may be 

 modified positively or negatively, may be increased, 

 augmented, or may be diminished, inhibited, by various 

 influences ; and in many cases these influences are of the 

 nature of gross nervous impulses brought to bear on the 

 unit by other afferent units. In such cases there can be 

 no question of a mere reflection. The individuality of 

 the efferent unit is stamped on all its work. 



