S90 SCIENCE PROGRESS 



discharge may be weak, although the excitatory afferents are 

 being excited strongly. This is because their influence on 

 the centre may be counteracted by concurrent operation of 

 inhibitory afferents. Under natural circumstances it is usual 

 not for one stimulus, but for groups of stimuli, to play con- 

 currently on the organism through its afferent paths. Their 

 opposed influences collide. Now, the depressor influence of 

 an inhibitory afferent is just as capable of graded intensity as 

 is the pressor influence of its excitatory antagonist. If while 

 a pressor afferent is acting on the motor centre a depressor 

 afferent is stimulated (fig. i) the motor discharge, as judged 

 by the muscles' contraction, can be curbed to any desired 

 amount by suitably grading the intensity of stimulation of 

 the inhibitory afferent. Representing the influence of the 

 pressor afferent by + and that of the inhibitory by — , the 

 contraction resulting from concurrent stimulation of the two 

 afferents appears as the algebraic sum of + and — quantities. 

 The muscle excited through a purely pressor afferent exhibits 

 for each intensity of stimulus theoretically but one particular 

 grade of intensity of contraction. But under the less simple 

 though more natural condition where several afferent channels 

 of opposed effect are concurrently at work on the centre any 

 particular grade of contraction may represent any one of many 

 different combinations of intensity of opposed stimuli. Even 

 where only two different channels are competing it is impossible 

 to know what state of interaction the state of the muscle 

 represents except by quantitative observation of one at least 

 of the stimuli. 



The two processes of reflex excitation and reflex inhibition 

 are to be regarded as co-equal in their importance for co-ordina- 

 tion. They are commonly combined, in the sense that the 

 accuracy of a muscular contraction delicately adjusted to the 

 extent and force of the movement which is required is usually 

 a result of the graded combination of both inhibitory and ex- 

 citatory influences coalescing upon the motor centres involved. 

 Each therefore is not only itself capable of finely graded adjust- 

 ment of intensity, but each forms also a means of finely grading 

 the intensity of the other (fig. 2). Reflex inhibition with its 

 gradation constitutes therefore a main means for co-ordinating 

 to the momentary requirements of the organism the intensity 

 of activity of its chief motor machinery, the skeletal musculature. 



