ADJUSTMENT THE NERVOUS SYSTEM 125 



effect on the discharging neurone, making it for the moment in- 

 capable of discharging. When the inhibitory influence ceases, the 

 neurone has been put into a state of rest in preparation for taking 

 its part in a new reflex act. Suppose that a motor neurone is dis- 

 charging under constant stimulus from a receptor. Inhibition 

 does not mean putting a block in the path of the stimuli, since the 

 motor neurone would continue its discharge for some time after- 

 wards. It is actually caused, by a direct influence, to stop dis- 

 charging practically instantaneously. The muscle contracting under 

 its discharge relaxes to its full length suddenly (p., pp. 410 and 4 14). 



Some of the ways in which inhibition works in nerve centres 

 may be realised from Fig. 6. The fibre C, when stimulated, inhibits 

 the motor neurone directly. A reflex through an intermediate 

 neurone, ^.an also be stopped by inhibiting this intermediate 

 neurone, as by F. An interesting case is when a reflex is being 

 elicited by stimulation of E. If, at the same time, H is being 

 stimulated, it sets the neurone F into activity, and this stops the 

 reflex. But the reflex can be restored if the inhibiting fibre G is 

 also stimulated along with the other two, since it stops the activity 

 of the inhibiting neurone, and leaves the neurone A free to convey 

 the exciting impulses from E. Thus an inhibitory nerve may 

 appear to start a reflex. 



Since the two processes of excitation and inhibition are 

 opposite, it is possible, stimulating them both in appropriate 

 strength, to make them mutually abolish one another, so that no 

 effect results. This can be shown in the case of certain muscles of 

 the thigh, which are caused to contract when a particular afferent 

 nerve of the opposite side is stimulated, and to relax if the corres- 

 ponding nerve of their own side is stimulated. By different relative 

 strengths of the two stimuli, various intermediate states between 

 full contraction and relaxation can be brought about. Similar 

 phenomena can be observed in the case of the nerves to the heart 

 and the blood vessels, about which more will be said in the next 

 chapter. 



An important aspect of muscular movements is that known as 

 reciprocal innervation. Suppose that a limb can be either bent or 

 straightened by the action of muscles, which are therefore antagon- 

 istic in their effects. It is found that when a reflex or voluntary 

 movement involves the contraction of one set, the antagonists are 

 concurrently relaxed by inhibition of their motor centres. It is 

 clear that for the exact performance of delicate movements, such 

 as those of the eye and the hand, the relaxation of the antagonistic 

 muscles must proceed step by step with the contraction of the 

 muscles producing the movement, and that, by this means, a very 

 accurate adjustment of the movement can be made. 



