THE MECHANISM OF CO-ORDINATED MOVEMENTS :;n 



tion. Every successfuLxefle^^.^oae which actually occurs. ,nh,b,ts 111 

 other reflexes which are not co-operative with the one which is taking place 

 We may, for instance, stimulate the area of skin which gives riJTtothe 

 scratch reflex, and at the same time apply a painful stimulus to the foot 

 The result is not a movement compounded of the two reflexes, but as a rule' 

 the flexor reflex preponderates. If, for instance, the scratch reflex be 

 proceeding and then the foot be pricked, the scratch reflex immediately 

 comes to an end, and the flexor reflex occurs. 

 When this in its turn has come to an end, 

 the scratch reflex may be once more re- 

 sumed (Fig. 170). 



One stimulus may reinforce another if 

 the reactions ensuing on the two stimuli 

 are allied i.e. tend to co-operate one with 

 another. In every other case, however, an 

 afferent impulse entering the cord and 

 spreading to a motor mechanism, so as 

 to produce a co-ordinate contraction of 

 various muscles, causes at the same time 

 inhibition of the muscle's antagonistic to the 

 movement, and a block, or inhibition, in all 

 other reflex arcs of the cord. 



The anatomical basis of the various 

 events involved in the carrying out of such 

 a reflex as that just studied is shown in the 

 diagram (Fig. 171). In this diagram the 

 nerve-fibre a represents the pain- receiving 

 or nociceptive nerve from the skin of the 

 foot. This passes by a posterior root into 

 the spinal cord, where it divides and gives 

 off a number of collaterals. These collaterals, 

 as we have already seen, pass in various 

 directions; some to the neighbouring 



grey matter, some to the centres in of a pathic stimulus to foot, 

 the higher parts of the nervous system. Signal A, stimulation of scratch 



XT i I- -i area. Signal B, stimulation of paw 



IN eglecting the latter and any intermediate by strong induction shock, 

 neuron which may be intercalated between 



the afferent fibre and the motor cell, we see that those collaterals which 

 affect the motor cells of the muscles of the two hind limbs can be 

 divided into two sets, one of which always produces during activity excitation 

 in certain efferent neurons, whilst the other produces inhibition of the 

 efferent neurons of the antagonistic muscles. The single afferent nerve fibre 

 is therefore, with regard to one set of its central terminal branches, specifically 

 excitor, and, in regard to another set of its central endings, specifically 

 inhibitor. In the case in point the central terminal branches of the 

 nerve a are excitor for the flexor muscles of the same side and inhibtor 



