940 



CENTRAL NERVOUS SYSTEM 



than the particular level of that reflex, will also be able to induce the 

 inhibition. In the case of the decerebrate cat this can be demonstrated 

 by stimulation of the lateral columns of the spinal cord; inhibition of 

 the extensor muscles of the elbow joint occurs, which is all the more 

 marked because in such a preparation these muscles are in a state of 

 hypertonicity. Through the pyramidal tract impulses may descend from 

 the cerebrum which exercise a marked inhibitory influence over the reflex 

 activities of the cord. 



Finally, it must be pointed out that this mechanism of reciprocal in- 

 hibition is by no means confined to the voluntary muscles. We have 



Fig. 240. Sherrington's diagram illustrating the mechanism of reciprocal inhibition. The 

 afferent fibers (d) from the skin of the leg and (a) from the flexor muscles of the knee (in 

 hamstring nerve) pass to the spinal cord, where each gives off a branch which divides into two 

 others, of which one in each case goes to a motor neuron of the extensor muscles () and the 

 other to a motor neuron (6) of the flexor muscles (F). Branches also pass across the median 

 line to similar motor neurons on the opposite side of the cord. As indicated by the plus and 

 minus signs, the afferent stimuli either stimulate or inhibit the activities cf the motor neurons, 

 the determination of the exact effect being a function of the synapsis. (From Sherrington.) 



already seen that it occurs in the case of the myenteric reflex. It is also 

 a most important function in the innervation of the blood vessels, dilata- 

 tion in one vascular area being accompanied by constriction in another. 

 These facts have been already sufficiently dwelt upon elsewhere (page 

 247). Sometimes also we may have reciprocal action between differently 

 acting nervous mechanisms, as for example in the case of the submaxil- 

 lary glands, which respond to stimulation of the chorda tympani nerve 

 by dilatation of the blood vessels, an inhibition of their tone occurring 

 along with stimulation of the activity of the gland cells. 



